90% draft - GHL Consultants Ltd.

Transcription

FPInnovations
570 Saint-Jean Blvd
Pointe-Claire (Québec) H9R 3J9
Technical Guide for the Design and Construction of
Tall Wood Buildings in Canada
90% DRAFT
August 30, 2013
This project was financially supported by the Canadian Forest Service
under the Contribution Agreement existing between
FPInnovations and Natural Resources Canada.
90% DRAFT
Notice
The “90% Draft” of Technical Guide for the Design and Construction of Tall Wood Buildings in Canada
was developed based on input from a broad group of experts. Although every reasonable effort has been
made to make this work accurate and authoritative, FPInnovations and the contributors to the document
do not warrant and assume no liability for the accuracy or completeness of the information or its fitness
for any particular purpose. It is the responsibility of users to exercise professional knowledge and
judgement in the use of the information.
The 1st edition of the Technical Guide for the Design and Construction of Tall Wood Buildings in Canada
is scheduled to be released on February, 28th, 2014. The 1st edition of the Guide, including the subjects
covered and the level of detail presented, may differ from the 90% Draft.
Inquiries
Please direct inquiries on this document to:
FPInnovations
2665 East Mall
Vancouver, BC
Canada V6T 1Z4
www.fpinnovations.ca
Erol Karacabeyli
(editor)
[email protected]
Conroy Lum
(co-editor)
[email protected]
© 2013 FPInnovations. All Rights reserved.
No part of this published Work may be reproduced, published, or transmitted for commercial purposes, in any form or by any means, electronic,
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The information contained in this Work represents current research results and technical information made available from many sources,
including researchers, manufacturers, and design professionals. The information has been reviewed by professionals in wood design including
professors, design engineers and architects, and wood product manufacturers. While every reasonable effort has been made to insure the
accuracy of the information presented, and special effort has been made to assure that the information reflects the state-of-the-art, none of the
above-mentioned parties make any warranty, expressed or implied, or assume any legal liability or responsibility for the use, application of,
and/or reference to opinions, findings, conclusions, or recommendations included in this published work, nor assume any responsibility for the
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This published Work is designed to provide accurate, authoritative information but is not intended to provide professional advice. It is the
responsibility of users to exercise professional knowledge and judgment in the use of the information.
90% DRAFT
Acknowledgements
The development of the Guide was supported by Natural Resources Canada (NRCan). The Guide is part
of the Canadian tall wood building initiative, which is overseen by a Steering Committee comprising
representatives from NRCan, Canadian Wood Council (CWC), Forestry Innovation Investment (FII),
National Research Council (NRC), Binational Softwood Lumber Council (BSLC), the wood industry and
FPInnovations.
A Working Group comprised of Erol Karacabeyli of FPInnovations, Michael Green of MGA, Eric Karsh
of Equilibrium, Andrew Harmsworth of GHL, Dave Ricketts of RDH, Joe Rekab of BTY, Kevin D.
Below of Douglas Consultants, Cameron McCartney of NRC, and Helen Griffin of CWC has overseen
the development of the Guide.
The contributions of the experts including staff of FPInnovations who devoted much time to the
development of the Guide are also greatly acknowledged.
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LIST OF CONTRIBUTORS
MGA
Equilibrium Consulting Inc.
Douglas Consulting
GHL Consultants Ltd.
RDH Building Engineering Ltd.
Fast+Epp
BTY
Canadian Wood Council
FPInnovations
National Research Council
mg-architecture.ca
Michael Green,
eqcanada.com
Eric Karsh, M.Eng, P.Eng., Struct.Eng, MIStructE, Ing
Ilana Danzig, P.Eng.
Robert Malczyk, M.A.Sc., P.Eng., Struct.Eng., MIStructE, MBA
Mahmoud Rezai, Ph.D., P.Eng., Struct.Eng., PE
douglasconsultants.ca
Kevin D. Below, ing., P.Eng., Ph.D. Structures
ghl.ca
Andrew Harmsworth, M.Eng., P.Eng., CP
Gary Chen, M.A.Sc, P.Eng.
rdhbe.com
Dave Ricketts, M.Sc., P.Eng.
Graham Finch, Dipl.T, M.A.Sc, P.Eng
fastepp.com
Gerry Epp, M.Eng., P.Eng., Struct.Eng., P.E.
Bernhard Gafner, P.Eng, MIStructE, C.Eng., Dipl. Ing. FH/STV
www.bty.com
Joe Rekab, MRICS, PQS, B.Sc.(Honours) QS
Olivier Barjolle, Wood Technology Engineer
Angela Lai, MRICS, PQS, BSc(Honours)QS, MSc (Building)
Ashley Perry, LEED Green Associate
cwc.ca
Helen Griffin, M.A.Sc., P.Eng.
Robert Jonkman, P.Eng.
Peggy Lepper, M.Sc.
Peter Moonen
Adam Robertson, M.A.Sc., P.Eng.
Jasmine Wang, Ph.D., P.Eng..
fpinnovations.ca
Erol Karacabeyli, M.A.Sc., P.Eng.
Christian Dagenais, Eng. M.Sc.
Sylvain Gagnon, ing., Eng.
Lin Hu, Ph.D.
Ken Koo, P.Eng., P.E.
Conroy Lum, M.A.Sc., P.Eng.
Mohammad Mohammad, Ph.D., P.Eng.
Paul Morris, Ph.D.
Chun Ni, Ph.D., P.Eng.
Jennifer O'Connor, M.Arch
Ciprian Pirvu, Ph.D., P.Eng.
Marjan Popovski, Ph.D., P.Eng.
Constance Thivierge, ing., M.Sc.
Jieying Wang, Ph.D.
www.nrc-cnrc.gc.ca
Cameron McCartney
Bruno Di Lenardo, P.Eng.
Brad Gover
Gary Lougheed
Michael Lacasse
Ghassan Marjaba
Joseph Su, Ph.D.
Michael Swinton
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Architect AIBC FRAIC AIA
Ausenco
ausenco.com
John Sherstobitoff,
Bird Construction
www.bird.ca
John Davidson
www.brantwoodreci.com
Helen Goodland,
carleton.ca
George Hadjisophocleous,
Brantwood Consulting
Carleton University
Consultant
CFT Engineering Inc.
CHM Fire Consultants Ltd.
RIBA MBA LEED AP
Michael McNaught,
www.cftengineering.com
Robert Heikkila,
chmfire.ca
Jim Mehaffey,
Deputy Chief Building Official
Colorado State University
P.Eng., M.Sc. Civil Eng.
Architect AIBC MRAIC
MBA, P.Eng., C.P.
Ph.D.
Tim Ryce, MPhil (Eng.), P.Eng.
colostate.edu
John van de Lindt,
edificeexperts.com
David Khudaverdian,
Empa
empa.ch
Stefan Schoenwald,
FCBA
fcba.fr
Jean-Luc Kouyoumji
Édifice Experts Inc.
Consultant
LEDCOR Group
LMDG Building Code Consultants Ltd.
McFarland Marceau Architects Ltd.
Morrison Hershfield
Nicola LogWorks Ltd.
PCL Constructors Westcoast Inc.
Perkins+Will Canada
Polygon Construction Management Ltd.
Quaile Engineering Ltd.
Read Jones Christoffersen Ltd.
Sereca Fire Consulting Ltd.
Ph.D., P.Eng.
MPhil (Eng), P.Eng.
ing.
Ph.D.
Jim Taggart
www.ledcor.com
Richard Aarestad
www.lmdg.com
Geoff Triggs,
AScT, Eng.L.
mmal.ca
Leung Chow,
architect AIBC, LEED AP
www.morrisonhershfield.com
Mark Lawton,
Mark Lucuik
www.loghome.com
John Boys
www.pcl.com
Vince Tersigni
www.perkinswill.ca
Kathy Wardle, LEED AP BD+C
Robert Drew, MAIBC, LEED BD+C
Jana Foit, Architect AIBC, LEED® AP BD+C
www.polyhomes.com
Bob Bryant
quaileeng.com
Steven Boyd,
www.rjc.ca
Ron DeVall, Ph.D., P.Eng.
Grant Newfield, B.Sc, M.Eng, P.Eng., Struct.Eng
Leslie Peer, BASc (Eng.), Ph.D., P.Eng., FEC, BEP, RRC, LEED AP
serecafire.com
Peter Senez,
90% DRAFT
P.Eng, FEC
P.Eng.
M.Eng., P.Eng.
Styxworks
Tango Management
styxworks.com
Jens Hackethal
www.tangomanagement.ca
John Bowser, GSC
Thomas Leung Structural Engineering
Thomas Leung,
P.Eng., Struct.Eng., MIStructE.
University of British Columbia
ubc.ca
Thomas Tannert,
University of New Brunswick
unb.ca
Y.H. Chui,
uwaterloo.ca
John Straube,
wsu.edu
Dan Dolan,
University of Waterloo
Washington State University
90% DRAFT
Ph.D.
Ph.D., P.Eng.
Ph.D.
Ph.D. P.E.
Foreword
This is the "90% Draft" of the 1st Edition of the Technical Guide for the Design and Construction of Tall
Wood Buildings in Canada. The 90% Draft is intended to be used by those design teams participating in
the “2013 Tall Wood Structure Demonstration Projects” Expression of Interest (EOI) that is currently
underway. The majority of the chapters have been peer reviewed; however, a few chapters or sections are
still undergoing review. The 1st Edition of the Guide is scheduled to be released by February 28, 2014
and will take into consideration the feedback from remaining peer reviews and those participating in the
EOI.
Tall wood buildings covered by the Guide are beyond the height and area limits currently found in the
National Building Code of Canada (NBCC). With the use of modern mass timber products such as Cross
Laminated Timber (CLT) and Structural Composite Lumber (SCL), "Tall Wood" is a goal that our
assembled team of experienced architects, engineers, cost consultants, contractors, and researchers
believe is achievable.
This Guide is intended to be used by experienced design and construction teams. The Guide provides
these teams with the concepts and background to questions that inevitably arise when designing beyond
the height and area limits prescribed by the NBCC.
We welcome your comments and suggestions on the Guide.
Editor: Erol Karacabeyli
Co-Editor: Conroy Lum
Links:
2013 Tall Wood Structure Demonstration Projects
Mass Timber
http://www.cwcdemoproject.ca
http://www.masstimber.com/
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Technical Guide for the Design and Construction of Tall Wood Buildings in Canada
Table of Contents
Foreword ...................................................................................................................................................................... vii
Table of Contents ........................................................................................................................................................ viii
List of Tables............................................................................................................................................................... xix
List of Figures ............................................................................................................................................................. xix
CHAPTER 1
Introduction .................................................................................................................................... 1
1.1 Defining “Tall Wood Building” ....................................................................................................................... 1
1.2 Why Wood In Tall Buildings? ....................................................................................................................... 2
1.2.1 A Renewable, Carbon Sequestering Alternative for Urban Structures........................................... 2
1.2.2 Cost Competitiveness .................................................................................................................... 3
1.3 Tall Wood Buildings to Date ......................................................................................................................... 3
1.4 High Rise Wood Demonstration Projects ..................................................................................................... 8
1.5 Guiding Principals ........................................................................................................................................ 9
1.6 Organisation of the Guide ............................................................................................................................ 9
1.7 References ................................................................................................................................................. 11
CHAPTER 2
The Building as a System ............................................................................................................ 12
Abstract................................................................................................................................................................. 12
2.1 Architecture and Structure.......................................................................................................................... 13
2.1.1 Selecting a Structural Approach .................................................................................................. 14
2.1.1.1
Building Program Considerations............................................................................... 15
2.1.1.2
Planning Considerations of Tall Wood Structures ...................................................... 15
2.1.1.2.1 Planning for Lateral Load Resistance: Vertical Circulation Core .............. 16
2.1.1.2.1.1 Mass Timber Panel Core .................................................... 16
2.1.1.2.1.2 Concrete Core .................................................................... 17
2.1.1.2.2 Planning for Lateral Load Resistance: Perimeter Shear and Load Bearing
Walls ........................................................................................................ 17
2.1.1.2.3 Planning for Lateral Load Resistance: Interior Shear and Load Bearing
Walls ........................................................................................................ 18
2.1.1.2.4 Planning for Lateral Load Resistance: Trusses ........................................ 18
2.1.1.2.5 Planning for Lateral Load Resistance: Moment Frames .......................... 19
2.1.1.2.6 Planning for Lateral Load Resistance: Diagonal Bracing ......................... 19
2.1.2 Selecting a Systems Integration and Aesthetic Considerations ................................................... 19
2.2 Integrating Systems ................................................................................................................................... 20
2.2.1 Mass Timber and Hybrid Mass Timber Concrete Ceilings ........................................................... 21
2.2.2 Structural Mass Timber Walls ...................................................................................................... 21
2.2.3 Floor Assemblies ......................................................................................................................... 21
2.2.4 Mechanical/Plumbing Systems .................................................................................................... 22
2.2.5 Electrical Systems........................................................................................................................ 23
2.2.6 Fire Suppression Systems ........................................................................................................... 23
2.3 Important Considerations ........................................................................................................................... 24
2.3.1 Acoustics ..................................................................................................................................... 24
2.3.1.1
Types of Sound .......................................................................................................... 24
2.3.1.1.1 Flanking Sound ........................................................................................ 24
2.3.1.2
Measuring Sound ....................................................................................................... 24
2.3.1.2.1 Sound Transmission Class....................................................................... 24
2.3.1.2.2 Impact Insulation Class ............................................................................ 24
2.3.1.3
Design Considerations ............................................................................................... 25
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2.4
2.5
2.6
2.7
2.3.1.3.1 Mass......................................................................................................... 25
2.3.1.3.2 Discontinuity ............................................................................................. 25
2.3.1.3.3 Resilient Connections .............................................................................. 25
2.3.1.3.4 Sound Absorbing Materials ...................................................................... 25
2.3.1.3.5 Assembly Components ............................................................................ 26
2.3.2 Energy Efficiency ......................................................................................................................... 27
2.3.3 Architectural Finishing.................................................................................................................. 29
2.3.4 Constructability ............................................................................................................................ 29
2.3.5 Costing......................................................................................................................................... 29
2.3.5.1
Cost Implications of Different Assemblies and Comparison to Traditional Assemblies29
2.3.5.2
Costs of Deconstruction, Salvaging, Recycling, Re-use and Waste Disposal ........... 29
Structural Capacity for Alterations .............................................................................................................. 30
Building Code Compliance ......................................................................................................................... 30
2.5.1 History of the National Building Code of Canada ......................................................................... 30
2.5.2 Objective Approach to Building Code Compliance ...................................................................... 30
2.5.2.1
Acceptance by Authorities Having Jurisdiction........................................................... 31
2.5.2.2
Objectives and Functional Statements....................................................................... 32
2.5.2.3
Level of Performance ................................................................................................. 33
2.5.2.4
Fire Implications ......................................................................................................... 33
2.5.2.4.1 Exposed Mass Timber ............................................................................. 33
2.5.2.4.2 Encapsulation........................................................................................... 34
2.5.2.4.3 Recommended Approach to Fire Protection ............................................ 34
2.5.2.4.4 Exterior Fire Spread ................................................................................. 34
2.5.2.4.5 Additional Considerations ........................................................................ 34
2.5.3 Alternative Solutions That May Be Required ............................................................................... 35
Examples of Tall Wood Building System Solutions .................................................................................... 35
2.6.1 All-Wood Systems........................................................................................................................ 36
2.6.1.1
FFTT .......................................................................................................................... 36
2.6.1.1.1 Structure................................................................................................... 36
2.6.1.1.2 Integration of Services ............................................................................. 38
2.6.1.1.3 Constructability......................................................................................... 40
2.6.1.1.4 Flexibility .................................................................................................. 40
2.6.1.2
Platform Approach: Stadthaus ................................................................................... 41
2.6.1.2.1 Structure................................................................................................... 41
2.6.1.2.2 Integration of Services ............................................................................. 43
2.6.1.2.4 Flexibility .................................................................................................. 43
2.6.2 Wood-Concrete Hybrid Systems .................................................................................................. 44
2.6.2.1
CREE (Creative Resource and Energy Efficiency) .................................................... 44
2.6.2.1.1 Structure................................................................................................... 44
2.6.2.1.2 Integration of Building Services ................................................................ 47
2.6.2.1.4 Flexibility .................................................................................................. 48
2.6.2.2
Concrete Jointed Timber Frame Solution................................................................... 48
2.6.2.2.1 Structure................................................................................................... 48
2.6.2.2.2 Service Integration ................................................................................... 51
2.6.2.2.4 Flexibility .................................................................................................. 51
References ................................................................................................................................................. 51
CHAPTER 3
Sustainability ................................................................................................................................ 52
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Abstract ........................................................................................................................................................................ 52
3.1 Introduction ................................................................................................................................................ 53
3.1.1 Defining Sustainability.................................................................................................................. 53
3.1.2 Sustainability and Tall Wood Building Systems ........................................................................... 53
3.2 Material Sourcing and Forest Impact ......................................................................................................... 54
3.2.1 Sustainable Forest Management ................................................................................................. 54
3.2.1.1
Defining Sustainable Forest Management ................................................................. 54
3.2.1.2
Sustainable Forest Management in Canada .............................................................. 55
3.2.2 Forest Certification ....................................................................................................................... 55
3.2.2.1
Canadian Standards Association’s Sustainable Forest Management Standards (CSA
SFM) .......................................................................................................................... 56
3.2.2.2
Forest Stewardship Council (FSC) ............................................................................. 56
3.2.2.3
Sustainable Forestry Initiative (SFI) ........................................................................... 56
3.2.3 Carbon Storage and Savings in Emissions .................................................................................. 57
3.2.4 Sourcing Regionally Available Materials ...................................................................................... 57
3.3 Transportation of Materials......................................................................................................................... 58
3.4 Construction Waste and By-product Use ................................................................................................... 58
3.5 Durability and Longevity ............................................................................................................................. 59
3.6 Re-use and End of Life .............................................................................................................................. 60
3.7 Impact on Human Health, Well-Being, and Comfort .................................................................................. 60
3.7.1 Indoor Air Quality and Toxicity ..................................................................................................... 60
3.7.1.1
Structural Adhesives .................................................................................................. 61
3.7.1.1.1 Formaldehydes ........................................................................................ 63
3.7.1.1.2 Adhesives and Fire Performance ............................................................. 63
3.7.1.2
Treatments for Wood-Destroying Organisms and Wood Rot ..................................... 63
3.7.1.2.1 Alkaline Copper Quaternary (ACQ) .......................................................... 64
3.7.1.2.2 Copper Azole (CA) ................................................................................... 64
3.7.1.2.3 Micronized Copper Azole (MCA) .............................................................. 64
3.7.1.2.4 Borates ..................................................................................................... 64
3.7.1.2.5 Wolman AG .............................................................................................. 64
3.7.1.2.6 Cyproconazole ......................................................................................... 64
3.7.1.2.7 Propiconazole .......................................................................................... 64
3.7.1.3
Fire Treatments.......................................................................................................... 65
3.7.1.4
Ventilation and Air Tightness ..................................................................................... 65
3.8 Tools to Measure, Evaluate, and Certify Sustainability .............................................................................. 65
3.8.1 Life Cycle Assessment................................................................................................................. 65
3.8.1.1
The Benefits of Life Cycle Assessment...................................................................... 66
3.8.1.2
Life Cycle Assessment Tools and Approaches .......................................................... 67
3.8.2 Green Building Certification Systems........................................................................................... 68
3.8.2.1
Leadership in Energy and Environmental Design (LEED) ......................................... 68
3.8.2.2
The Living Building Challenge.................................................................................... 70
3.8.2.3
BuiltGreen High Density Program .............................................................................. 71
3.8.2.4
Green Globes Design for New Buildings.................................................................... 71
3.8.3 The Carbon Calculator ................................................................................................................. 72
3.8.4 The Wood Calculator ................................................................................................................... 72
3.8.5 Environmental Product Declarations............................................................................................ 72
3.9 References ................................................................................................................................................. 73
CHAPTER 4
Structural and Serviceability ........................................................................................................ 76
4.1 Recommendations for Conceptual Design ................................................................................................. 77
Abstract................................................................................................................................................................. 77
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4.1.1
4.1.2
Introduction .................................................................................................................................. 77
Tall Wood Case Studies .............................................................................................................. 79
4.1.2.1
Historical Case Studies .............................................................................................. 79
4.1.2.1.1 Ancient Pagodas ...................................................................................... 79
4.1.2.1.2 Churches and Monasteries ...................................................................... 80
4.1.2.1.3 Early Post and Beam Timber Structures .................................................. 80
4.1.2.2
Modern Case Studies................................................................................................. 81
4.1.2.2.1 Light Framing ........................................................................................... 82
4.1.2.2.2 Tall Timber Structures with Mass Timber ................................................. 82
4.1.2.2.3 Hybrid Structures ..................................................................................... 86
4.1.2.2.4 Structures on a Single Storey Concrete Podium ...................................... 91
4.1.3 Conceptual Structural Systems .................................................................................................... 93
4.1.3.1
Structural Materials .................................................................................................... 93
4.1.3.2
Gravity Load Systems ................................................................................................ 98
4.1.3.3
Lateral Loads and Complete Building Systems........................................................ 100
4.1.3.4
Tall Buildings on a Podium....................................................................................... 108
4.1.4 Practical Guidelines for Given Heights....................................................................................... 109
4.1.5 Foundations for Tall Wood Buildings ......................................................................................... 110
4.1.6 Conclusion ................................................................................................................................. 110
4.1.7 References................................................................................................................................. 111
4.2 Design Considerations and Input Parameters for Connections and Assemblies ..................................... 113
Abstract............................................................................................................................................................... 113
4.2.1 Introduction ................................................................................................................................ 114
4.2.2 Wood-Related Analysis and Design Considerations.................................................................. 115
4.2.2.1
Mechanical Properties of Wood ............................................................................... 115
4.2.2.2
Size Effect ................................................................................................................ 116
4.2.2.3
Compression Perpendicular to Grain ....................................................................... 117
4.2.2.4
Shrinkage and Swelling............................................................................................ 120
4.2.2.5
Tension Perpendicular to Grain ............................................................................... 123
4.2.2.6
Duration of Load and Creep ..................................................................................... 124
4.2.2.7
Punching Shear ....................................................................................................... 125
4.2.2.8
Transverse Reinforcement of Connections .............................................................. 125
4.2.2.9
Fire Performance (also refer to Section 5) ............................................................... 127
4.2.2.10 Cost Considerations (also refer to Chapter 8) .......................................................... 129
4.2.3 Input Data for Connections and Assemblies .............................................................................. 130
4.2.3.1
Strength ................................................................................................................... 130
4.2.3.2
Stiffness ................................................................................................................... 131
4.2.3.3
Ductility .................................................................................................................... 132
4.2.3.4
Damping................................................................................................................... 133
4.2.3.5
Evaluating, Testing and Detailing of Connections and Assemblies ......................... 135
4.2.3.5.1 Evaluation of Connections...................................................................... 135
4.2.3.5.2 Evaluation of wall, floor or roof assemblies ............................................ 136
4.2.3.6
Deriving Design Values for Connections and Assemblies based on Test Data or
Design Data from Other Jurisdictions ...................................................................... 137
4.2.3.6.1 Connections ........................................................................................... 137
4.2.3.6.2 Wall, Floor and Roof Assemblies ........................................................... 138
4.2.3.7
Requirements for Proprietary Connections .............................................................. 139
4.2.4 References................................................................................................................................. 140
4.3 Advanced Analysis and Testing of Systems for Design ........................................................................... 142
Abstract............................................................................................................................................................... 142
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4.3.1
4.3.2
4.3.3
4.3.4
National Building Code of Canada ............................................................................................. 143
4.3.1.1
Objectives and Functional Statements..................................................................... 143
4.3.1.2
Building Code Compliance ....................................................................................... 143
4.3.1.3
Performance Levels ................................................................................................. 143
Analysis and Design for Gravity Loads ...................................................................................... 145
4.3.2.1
General Analysis and Design Approach .................................................................. 145
4.3.2.2
Structural Integrity and Progressive/Partial Collapse ............................................... 145
4.3.2.2.1 Progressive and Disproportional Collapse ............................................. 145
4.3.2.2.2 Design Alternatives ................................................................................ 146
4.3.2.2.3 General Building Design Guidelines for Better Structural Integrity ......... 147
4.3.2.2.4 United Kingdom Regulations on Disproportionate Collapse................... 147
4.3.2.2.5 Blast Protection of Buildings................................................................... 149
4.3.2.3
Wall/column to Foundation Interface........................................................................ 149
4.3.2.4
Testing to Support Gravity Load Analyses and Design ............................................ 150
4.3.2.5
Compatibility of Gravity System for Lateral Load Demand....................................... 150
Analysis and Design for Earthquake Loads ............................................................................... 151
4.3.3.1
Seismic Force Resisting Systems SFRS and Force modification factors ................ 151
4.3.3.1.1 FEMA P-695 Procedure ......................................................................... 153
4.3.3.1.2 FEMA P-795 Procedure ......................................................................... 154
4.3.3.1.3 AC-130 Equivalency Approach .............................................................. 155
4.3.3.1.4 Other Procedures ................................................................................... 155
4.3.3.1.5 R-factors for Dual and Hybrid Systems .................................................. 156
4.3.3.2
Methods for Seismic Analysis .................................................................................. 157
4.3.3.2.1 Equivalent Static Procedure ................................................................... 157
4.3.3.2.2 Linear Dynamic Analyses ....................................................................... 158
4.3.3.2.2.1 Modal analysis .................................................................. 158
4.3.3.2.2.2 Linear response history analysis....................................... 159
4.3.3.2.3 Nonlinear Static Analyses ...................................................................... 160
4.3.3.2.4 Nonlinear Dynamic Analysis................................................................... 161
4.3.3.2.5 Input needed for the Analyses................................................................ 163
4.3.3.3
Analytical Models, Software AND Model Verification ............................................... 164
4.3.3.3.1 Software and Analytical Models ............................................................. 164
4.3.3.3.2 Model Verification and Comparison of Results ...................................... 165
4.3.3.4
Methods of Seismic Design...................................................................................... 165
4.3.3.4.1 Force-Based Design .............................................................................. 165
4.3.3.4.2 Displacement-Based Design .................................................................. 167
4.3.3.4.3 Performance-Based Design ................................................................... 169
4.3.3.5
Capacity-Based Design Procedures ........................................................................ 176
4.3.3.6
Diaphragm flexibility and its Influence on Seismic Response .................................. 179
4.3.3.7
Discontinuities in Plan and Elevation ....................................................................... 180
4.3.3.8
Lateral Drifts............................................................................................................. 181
4.3.3.9
Testing needed to Support Seismic Load Analyses and Design .............................. 181
Analysis and Design for Wind Loads ......................................................................................... 181
4.3.4.1
Static Analysis.......................................................................................................... 181
4.3.4.2
Dynamic Analysis ..................................................................................................... 181
4.3.4.3
Vortex Shedding ...................................................................................................... 182
4.3.4.4
Experimental Analysis and Testing .......................................................................... 183
4.3.4.5
Deflections and Wind Induced Vibrations-Controlled Design ................................... 183
4.3.4.5.1 Deflection Controlled Design .................................................................. 183
4.3.4.5.1.1 Design Criterion ................................................................ 183
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4.3.4.5.1.2 Design Values of the Building Components...................... 184
4.3.4.5.2 Wind-Induced Vibration Controlled Design ............................................ 184
4.3.4.6
Testing needed to Support Wind Load Analyses and Design .................................. 184
4.3.5 Design Methodologies for Low Seismic Damage....................................................................... 185
4.3.5.1
Passive and Active Seismic Isolation and Vibration Control .................................... 185
4.3.5.2
Rocking Self Centering Post Tensioned Systems .................................................... 186
4.3.6 Quality Assurance ...................................................................................................................... 189
4.3.7 Recommendations for Future Work ........................................................................................... 190
4.3.8 References................................................................................................................................. 190
4.4 Building Sound Insulation and Floor Vibration Control ............................................................................. 198
Abstract............................................................................................................................................................... 198
4.4.1 Building Sound Insulation .......................................................................................................... 199
4.4.1.1
Scope ....................................................................................................................... 199
4.4.1.2
Terms and Definitions .............................................................................................. 199
4.4.1.3
NBC and Other Code Requirements........................................................................ 200
4.4.1.4
Principles for Building Sound Insulation Design ....................................................... 201
4.4.1.5
Wood–Based Wall Sound Insulation ........................................................................ 202
4.4.1.5.1 Light-Frame Wood Stud Walls ............................................................... 202
4.4.1.5.2 CLT Walls............................................................................................... 203
4.4.1.5.3 Other Wall Construction ......................................................................... 204
4.4.1.6
Wood- Based Floors Sound Insulation..................................................................... 204
4.4.1.6.1 Light-Frame Joisted Wood Floors .......................................................... 204
4.4.1.6.2 CLT Floors ............................................................................................. 205
4.4.1.6.3 Massive Timber Floors ........................................................................... 207
4.4.1.6.4 Wood Concrete Composite Floors ......................................................... 208
4.4.1.6.5 Other Floor Construction ........................................................................ 208
4.4.1.7
Wooden Building Sound Insulation System Performance ........................................ 208
4.4.1.7.1 Wood Frame Buildings ........................................................................... 209
4.4.1.7.2 CLT Buildings ......................................................................................... 210
4.4.1.7.3 Massive Wood and Wood-Hybrid Buildings ........................................... 210
4.4.1.8
Best Practices for Ensuring End Users’ Satisfaction – Step by Step Guide ............. 211
4.4.1.8.1 Step 1: Selecting Construction Solutions for FSTC and FIIC at Least 50211
4.4.1.8.2 Step 2: Eliminating Avoidable Flanking Paths ........................................ 211
4.4.1.8.3 Step 3: Measuring FSTC and FIIC after Finishing ................................. 212
4.4.1.8.4 Step 4: Subjective Evaluation by Architects, Designers, Builders and
Contractors............................................................................................. 212
4.4.2 Floor Vibration Control ............................................................................................................... 212
4.4.2.1
Scope ....................................................................................................................... 212
4.4.2.2
Terms and Definitions .............................................................................................. 213
4.4.2.3
Control of Vibration Induced by Footsteps for Occupant’ Comfort ........................... 213
4.4.2.3.1 Design Principles for Control of Floor Vibrations induced by Footsteps. 213
4.4.2.3.2 Light-Frame Joisted Floors..................................................................... 214
4.4.2.3.3 Light-Frame Joisted Floors with Heavy Topping .................................... 216
4.4.2.3.4 CLT Floors ............................................................................................. 216
4.4.2.3.5 Massive Timber Beam Floors................................................................. 216
4.4.2.3.6 Hybrid Steel Truss and Thick Wood Deck Floors ................................... 217
4.4.2.3.7 Wood-Concrete Composite Floors ......................................................... 217
4.4.2.3.8 Other Innovative Wood-Based Floors .................................................... 218
4.4.2.4
Control of Vibration Induced by Machine for Occupant’ Comfort ............................. 218
4.4.2.5
Best Practices .......................................................................................................... 219
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4.4.3
CHAPTER 5
4.4.2.5.1 Proper Supports ..................................................................................... 219
4.4.2.5.2 Subjective Evaluation on Floors by Architects, Designers, Builders and
Contractors............................................................................................. 220
4.4.2.5.3 Field Measurement Before and After Finishing ...................................... 220
References................................................................................................................................. 220
Fire Safety and Protection ......................................................................................................... 223
Abstract ...................................................................................................................................................................... 223
5.1 Introduction .............................................................................................................................................. 224
5.1.1 Acceptable Solutions for Fire Safety .......................................................................................... 224
5.1.2 Alternative Solutions for Fire Safety ........................................................................................... 224
5.1.3 Acceptance by Authority ............................................................................................................ 225
5.1.4 Objectives and Functional Statements ...................................................................................... 226
5.1.5 Level of Performance ................................................................................................................. 227
5.1.6 Fire Dynamics and Engineering Design ..................................................................................... 227
5.2 Development of a Fire Safe Alternative Solution...................................................................................... 229
5.2.1 Approach to an Alternative Solution for Fire Safe Tall Wood Buildings ..................................... 229
5.2.1.1
Other Fire Safety Objectives and Unknown Fire Risks ............................................ 231
5.2.2 Level of Performance in the Areas Defined by Objectives and Functional Statements ............. 231
5.2.2.1
Objectives and Functional Statements related to Noncombustible Construction ..... 232
5.2.2.2
Scope of Proposed Alternative Solution................................................................... 232
5.2.2.3
Combustible Components Explicitly Permitted by Division B of the NBCC .............. 233
5.2.3 Assessment of Performance Level (Division B vs. Alternative Solution).................................... 233
5.2.3.1
Encapsulation .......................................................................................................... 233
5.2.3.2
Complete Encapsulation .......................................................................................... 234
5.2.3.3
Fully Exposed .......................................................................................................... 235
5.2.3.4
Limited Encapsulation .............................................................................................. 236
5.2.3.5
Suspended Membrane Type Encapsulation ............................................................ 237
5.2.3.6
Exposed Mass Timber within Occupied Spaces ...................................................... 238
5.2.3.7
Automatic Sprinklers ................................................................................................ 238
5.2.3.8
Non-Standard Fire Exposure ................................................................................... 239
5.2.3.9
Protection in Depth................................................................................................... 239
5.2.3.10 Practical Considerations .......................................................................................... 240
5.3 Provisions for High Buildings (Part 3 of Division B) .................................................................................. 240
5.3.1 What is Stack Effect? ................................................................................................................. 241
5.3.2 Design of Tall Shafts to Resist Movement of Smoke to an Acceptable Level ............................ 241
5.4 Fire-Resistance of Assemblies and Components..................................................................................... 242
5.4.1 What is Fire-Resistance ............................................................................................................. 242
5.4.2 Standard Fire vs. Design Fire Scenarios ................................................................................... 244
5.4.3 Behaviour of Wood at High Temperatures (Charring)................................................................ 246
5.4.4 Fire-Resistance of Timber Structure – Structural Criteria .......................................................... 250
5.4.4.1
Massive and Glued-Laminated Timber .................................................................... 250
5.4.4.2
Structural Composite Lumber .................................................................................. 251
5.4.4.3
Cross-Laminated Timber.......................................................................................... 251
5.4.4.4
Timber-Concrete Composite Structure .................................................................... 252
5.4.4.5
Fasteners and Connections ..................................................................................... 253
5.4.4.6
Structural Adhesive .................................................................................................. 255
5.4.5 Fire-Resistance of Timber Structure – Integrity Criteria ............................................................. 256
5.4.6 Fire-Resistance of Timber Structure – Insulation Criteria .......................................................... 256
5.4.7 Fire-Resistance of Gypsum Board Membranes ......................................................................... 257
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5.5
5.6
5.7
5.8
5.9
5.10
5.11
5.12
5.13
5.14
5.15
Flame Spread Rating of Exposed Timber ................................................................................................ 258
5.5.1 What is Flame Spread? ............................................................................................................. 258
5.5.2 Fire Safety Strategies in a Pre-Flashover Compartment ........................................................... 259
5.5.3 Impact of Exposed Timber on these Fire Safety Strategies ....................................................... 260
Fire Separation......................................................................................................................................... 261
5.6.1 What is a Fire Separation? ........................................................................................................ 261
5.6.2 Integrity of Fire Separations ....................................................................................................... 262
Fire Protection of Service Penetrations and Construction Joints ............................................................. 262
5.7.1 Fire Stopping.............................................................................................................................. 262
5.7.2 Availability of Fire Stop for Mass Timber Assemblies ................................................................ 262
Concealed Spaces ................................................................................................................................... 263
5.8.1 What are Concealed Spaces and the Concern with Them? ...................................................... 263
5.8.2 Performance of Combustible Concealed Spaces of Mass Timber ............................................. 264
5.8.3 Building Code and NFPA 13 Provisions for Concealed Spaces ................................................ 264
5.8.4 Methods of Protecting Concealed Spaces ................................................................................. 264
Spatial Separation and Exposure Protection............................................................................................ 265
5.9.1 Assumptions behind the Current Spatial Separation Provisions ................................................ 265
5.9.2 Exterior Cladding ....................................................................................................................... 266
5.9.3 Wildfire ....................................................................................................................................... 268
5.9.4 Roof Construction ...................................................................................................................... 268
Firefighting Assumptions .......................................................................................................................... 268
5.10.1 Firefighting Considerations in Tall Wood Buildings .................................................................... 268
Provisions for Mobility Impaired Occupants ............................................................................................. 270
Consideration for Major Natural Disasters ............................................................................................... 270
Fire Safety during Construction ................................................................................................................ 271
5.13.1 Fire Risk Factors and the Fire Problem during Construction ..................................................... 271
5.13.2 Management of Risk .................................................................................................................. 271
5.13.3 Considerations in Fire Safety during Construction and Renovations ......................................... 272
5.13.4 Construction Fire Safety Plan .................................................................................................... 272
5.13.5 Construction Fire Safety Coordinator ......................................................................................... 272
5.13.6 Pre-Construction Meeting .......................................................................................................... 273
5.13.7 Fire Watch during Off-Hours ...................................................................................................... 273
5.13.8 Firefighting Water Supply........................................................................................................... 273
5.13.9 Early Fire Compartmentalization ................................................................................................ 273
5.13.10 Exposure Protection from Wildfires ............................................................................................ 274
5.13.11 Exterior Exposure ...................................................................................................................... 274
5.13.12 Fire Safety during Renovations.................................................................................................. 274
Conclusion ............................................................................................................................................... 274
References ............................................................................................................................................... 275
Appendix 5A Fire Risk Assessment .......................................................................................................................... 279
CHAPTER 6
Building Enclosure Design ......................................................................................................... 283
Abstract ...................................................................................................................................................................... 283
6.1 Introduction .............................................................................................................................................. 284
6.1.1 Building Enclosure Systems ...................................................................................................... 284
6.2 Building Enclosure Loads ......................................................................................................................... 287
6.2.1 Climate Considerations and Environmental Loads .................................................................... 287
6.2.2 Building Movement and Structural Considerations .................................................................... 288
6.3 Building and Energy Code Considerations............................................................................................... 289
6.3.1 Canadian Building Code Considerations ................................................................................... 289
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6.3.2 Canadian Energy Code Considerations ..................................................................................... 289
Building Enclosure Design Fundamentals................................................................................................ 292
6.4.1 Moisture Control......................................................................................................................... 292
6.4.1.1
Wetting, Drying, and Safe Storage........................................................................... 292
6.4.1.2
Control Layers & Critical Barriers – Assembly Design and Detailing ....................... 293
6.4.1.3
Control of Rainwater and Assembly and Detail Design............................................ 296
6.4.1.4
Accidental Sources of Moisture................................................................................ 299
6.4.1.5
Heat Flow Control & Thermal Bridging..................................................................... 299
6.4.2 Condensation Control ................................................................................................................ 301
6.4.3 Air Flow Control ......................................................................................................................... 302
6.4.4 Noise Control ............................................................................................................................. 303
6.4.5 Fire Control ................................................................................................................................ 304
6.5 Building Enclosure Assemblies and Details ............................................................................................. 305
6.5.1 Wall Assemblies......................................................................................................................... 305
6.5.1.1
Structure and Insulation ........................................................................................... 305
6.5.1.2
Claddings and Cladding Attachment ........................................................................ 307
6.5.1.3
Appropriate Air Barrier Systems for Tall Wood Buildings ......................................... 307
6.5.1.4
Fenestration Selection and Installation Considerations ........................................... 310
6.5.2 Roof Assemblies ........................................................................................................................ 310
6.6 Protection and Wood Durability ................................................................................................................ 312
6.6.1 On-site Moisture Management................................................................................................... 312
6.6.2 Exterior Wood and Preservative Treatment ............................................................................... 314
6.7 Concluding Remarks ................................................................................................................................ 315
6.8 References ............................................................................................................................................... 315
CHAPTER 7
Prefabrication and Inspection of Assemblies ............................................................................. 317
6.4
Abstract ...................................................................................................................................................................... 317
7.1 Preamble .................................................................................................................................................. 318
7.2 General .................................................................................................................................................... 318
7.2.1 Qualification of Personnel .......................................................................................................... 319
7.2.2 Quality Assurance Programs ..................................................................................................... 319
7.2.3 Design Criteria for Prefabricated Assemblies ............................................................................ 319
7.2.4 Coordination and Fabrication Drawings / 3D Modelling ............................................................. 320
7.2.5 Testing for Design ...................................................................................................................... 320
7.2.6 Submittals .................................................................................................................................. 320
7.3 Fabrication ............................................................................................................................................... 321
7.3.1 General ...................................................................................................................................... 321
7.3.2 Qualification Procedures ............................................................................................................ 322
7.3.3 Quality Control Procedures ........................................................................................................ 322
7.3.4 Storage ...................................................................................................................................... 323
7.4 Execution ................................................................................................................................................. 323
7.4.1 Coordination............................................................................................................................... 323
7.4.2 Transportation ............................................................................................................................ 323
7.4.3 Site Modifications ....................................................................................................................... 324
7.4.4 Erection...................................................................................................................................... 324
7.5 Inspection and Records ........................................................................................................................... 324
7.6 References ............................................................................................................................................... 325
Appendix 7A Qualification and Quality Control Principles for On-Site Prefabrication of Structural Glued Wood
Assemblies.......................................................................................................................................................... 326
CHAPTER 8
Project and Construction Costing .............................................................................................. 328
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Abstract ...................................................................................................................................................................... 328
8.1 Introduction .............................................................................................................................................. 329
8.2 Knowledge Gaps in Costing Tall Wood Buildings .................................................................................... 329
8.2.1 Availability of Data ..................................................................................................................... 330
8.2.2 Cost of Innovation ...................................................................................................................... 330
8.2.3 Market Premium for Learning Curve .......................................................................................... 331
8.3 Procurement............................................................................................................................................. 331
8.3.1 Procurement of Contractors ....................................................................................................... 331
8.3.2 Procurement of Material............................................................................................................. 332
8.3.2.1
Availability of Material .............................................................................................. 332
8.3.2.2
Transportation of Material ........................................................................................ 332
8.3.2.3
Construction Site Limitations and Considerations .................................................... 333
8.3.2.4
Storage of Materials ................................................................................................. 333
8.3.2.5
Temporary Protection during Construction .............................................................. 333
8.4 Quantifying Schedule Benefits of Prefabrication ...................................................................................... 334
8.5 Disposal Costs and Opportunity for Re-Use/Recycling ............................................................................ 334
8.6 Planning/Design Process/Soft Costs........................................................................................................ 335
8.7 How to Develop the Cost Estimate........................................................................................................... 335
8.7.1 Construction Costs..................................................................................................................... 335
8.7.2 Preparing an Elemental Cost Estimate ...................................................................................... 338
8.7.3 Soft Costs .................................................................................................................................. 338
8.8 Insurance Costs ....................................................................................................................................... 340
8.8.1 Professional Indemnity............................................................................................................... 340
8.8.2 Course of Construction .............................................................................................................. 340
8.8.3 Property Insurance..................................................................................................................... 340
8.9 List of References and Appendices.......................................................................................................... 341
Appendix 8A List of Acknowledgements and Questionnaires ................................................................................... 342
CHAPTER 9
Monitoring and Maintenance...................................................................................................... 356
Abstract ...................................................................................................................................................................... 356
9.1 Introduction .............................................................................................................................................. 357
9.2 Short-term Performance Tests ................................................................................................................. 357
9.2.1 Airtightness ................................................................................................................................ 359
9.2.2 Building Natural Frequencies, Mode Shapes, and Damping Ratios .......................................... 359
9.2.3 Sound Insulation Performance of Floors and Walls ................................................................... 360
9.2.4 Floor Vibration Performance ...................................................................................................... 360
9.2.5 Thermal Resistance Testing ...................................................................................................... 361
9.3 Long-term Performance Monitoring Studies............................................................................................. 362
9.3.1 Durability Performance .............................................................................................................. 364
9.3.2 Differential Movement ................................................................................................................ 364
9.3.3 Time History of Accelerations of Wind-induced Lateral Vibrations............................................. 365
9.3.4 Energy Consumption Monitoring ................................................................................................ 366
9.4 Building Maintenance ............................................................................................................................... 366
9.4.1 Design Considerations ............................................................................................................... 367
9.4.1.1
Access ..................................................................................................................... 367
9.4.1.2
Wall Cladding Systems ............................................................................................ 367
9.4.1.3
Fenestration ............................................................................................................. 368
9.4.1.4
Dryer and other Exhaust Vents ................................................................................ 368
9.4.1.5
Material Selection .................................................................................................... 368
9.4.2 Maintenance Planning ............................................................................................................... 369
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9.5
9.6
9.4.3 Routine Inspection, Clean, Repair, and Renewal ...................................................................... 370
Summary .................................................................................................................................................. 370
References ............................................................................................................................................... 371
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List of Tables
Table 1
Table 2
Table 3
Table 4
Table 5
Table 6
Table 7
Table 8
Table 9
Table 10
Table 11
Table 12
Table 13
Table 14
Table 15
Table 16
Table 17
Table 18
Table 19
Table 20
Comparison of Different Wood Products ................................................................................................ 97
Comparison of Different Materials .......................................................................................................... 98
Typical EMC for different regions of Canada ........................................................................................ 121
Proposed ductility classes for connections by Smith et al. (2006) ........................................................ 133
IBC’s minimum requirements for sound insulation of demising walls and floor/ceiling assemblies ...... 201
ICC grades of field acoustical performance recommendations ............................................................ 201
ICC grades of laboratory acoustical performance recommendations ................................................... 201
Perceptible change due to the change in sound level (dB) (Pope 2003) .............................................. 202
STC 60 CLT wall assembly .................................................................................................................. 203
CLT wall assembly of FSTC/ASTC 50.................................................................................................. 204
STC 67 and IIC 72 CLT floor-ceiling assemblies .................................................................................. 206
FSTC and FIIC 53 field CLT floor-ceiling assemblies ........................................................................... 207
FSTC and IIC of a massive timber floor................................................................................................ 208
Flanking path checklist and treatment .................................................................................................. 212
Summary of design methods for light-weight joisted floors in literatures .............................................. 214
Design charring rates of timber as specified in Eurocode 5: Part 1-2 ................................................... 248
Flame spread rating of massive timber assemblies .............................................................................. 260
Building performance tests, parameters, and timelines of testing ........................................................ 358
NBC recommended acceleration limits for vibrations caused by rhythmic activities............................. 361
Building performance monitoring, parameters, major instruments, and timelines of instrumentation
installation............................................................................................................................................. 363
List of Figures
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Stadthaus (Waugh Thistleton Architects) ................................................................................................. 4
LCT One Tower (CREE) ........................................................................................................................... 5
Earth Sciences Building (Perkins + Will)................................................................................................... 6
Wood Innovation Design Centre (MGA | Michael Green Architecture) ..................................................... 7
Wind Turbine in Hanover, Germany ......................................................................................................... 7
Pyramidenkogel ........................................................................................................................................ 8
Diagram of the Technical Guide for Tall Wood Buildings in Canada (MGA | Michael Green Architecture)
................................................................................................................................................................ 10
WIDC Typical Floor Plan and Rendering (Michael Green Architecture) ................................................. 17
LifeCycle Tower One Typical Floor Plan and Rendering (CREE) ........................................................... 17
Stadthaus Floor Plan and Axonometric (Waugh Thistleton) ................................................................... 18
Services Integration: WIDC Approach (Michael Green Architecture) ..................................................... 22
WIDC’s “Slab” Assemblies (Michael Green Architecture) ....................................................................... 27
Summary of the two compliance paths in the NBCC .............................................................................. 31
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Figure 14
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Figure 21
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Figure 23
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Figure 25
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Figure 28
Figure 29
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Figure 40
Figure 41
Figure 42
Figure 43
Figure 44
Figure 45
Figure 46
Figure 47
Figure 48
Figure 49
Figure 50
Figure 51
Figure 52
Figure 53
FFTT Structural Configuration (Michael Green Architecture) ................................................................. 37
North Vancouver City Hall Structural Configuration (Michael Green Architecture) ................................. 37
WIDC Structural Configuration (Michael Green Architecture)................................................................. 37
“W” Floor System (Michael Green Architecture) ..................................................................................... 38
Services Integration: Encapsulation Approach (The Case for Tall Wood Buildings, Michael Green) .....39
Services Integration (The Case for Tall Wood Buildings, Michael Green) .............................................. 40
CLT Panel Structure (Waugh Thistleton) ................................................................................................ 42
CLT Panel Structure (Waugh Thistleton) ................................................................................................ 42
Encapsulated CLT Panel Structure (Waugh Thistleton) ......................................................................... 43
Column and Wood-Concrete Hybrid Slab Structure (CREE) .................................................................. 45
Service Integration Between Beams (CREE) ......................................................................................... 46
Column to Slab Connection (CREE) ....................................................................................................... 46
Façade Panels (CREE) .......................................................................................................................... 47
Fabrication of Façade Panels (CREE) .................................................................................................... 48
Floor Slabs Lifted Into Place (CREE) ..................................................................................................... 48
Concrete Jointed Timber Frame (SOM) ................................................................................................. 49
Concrete Jointed Timber Frame (SOM) ................................................................................................ 50
Environmental Impact of Structural Typologies ...................................................................................... 54
Transportation Impact ............................................................................................................................. 58
Adhesives Used in Wood Products ........................................................................................................ 62
The Life-Cycle Approach ........................................................................................................................ 66
8-storey brick-and-beam building built in Toronto in the 1920s .............................................................. 78
9-storey brick-and-beam building built in Vancouver in 1905 ................................................................. 78
Post and Beam (Left), Light Framing (Centre), Solid Construction (Right) ............................................. 79
Horyu-Ji Temple Pagoda (left) (Nakahara, Hisatoku, Nagase, & Takahashi, 2000), Yingxian Pagoda
(right) (Lam, He, & Yao, 2008)................................................................................................................ 80
Urnes Stavkirke (left) (UNESCO, n.d.), Barsana Monastery (Green, 2012) ........................................... 80
Leckie Building in Vancouver, BC........................................................................................................... 81
The Eslov Building in Sweden Built in 1918 (Source: to be determined) ................................................ 81
Marselle in Seattle, Washington (courtesy of Matt Todd Photography) .................................................. 82
Gastonia Bell Tower during Construction (courtesy of WoodWorks) ...................................................... 83
Bridport House (Lehmann, 2012) ........................................................................................................... 83
Timber Tower (courtesy of TimberTower GmbH) ................................................................................... 84
Via Cenni 9 Storey Buildings (courtesy of Prof. Arch. Fabrizio Rossi Prodi) .......................................... 84
The Studentenwohnheim in Norway (Copyright Raimund Baumgartner GmbH (Hausegger, 2013a)) ...85
Rendering of WIDC (courtesy of MGA) .................................................................................................. 85
Model of Shiang-Yang Woodtek Office Building (courtesy of Equilibrium Consulting) ...........................86
District 03 residential building, Québec City (Source: to be determined) ................................................86
Scotia Place (Moore, 2000) .................................................................................................................... 87
6-Storey Hybrid Québec City Building .................................................................................................... 88
8 Storey Timber Building in Bad Aibling (courtesy of Woodworks) ......................................................... 88
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Figure 54
Figure 55
Figure 56
Figure 57
Figure 58
Figure 59
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Figure 61
Figure 62
Figure 63
Figure 64
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Figure 75
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Figure 77
Figure 78
Figure 79
Figure 80
Figure 81
Figure 82
Figure 83
Figure 84
Figure 85
Figure 86
Figure 87
Figure 88
UBC Earth Sciences Building (courtesy of Equilibrium Consulting) ........................................................ 89
LCT One Tower by CREE (courtesy of CREE by Rhomberg) ................................................................ 89
CREE's IZM Building in Austria (courtesy of CREE by Rhomberg) ........................................................ 90
Carinthia Lookout Tower, Austria (courtesy of Marcus Fischer, Rubner Holzbau GmbH) ......................90
One of the 8 storey Limnologen buildings in construction ...................................................................... 91
Stadhaus Building (Lehmann, 2012) ...................................................................................................... 92
10-Storey Forté Building (courtesy of Lend Lease) ................................................................................ 92
Bullitt Center (left, middle: courtesy of John Stamets, right: courtesy of Ben Benschneider) .................93
Glulam beam (left), Glulam Columns in Prince George Airport Expansion (right, courtesy of MGA)......94
Parallel Strand Lumber Beam................................................................................................................. 94
CLT Panels (left), CLT framing of UBC Okanagan Wellness Centre (right) (Photos courtesy of
McFarland Marceau Architects) .............................................................................................................. 95
Laminated Veneer Lumber ..................................................................................................................... 96
Laminated Strand Lumber ...................................................................................................................... 96
Limnologen Building Vertical Deformation (left) and Platform Framing (right) (Serrano, 2009) ..............99
HBV Connectors for TCC Slab of UBC Earth Sciences Building (courtesy of Equilibrium Consulting) 100
FPInnovations Testing .......................................................................................................................... 101
Ductile Steel Connection at Chevron Brace (left, photo courtesy of Equilibrium Consulting), UBC
Bioenergy Research & Demonstration Facility Moment Frames (right, photo courtesy of Don Erhardt)
.............................................................................................................................................................. 102
Six-Storey Québec City Hybrid Building (courtesy of Nordic Engineered Wood) ................................. 102
CREE’s LCT Panelized System (CREE by Rhomberg, 2012) .............................................................. 103
CEI's Wood Concrete Hybrid System (Bevanda, 2012)........................................................................ 103
Steel-timber hybrid in Spain (left), Scotia Place in New Zealand (middle), Linea Nova in Holland (right)
.............................................................................................................................................................. 104
30 Storey FFTT Rendering (courtesy of MGA) ..................................................................................... 105
SOM's Concrete Jointed Timber Frame System (Skidmore Owings & Merrill, 2013b) ......................... 106
Timber Frame self centering system (Newcombe, Pampanin, Buchanan, & Palermo, 2008) .............. 107
Timber Coupled Shearwalls (Holden, Devereux, Haydon, Buchanan, & Pampanin, 2012) .................. 107
Typical Load-deformation relationships for wood parallel and perpendicular to grain .......................... 116
Post to beam connection detail avoiding excessive compression perpendicular to grain due to gravity
loads ..................................................................................................................................................... 117
Example of continuous posts in mid-rise wood building ....................................................................... 118
Minimizing compression perpendicular to grain and shrinkage using different strategies in massive
construction (Source: Eurban) .............................................................................................................. 119
(a) CREE hybrid concrete-wood system and (b) FFTT system for tall wood buildings ......................... 119
Typical shrinkage values of wood in the three different orientations .................................................... 120
Example of poor detailing practice and suggestions for improvement to avoid splitting (Source CWC
Wood Design Manual, CWC 2010)....................................................................................................... 122
Massive-wood floor plate on posts with potential punching-shear issue (Source: KLH) ....................... 125
Transverse reinforcement of bolted connections using self-tapping screws ......................................... 126
Concealed post to beam connection systems ...................................................................................... 128
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Figure 90
Figure 91
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Figure 120
Figure 121
A post-to-beam connection detail in a seismically upgraded historical tall wood building in North
America ................................................................................................................................................ 129
Definitions of various ductility parameters ............................................................................................ 132
Structural damping hysteresis loops a) idealized and b) simplified (Labonnote, N. 2012).................... 134
Envelope curves of cyclic test data for assemblies............................................................................... 137
An example of sudden column loss in a multi-storey building. ............................................................. 149
Six storey wood-frame plus steel podium building tested as a part of the NEESWood project ............ 151
Six storey plus attic CLT building tested as a part of the SOFIE project ............................................. 153
Schematic flowchart of FEMA P-695 methodology for system performance assessment .................... 154
Conceptual boundaries defined by applicability criteria of the Component Methodology ..................... 155
Typical story of a CLT structure with various connections between the panels (drawing courtesy of A.
Ceccotti) ............................................................................................................................................... 162
Single degree of freedom simulation of a multi-storey structure; b) Determining the effective stiffness of
the structure.......................................................................................................................................... 168
Simplified diagram of the performance-based seismic design procedure............................................. 171
Typical push-over curve with the structural performance levels ........................................................... 172
NEHRP 1997 Performance objectives.................................................................................................. 173
Qualitative performance levels of FEMA 273/356................................................................................. 173
Potential choices for plastic hinges: a) hinges in columns can lead to a soft storey mechanism; b)
hinges in beams can lead to desirable weak beam - strong column design ......................................... 177
Typical storey of a multi-storey CLT structure with various connections between panels (courtesy of A.
Ceccotti); Connections 1, 2 and 3 to be elastic, while 4 to be ductile ................................................... 179
Post tensioning details of: a) beam-column frame structure, b) wall system ........................................ 186
Self centering, energy dissipation and hybrid system hysteresis for PRES-Lam system (CERC, 2012)
.............................................................................................................................................................. 187
A two-thirds-scale two-storey frame consisting of beam and wall Press-Lam elements....................... 188
A detail of wood-based rocking wall system with energy dissipater ..................................................... 188
Direct and flanking sound transmission for the floor-wall junction between two side-by-side rooms (Path
naming convention according to ISO 15712: “D”, “d”: direct element; “F”, “f”: flanking element; Source
room: Capital letter; Receiving room: lowercase) ................................................................................. 209
Summary of the two compliance paths in the NBCC ............................................................................ 225
Typical stage of fire development (Buchanan A. H., 2002)................................................................... 228
Examples of encapsulation methods for structural steel and concrete components ............................ 234
Complete timber encapsulation used in London, England (credit: Karakusevic Carson Architects.) .... 235
Approaches to encapsulation creating concealed spaces .................................................................... 237
Normal stack effect in high buildings .................................................................................................... 241
Fire-resistance criteria per ULC S101 .................................................................................................. 244
ULC S101 standard time-temperature curve ........................................................................................ 245
Observed fire time-temperature curves in several fire experiments compared to a one-hour fire
exposure in compliance with ULC S101 (Bwalya, Gibbs, Lougheed, & Kashef, 2013) ........................ 246
Char layer formed during a small-scale flame test per CSA O177 (2011) ............................................ 247
Wood properties below the char layer (Buchanan A. H., 2002) ............................................................ 247
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Figure 122
Figure 123
Figure 124
Figure 125
Figure 126
Figure 127
Figure 128
Figure 129
Figure 130
Figure 131
Figure 132
Figure 133
Figure 134
Figure 135
Figure 136
Figure 137
Figure 138
Figure 139
Figure 140
Figure 141
Figure 142
Figure 143
Flow chart for advanced calculations for structural fire-resistance of elements .................................... 249
Charred timber cross-section exposed to fire from 3 sides (left) and 4 sides (right) ............................. 250
Examples of timber-concrete composite systems................................................................................. 252
Reduced cross-section of a timber-concrete composite structure ........................................................ 253
Connection in historical Leckie Building in Vancouver.......................................................................... 254
Protected connections for enhanced fire performance ......................................................................... 255
Concealed connections for enhanced fire performance ....................................................................... 255
CLT smoke leakage paths (credit: RDH Consulting) ............................................................................ 263
CAN/ULC S134 Full-scale test of exterior window plume ..................................................................... 267
Types of exterior wall enclosure systems utilizing wood components .................................................. 285
Climate Maps of Canada showing general climate zone classification (left) and annual rainfall levels
(right). Rainfall classifications - Extreme over 1500 mm/y, High between 1000 – 1500 mm/y, Moderate
between 500-1000 mm/y and Low less than 500 mm/y. Maps from the “Guide for Designing EnergyEfficient Building Enclosures” (2013) adapted from several industry references. ................................. 287
Minimum Effective R-Value Requirements for Above Grade Wall Assemblies within 2011 NECB and
ASHRAE 90.1-2010 (left) and NECB and ASHRAE 90.1 Climate Zones (right) (Note that ASHRAE 90.1
includes Climate Zone 4, Lower Mainland and Victoria, BC with Climate Zone 5 in Canada) .............. 291
List of Primary Building Enclosure Control Layers & Associated Critical Barrier Function ................... 293
Options for placement of thermal insulation within wood-frame wall assemblies. ................................ 300
Options for placement of thermal insulation within low slope roof assemblies. .................................... 301
Split insulated wood stud frame (left and middle) and exterior insulated CLT wall (right)..................... 306
Photographs of some of the unique air barrier detail considerations required for CLT panel assemblies
when utilized within tall wood buildings. Gaps between lumber plies and connections (left), structural
anchors interfering with installation of air barrier membrane (centre) and protruding structural elements
(right). ................................................................................................................................................... 309
Sketches showing potential air leakage paths and need for continuous adhered air barrier membranes
and transitions for CLT wall and roof details. ........................................................................................ 309
Low slope conventional roof (left) and protected membrane roof (right) .............................................. 311
Basic framework for developing an integrated cost estimation and cost control system ...................... 336
Elemental classification ........................................................................................................................ 337
Guideline on project cost estimate........................................................................................................ 339
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Introduction
CHAPTER 1 Introduction
Lead Author:
Co-Authors:
Michael Green
Erol Karacabeyli, Eric Karsh
The Technical Guide for the Design and Construction of Tall Wood Buildings in Canada has been
prepared to assist architects, engineers, code consultants, developers, building owners, and Authorities
Having Jurisdiction (AHJ) understand the unique issues to be addressed when developing and
constructing tall wood buildings. In addition, the Guide is designed to provide an understanding of the
broader context of why it is worth investing in wood technologies when choosing the structure for a
midrise-to-tall building.
The Guide is not specific to any one structural solution. Rather, it establishes the parameters and
resources necessary for a capable team to design a tall wood building that meets the performance
requirements of current building codes and the competitive building marketplace. The Guide is not an
answer key with specific details and solutions; instead it is organized to provide the broad information
and concepts that design teams will need to consider, address, and further develop within projects that are
specific to local jurisdictions, functional requirements, and the site and regional contexts. As such, there
are three pervasive discussions found throughout the Guide:
•
•
•
How wood structures are a practical, safe, and realistic choice for tall buildings.
The tall wood building, its structure, and its systems.
Metrics to consider for a successful project.
A Working Group comprised of design consultants and experts from FPInnovations, NRC, and CWC has
overseen the development of the Guide. Under the guidance of this Working Group, more than 60
technical professionals, including architects, structural engineers, cost consultants, and experts from
universities, NRC, CWC, and FPInnovations, have contributed to the Guide and been involved in its
development. It is expected that owners, design teams, and authorities will expand on the Guide with the
specifics appropriate to their projects and that future guides will contribute increasing detail as the
industry grows and more efficient systems are developed.
1.1
Defining “Tall Wood Building”
There is currently no clear definition as to what constitutes a tall “wood” building other than the general
intent that the building structure must include a “reasonable” percentage of wood. This percentage of
wood can vary drastically, depending on the particular solution and whether the design is an “all wood” or
“wood hybrid” approach. In most cases, it is likely that concrete and steel will continue to be used in the
building foundation and may play a role in the above grade structure as well. The Guide does not
recommend any particular proportion of the structure that can or should be wood, and instead encourages
designers to select the most appropriate building material for their particular application.
For the purposes of this Guide, “tall” is defined as a building height that is significantly beyond the
current limits in the National Building Code of Canada (NBCC) and what was permitted in the past using
traditional sawn timber members. As discussed below and in Section 4.1, heavy timber buildings up to 9
storeys were common in urban centres around 100 years ago (Koo, 2013), and wood frame construction
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up to 6-storey in some Canadian jurisdictions are now accepted. “Tall” should, therefore, be defined as a
height significantly above 10 storeys.
1.2
Why Wood In Tall Buildings?
After more than a century of modern urban building in steel and concrete, we are entering a new era of
building — an era where engineered wood offers an alternative way to build safe, cost effective structures
with a great environmental story and at increased heights.
To date, wood buildings have been comprised predominantly of light wood frame construction techniques
and, to a much lesser extent, of solid wood and glulam framing. In most regions, light wood frame
construction is limited to 4 to 6 storeys in height. Above 6 storeys, more massive structural sizes of wood
are necessary and the shift to engineered wood as mass timber elements and to Mass Timber Panels
(MTP) in particular, becomes a relevant alternative.
This guide will demonstrate that, while there are many reasons to consider a wood structure, two
fundamental motivations prevail:
•
•
1.2.1
The ability for wood structures to reduce the embodied energy and carbon footprints of buildings.
The ability for wood structures to be cost competitive with steel and concrete structures.
A Renewable, Carbon Sequestering Alternative for Urban Structures
Over the last decade we have seen an increasing awareness that we should turn from fossil fuel energy
solutions to renewables to meet our global energy needs. This same understanding applies to the materials
we choose to build with. Fed by the sun, wood offers us a renewable, carbon sequestering resource for the
structure of our buildings. Although wood buildings have traditionally been constructed at smaller scales,
new engineered wood products allow us to build taller, expanding the potential for utilizing renewable
materials in larger, urban-scaled buildings.
At present, the building industry is the single largest source of greenhouse gas emissions and energy
consumption in Canada and in developed countries around the world (RAIC and Architecture2030).
Currently, approximately 85% of the total energy footprint ( (US Department of Energy, 2009)) and twothirds of the total carbon footprint of buildings (GGLO LLC, 2010) is related to building operations, such
as heating, cooling, and electricity use. As energy codes continue to evolve and address the operational
side of the equation, the embodied energy and greenhouse gas emissions associated with building
materials and construction will form an increasing proportion of the overall environmental footprint of
buildings in the future. Numerous studies worldwide indicate that an increased substitution of wood for
other structural materials would lead to a reduction of greenhouse gas emissions (Sathre & O'Connor,
2010) and, in fact, the Intergovernmental Panel on Climate Change advocates for wood substitution
(IPCC, 2007)).
With a rapidly urbanizing world, it is important that low-carbon solutions are developed and implemented
across all scales of building. Tall wood buildings are one important strategy for cities to consider as they
aim to lower their greenhouse gas emissions, while at the same time providing for the building needs of a
dense urban population.
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1.2.2
Cost Competitiveness
Broad market studies across many industries commonly quote a concept that 95% of people say they
would pay more for a “green” product, but only 5% actually do. This concept illustrates that, ultimately,
the marketplace will determine the success of building tall in wood, not simply a desire to build with
natural products and in a more sustainable way. As a result, the cost competitiveness of tall wood systems
with alternative solutions is an integral part of this discussion and remains a priority throughout the
Guide.
To achieve cost competitiveness, design teams look for a systemic optimization of the structure, design,
and building systems over alternative construction approaches, as well as opportunities to utilize efficient
prefabrication and rapid assembly processes, which can further lower the overall costs to the owner. The
Guide will illustrate how these issues of cost and constructability, among others, inform decision–making
throughout the design and development process.
1.3
Tall Wood Buildings to Date
Building tall in wood is not a new phenomenon. In fact, tall wood buildings have existed for centuries,
reaching as high as 67 meters (220 feet). 1,400 years ago, tall pagodas in Japan were built to 19 storeys in
wood and are still standing today in high seismic and wet-climate environments. In the Maramures
Region of Northern Transylvania, the Barsana Monastery has been standing at 56 meters (184 feet) tall
since the year 1720. Furthermore, several countries around the world, including Canada, have a history of
constructing tall wood buildings out of heavy timber elements, reaching up to 9 storeys. These buildings
have been standing for approximately a hundred years (Koo, 2013).
More recently we have started to see an increase in the number of modern tall wood buildings worldwide.
In 2008, Waugh Thistleton’s Stadthaus project in London (Figure 1) was the impetus for continued
innovation in “all wood” building solutions. This 9-storey residential building used Cross Laminated
Timber (CLT), a mass timber product that emerged in Europe in the late 20th century, for its structure
above grade. Other examples use a “wood hybrid” approach, such as the 8-storey LCT One Tower by
CREE (Figure 2), located in Austria. Additional projects in the 7 to 10 storey range have already been
completed in Sweden, Australia, Norway, Switzerland, France, Italy, and New Zealand (see Section 0 for
more information about these buildings).
In Canada, in addition to 5 to 6 storey light wood frame buildings, some notable mass timber buildings
have been or are being constructed. The Earth Sciences Building at the University of British Columbia is
one good example of a mass timber and reinforced concrete and steel hybrid application (Figure 3). The
Wood Design and Innovation Centre (WIDC) in Prince George, British Columbia, under construction at
the time of this publishing, will demonstrate an “all wood” approach, utilizing CLT, Laminated Strand
Lumber (LSL), Laminated Veneer Lumber (LVL), and Parallel Strand Lumber (PSL) in innovative ways.
Upon completion, this academic/office building will reach 6-storeys and 30 meters in height (Figure 4).
Also under construction, a 4 and 6-storey residential complex in Québec City, built out of CLT will be the
first of its kind in North America.
Going forward, many conceptual designs are being proposed for tall wood buildings above 10 storeys,
including all wood approaches, such as FFTT by Michael Green and Eric Karsh from Canada, and woodconcrete or wood-steel hybrids by CEI from Canada, CREE by Rhomberg from Austria, RAA from
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Norway, and SOM from the United States. These projects are discussed in greater detail in 0 and Section
0.
Finally, providing insight into how high we can build with wood are two recently constructed mass timber
towers reaching 100 meters in height, as high as the tallest trees in the world: a wind turbine in Hanover,
Germany (Figure 5) and Pyramidenkogel, a lookout tower in Austria (Figure 6).
Figure 1
Stadthaus (Waugh Thistleton Architects)
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Figure 2
LCT One Tower (CREE)
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Figure 3
Earth Sciences Building (Perkins + Will)
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Figure 4
Wood Innovation Design Centre (MGA | Michael Green Architecture)
Figure 5
Wind Turbine in Hanover, Germany
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Figure 6
1.4
Pyramidenkogel
High Rise Wood Demonstration Projects
Recent efforts to expand the height and area limits for wood construction, augmented by the green
building movement, have increased interest within the design and construction community to develop
taller and larger wood buildings. Early adopters that have seen great potential for using wood systems in
high-rises have triggered an initiative by Natural Resources Canada (NRCan) to create tall wood building
demonstration projects. This initiative, designed to enhance Canada’s position as a global leader in tall
and large wood building construction by showcasing the application and performance of advanced wood
technologies, is overseen by a Steering Committee comprised of representatives from NRCan, the
Canadian Wood Council (CWC), Forestry Innovation Investment (FII), the National Research Council
(NRC), Binational Softwood Lumber Council (BSLC), FP Innovations, and the industry.
A request for an Expression of Interest (EOI) to design and construct a 10 storey or taller High Rise
Wood Demonstration Project in Canada has been issued by the Canadian Wood Council (for more
information, see www.cwcdemoproject.ca) . As tall wood buildings currently fall outside the scope of
acceptable solutions in North American building codes and design manuals for wood structures, design
teams of wood buildings 10 storeys and taller will have very specific design, construction, and
maintenance challenges that will need to be addressed, and those responsible for the first demonstration
buildings will have additional considerations. For this reason, this Technical Guide has been developed to
systematically address these challenges in demonstration projects, and proponents are advised to use this
Technical Guide in their submissions.
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Introduction
1.5
Guiding Principals
In the development of the Technical Guide, the working group and the lead authors have adhered to the
following guiding principles:
•
•
•
•
•
•
•
•
•
•
•
1.6
For tall wood buildings, be consistent with the underlying code objectives for tall steel and
concrete buildings in the National Building Code of Canada (NBCC 2010) and National Energy
Code of Canada for Buildings (NECB 2011).
Follow a performance-based philosophy and suggest performance criteria and preferred
methodologies where applicable.
Follow a generic approach that can be applied to a variety of innovative systems. Brief examples
for specific systems may be given.
Develop recommendations for designing for redundancy and resiliency.
Make recommendations for addressing attributes and/or issues that are not explicitly covered in
the Canadian codes.
Develop recommendations on how to assess and specify proprietary connections, products,
assemblies, and systems.
Develop recommendations on how to assess whole building energy efficiency.
Develop recommendations on how to design for durability.
Strive to address pressing issues with a multi-disciplinary approach and make reference to
existing technical information.
Focus on essential items that may form the basis of a “best practices guide” or code change
proposal for broader acceptance.
Enable the monitoring and collection of feedback on the performance of demonstration projects
to help update the Guide.
Organisation of the Guide
The sections of this multi-disciplinary guide are described below and illustrated in Figure 7:
Following this Introduction, 0, The Building as a System, deals with systems integration and includes
discussion on code compliance.
0, Sustaintability, covers a range of sustainability issues and provides guidance on how to measure the
environmental performance of tall wood buildings.
Structural and Serviceability are integrated in 0, as structural calculations are necessary for both strength
and serviceability design. This Section is grouped into 4 important sub-sections. Sub-section 0 provides
recommendations for conceptual design, while Sub-section 0 is intended to give guidance for the
development of the input data for advanced analysis in Sub-section 0. The latter deals with determining
stiffness of the building and performing dynamic analysis, one of the most pressing issues for tall wood
buildings related to both structure and serviceability.
Chapter 5 covers all pertinent topics related to fire safety and fire protection, and provides guidance for
developing a fully encapsulated assembly as a starting point.
0 addresses design considerations for the building enclosure and durability of tall wood buildings.
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0, Prefabrication and Inspection of Assemblies, seeks to establish best practices and standards, which can
provide confidence that what is designed in accordance with the intent of relevant building codes can in
fact be built to high standards of quality.
Chapter 8, Project and Construction Costing, provides guidelines for consultants tasked with estimating
costs associated with tall wood buildings higher than 6 storeys.
Chapter 9, Monitoring and Maintenance, includes recommendations for performance testing and
monitoring and provides guidance on building maintenance to help building owners avoid unexpected
high repair and replacement costs during operation.
Figure 7 Diagram of the Technical Guide for Tall Wood Buildings in Canada (MGA | Michael
Green Architecture)
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Introduction
1.7
References
GGLO
LLC. (2010). Embodied Carbon at the Building Scale. Retrieved from
http://www.gglo.com/insight/embodiedcarbon.aspx
IPCC. (2007). Chapter 9: Forestry. Fourth Assessment Report: Climate Change 2007. Intergovernmental
Panel on Climate Change. Working Group III: Mitigation of Climate Change.
Koo, K. (2013). A Study on Historical Tall-Wood Buildings in Toronto and Vancouver. FPInnovations.
Sathre, R., & O'Connor, J. (2010). A Synthesis of Research on Wood Products and Greenhouse Gas
Impacts, 2nd Edition. Vancouver, British Columbia: FPInnovations.
US Department of Energy. (2009, March). Annual Energy Outlook 2009. Energy Information
Administration. Retrieved from www.eia.doe.gov/oiaf/aeo/
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The Building as a System
CHAPTER 2 The Building as a System
Lead Author:
Co-Authors:
Peer Reviewers:
Michael Green
Eric Karsh, Andrew Harmsworth, Dave Ricketts, Joe Rekab
Jim Taggart, Mark Lucuik, Jana Foit, Conroy Lum
Abstract
The design of a tall wood building requires a much broader perspective than simply the development of a
structural approach. Design teams must consider the integration of all building systems, the building
envelope, and performance detailing, as well as architectural form, function, and flexibility from the
outset of the design process. This chapter will discuss these aspects of tall wood buildings and present
principles and potential solutions to help designers, owners, and construction teams navigate through this
integration process.
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2.1
Architecture and Structure
The design of a tall wood building requires a much broader perspective than a mere shift in structural
approach. The structure and overall design must consider the integration of all building systems, the
building envelope, performance detailing, and naturally the architectural form, function and flexibility
from the outset of the design process. No component of the building can or should be developed in
isolation of the next and, if not coordinated properly, any one of the building’s components can tip the
design solution out of balance with respect to cost, constructability, performance, or even market
acceptance. While the building as a system can be a very broad discussion, it is the intent of this guide to
provide a catalogue of considerations and possible solutions to help designers, owners, and construction
teams navigate through the process. The focus will be on “how best can this be done in wood”.
Each of the following broad considerations should be taken into account in the design and construction of
a fully integrated tall wood structure:
•
•
•
•
Selecting a full consultant team with broad experience in systems-integrated wood design
solutions, and in the development and presentation of alternative solutions to the Authority
Having Jurisdiction.
Selecting or developing a wood or wood hybrid structural system appropriate to:
o The building’s architecture, including its:
 Function and Program
 Intended Building Form and Massing
 Region, Context, Architectural Style, and Vision
 Market and Client Ambitions
 Flexibility Goals for Design and Post-Construction
 Site Requirements
 Geotechnical Conditions
o The building’s performance expectations and goals including:
 Building Code Compliance
 The Performance of Assemblies
 Fire Protection
 Acoustics
 Vibration Mitigation
 Thermal Performance
 Cost Competitiveness
 Constructability
 Human Health and Well-Being
 Sustainability and Green Building Goals
Integration of the building services (concealed or visible) into the structural and architectural
design
o Mechanical
o Electrical
o Plumbing
o Fire Suppression
o IT and Other Communication Systems
Delivering efficient, constructible, and cost effective solutions
Many of the above items are covered in the Guide. This Chapter provides an overview of potential
conceptual approaches to building tall with wood that will enable effective integration of building
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systems. At the outset of the design process, a host of decisions will need to be made to set the project
direction including selecting a structural and systems integration approach.
2.1.1
Selecting a Structural Approach
There are effectively three strategies for developing the structure of a tall wood building:
1. Start with a structural system as the driver of the building’s design and let the architecture work
to that system:
When selecting a relatively prescriptive structural solution, the building’s architecture will have a
set of clear defining parameters to work to from the outset that will inform the optimal column
spacing, building massing, and building envelope solutions for that structural system. While
deviations are possible, the system itself may create clear rules for the architectural language of
the building.
2. Start with an architectural strategy and then apply a structural approach
For example: the architect generates a building form, plan and massing that the structural
engineer adapts a structural system or approach to. This may imply the most flexibility for the
architectural design but may also generate higher cost, system inefficiencies and engineering
challenges depending on the formal response.
3. A Hybrid of Options 1 and 2
Most owners and design teams will want to select a hybrid of options 1 and 2 and keep an open
mind to the appropriateness of any structural approach until the full parameters of the project are
established. A back and forth exercise between the architecture and structure with an
understanding of the diverse range of potential structural solutions will help teams discover the
optimal solution for a particular building.
Development of the following issues will help design teams ultimately select and refine the best structural
approach for their project early in the design process:
1. Developing a lateral load resisting approach
• Establish the design and layout of the building core(s),
• Establish the building’s bracing/ shear /moment frame systems
• Coordinate these major structural elements with the building plan and architecture.
2. Establish the required and/or optimum column spacing and beam depths (if beams are employed)
3. Establish the floor-to-floor height and “slab” depth, with services integrated and acoustic / fire
assemblies considered
4. Establish the floor and ceiling assembly strategy for acoustic performance and building service
integration
5. Review design solutions relative to the various performance criteria
Given that the use of wood will be important in the tall wood building, it is useful to acquire as
background knowledge of what approaches are been developed to address the various performance
attributes of wood systems. One of the objectives of this Guide is intended to assist with this.
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2.1.1.1
Building Program Considerations
Selecting a structural typology that is suitable to the building’s intended use is essential. The unique
structural qualities of a wood structure may dictate ideal column spacing and beam depths, for instance,
that will drive the height and potentially planning flexibility of the building’s overall design.
Opportunities for locating continuous, well-proportioned and well distributed lateral load resisting
elements will be largely informed by building use, much in the same way that they are with other
materials.
The selection of a structural system with a lateral bracing strategy that is flexible to the intended use is
also important. Internal shear walls or bracing may interfere with planning flexibility and are likely to
dictate the bracing strategies for the end use, whether that be for office or institutional or academic or
residential. In other words, while the core of a tall wood building is likely to be a major lateral loadresisting element that clearly needs to be integral on all levels to perform, the addition of other internal
shear walls may be problematic for the program. In those instances other lateral load resisting systems
including exterior wall systems or moment frames might be better choices.
As is true in tall buildings of any structure, ideally columns are continuous vertically through the building
and into below grade parking etc. Some mixed-use building programs may require changing bay sizes to
accommodate larger spaces without column obstructions. While it is possible to transfer columns in
wood structures, this can become an expensive and challenging problem for a cost effective solution.
Where possible longer span spaces should be located on the top levels of the building instead of the lower
levels, or located adjacent to the tower itself to optimize the vertical loads.
Project teams should consider the following spatial requirements and their implications, as applicable to
their specific project:
•
•
•
•
•
•
•
Spaces requiring long spans
Wall free spaces (limited by internal shear walls for instance)
The ability to renovate and move internal program elements (limited with internal shear walls)
Podiums and Lobbies with transfer column requirements to encourage longer span program
functions
Spaces with high performance acoustic demands
Spaces with higher fire risk
Parking Garages and Foundations
The use of wood, in general, for some building uses and for some applications may be challenging as
well, though not necessarily impossible depending on the circumstance:
•
•
•
•
2.1.1.2
Extremely wet conditions and high humidity locations (though not always - wood is often a good
choice for swimming pools for example)
Programs / spaces / proximities of unusually high risk for fire
Programs requiring unique sterile and clean room requirements (ex. Some Hospital spaces, Labs
etc)
Exterior applications in areas at high risk for vandalism, abuse, or damage
Planning Considerations of Tall Wood Structures
The desired massing of the building will significantly inform the structural design, as the overall
proportion (length, width, and height) and effective stability of the building will influence the lateral load
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resisting strategies required. The addition of higher lateral loads due to seismic conditions, wind loading,
or an increased height will place higher demands on the structural system and may increase the need for
shear walls, moment frames, or diagonal bracing. Lateral load resistance can present a significant
challenge to the flexibility and functionality of a design, and should be developed from the earliest studies
of building siting, architectural form and foundation concepts, and budgeting.
Generally speaking, there are four bracing strategies typical of tall wood buildings: the use of the vertical
circulation core(s), perimeter walls, interior shear walls, trusses, moment frames, diagonal bracing, or a
combination of these.
2.1.1.2.1 Planning for Lateral Load Resistance: Vertical Circulation Core
Using the vertical circulation core for lateral load resistance is typical in most tall buildings and therefore
tall wood structures. In some buildings, only a central core will be necessary for bracing, depending on
the overall height, massing, and wind or seismic load requirements. With only a central (or in some cases
non-central asymmetric core as seen in CREE’s LCT) core, designers have greater freedom in planning
the building and adjusting the design throughout the process. Depending on regional requirements,
vertical circulation cores can be constructed of mass timber panels, concrete, or braced wood or steel
frame.
2.1.1.2.1.1
Mass Timber Panel Core
If selecting a mass timber core, this must be established from the earliest stages of the design process to
ensure that planning integrity is maintained throughout the building. This strategy was recently employed
in the Wood Innovation and Design Centre (WIDC) project in Prince George, with a centralized core with
switchback exit stairs of timber and an otherwise open floor plan (Figure 8).
Architects should work carefully in planning a wood core to work to the structural engineers design.
Door openings in the core may be quite limited as the engineer works to ensure continuity around the
corners and to provide enough panel length to achieve the required strength and stiffness.
An all-wood core may prove advantageous where the building’s other vertical structural elements are also
wood. The main advantage of an all-wood core is prefabrication, use of a single trade and, potentially,
speed of erection.
Several tall buildings that have mixed concrete cores with steel columns have seen issues arising from
differential movement (e.g. shrinkage in one material but not the other) which results in floor levelness
issues over time. This concern exists in mixing a variety of structural materials for vertical load bearing
elements and should be considered early in the design stage.
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Figure 8
WIDC Typical Floor Plan and Rendering (Michael Green Architecture)
2.1.1.2.1.2
Concrete Core
A traditional concrete core may also be utilized, as was recently done in CREE’s LifeCycle Tower One
and the Earth Sciences Building at UBC. In the CREE example, the floor slabs on the core side connect
directly into the concrete. The LifeCycle core is asymmetrically planned and complemented by two
additional perimeter shear walls (Figure 9. The scale of lateral loads and planning goals will inform the
ultimate location of the core and other shear wall location, where centralized cores generally represent the
simplest approach to avoid torsional issues in high loading conditions, particularly in high seismic zones.
A traditional concrete core may also be utilized, as was recently done in CREE’s LifeCycle Tower One.
In this example, the floor slabs on the core side connect directly into the concrete, providing stability. The
LifeCycle core is asymmetrically planned. Lateral loads will inform the location of the core, where
centralized cores generally perform with greatest ease as loads increase.
Figure 9
LifeCycle Tower One Typical Floor Plan and Rendering (CREE)
2.1.1.2.2 Planning for Lateral Load Resistance: Perimeter Shear and Load Bearing Walls
Perimeter bracing requires an integrated approach with the building envelope and typically results in a
less transparent exterior. The Schematic designs in The Case For Tall Wood Buildings concluded that
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perimeter bracing strategies offer interior planning flexibility at greater heights than core-only solutions.
If this strategy is used, the perimeter bracing walls require careful coordination with the architectural
fenestration and design. Presumably the more “solid” appearance of an externally braced building will
respond well to increasing demands on envelope energy performance with more opportunities for
insulated exterior walls.
2.1.1.2.3 Planning for Lateral Load Resistance: Interior Shear and Load Bearing Walls
Interior shear walls are a reasonable solution for bracing in buildings with fixed plans where structural
walls can be coordinated and future flexibility to remove these walls is not needed. An interior shear wall
approach generally works best in residential applications and is less likely to work well in office,
academic, or other applications. One example of this strategy can be seen in Waugh Thistleton’s
Stadthaus, in which a honeycomb-like structure was framed floor-by-floor (Figure 10).
Interior shear walls may limit the flexibility of planning in some residential buildings and may limit the
ability to make changes late in the design process or after construction. This may be less desirable for
market housing where some developers depend on a degree of flexibility in the plan. Internal shear walls
are not practical for most office, institutional or academic applications where, again, renovations, or retenanting in the future will be limited by the structural layout.
Figure 10 Stadthaus Floor Plan and Axonometric (Waugh Thistleton)
2.1.1.2.4 Planning for Lateral Load Resistance: Trusses
A recent proposal put forth by CEI Architecture for a 40-storey office building makes use of mass wood
trusses. In their wood-concrete hybrid approach, concrete piers are positioned at the perimeter of the
building to support glulam trusses. These trusses span from floor to ceiling, and support the CLT floor
structure, which ties back into a central concrete core. This configuration frees the floor plan or columns.
Additional information on this system may be found in Section 4.1.
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2.1.1.2.5 Planning for Lateral Load Resistance: Moment Frames
Wood moment frames are challenging at the scale of tall buildings and built examples are not yet found,
although the FFTT system, described in The Case for Tall Wood and also reviewed in Section 4.1, does
introduce a concept to achieve wood panel/steel beam hybrid perimeter moment frames.
2.1.1.2.6 Planning for Lateral Load Resistance: Diagonal Bracing
This strategy is used in the University of British Columbia’s Earth Sciences Building, in which a glulam
chevron brace with steel connections is incorporated in the east exterior wall of the office wing to
complement the asymmetrically located concrete core located near the west end of the wing. Additional
information on this system may be found in Section 4.1.
2.1.2
Selecting a Systems Integration and Aesthetic Considerations
The project team will need to make another fundamental choice early in the process with regard to
systems integration and aesthetic ambition of the wood building structure:
1. Will the wood structure be exposed, partially exposed, or concealed?
•
•
•
Exposed wood structures (ex. CREE LCT and WIDC solutions) will:
o Establish expected amount of char
o Require additional care in detailing to maintain fire separations, smoke separation and
exposure risks.
o Require additional care in acoustic detailing
o Likely consider a solution that integrates the building services and structure for a unified
aesthetic.
o Likely consider Architectural Grade mass timber panel in lieu of Structural Grade.
Partially exposed wood structures (ex. SOM solution: exposed columns, concealed floor/ceiling)
will:
o Assume a mix of exposed and encapsulation methods for fire ratings
o Require additional care in detailing for fire and acoustics
o May not require a fully systems integrated approach in that most systems can hang below
the structure and be concealed by a dropped ceiling, as would be common in most
concrete and steel buildings
o Will not result in exposed wood ceilings.
Concealed wood structures (ex. Stadthaus in Murray Grove) will:
o Fully encapsulated wood structure means the structure itself is ultimately not part of the
building aesthetic.
o May be relevant for some building uses, higher performance demands of acoustics and
fire, or for market or design intent, as was the case in Murray Grove
2. Will the building systems need to be integrated into the structural design as part of a complete
system or will the systems be independent of the structural design (though still requiring
coordination)?
•
Fully integrated systems (ex. WIDC and CREE LCT):
o Typical of higher architectural finish exposed wood structures
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Floor/ceiling structural assemblies integrate with mechanical, electrical, fire suppression
systems etc. in cavities, coffers or within hybrid concrete slabs.
o Exposed wood ceiling structures require careful coordination of services
Partially integrated systems:
o Typical of concealed or partially concealed wood structures
o Typically mechanical and electrical will be run in the floor system of the building and
require some structural integration.
o Dropped ceilings will simplify the distribution of services though coordination with the
structure from location to location will be required as is typical of all structures.
Non Integrated Solutions
o Systems hang below the structure and are;
 Visible and exposed as might be typical in more industrial buildings
 Concealed by dropped ceilings
o
•
•
3. Which Mass Timber Panel (MTP) products will be used?
This question is particularly relevant to buildings with exposed wood structures where the choice of
material is one of aesthetic appropriateness as well as structure. The project team should consider the
following when selecting the appropriate MTP product:
•
•
•
•
•
•
Architectural aesthetic intent
Panel Dimensions
o Different MTP are made in different panel sizes and these sizes will impact structural bay
spacing and even the available finishes of the panels. For example: CLT is currently
available in Canada at 3m wide in structural grade, but only at 2.4m in an architectural or
appearance grade
o Material thicknesses available potentially informing the span capability or strategy of the
structure.
Material handling and exposure to weather
Material cost
o For example: appearance grade CLT is more costly than structural grade, and the
omission of grading stamps or removal and sanding may be required for certain
engineered wood products.
Material availability
o Consideration of substitution of alternative MTP types may be appropriate to promote
competitive bidding of the project.
Sustainable objectives
There are several mass timber panel products available for the use in tall wood buildings. These are
discussed in greater depth in Section 4.1.
2.2
Integrating Systems
Throughout the design the project team will need to consider the routing of services between floors,
within ceiling spaces and within walls. This can be done as a fully integrated solution or as a partially
integrated solution.
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2.2.1
Mass Timber and Hybrid Mass Timber Concrete Ceilings
Fully Integrated solutions allow for the underside of the timber structure to be left exposed, which is often
preferable (aesthetically and for marketing of the building) to concealed ceiling systems in tall wood
buildings. This said exposed structure ceilings have limited places to conceal the primary services
required including sprinklers, smoke detectors, and lighting and potentially air handling or radiant heating
and cooling systems.
Many existing wood structural systems are directional in their layout. By using linear panels and beams
many systems have a primary direction for laying services that may pose an increased challenge at 90
degrees in the secondary direction. Examples can be seen in the CREE LCT system and the WIDC
projects. In both cases panels are staggered with recessed coffers that create raceways for building
services. These coffers work well in one direction but can be limiting to services that need to run in the
perpendicular direction. Often the solution requires a drop ceiling for some areas to conceal transitions in
direction and allow services to run under primary beams rather than through them. Mono directional
systems are also typically challenged in reaching services to the corners of the building without
decreasing the efficiency of the piping, conduit or duct runs with 90 degree routing.
Alternatively a dropped ceiling solution that conceals the structure significantly simplifies these issues
allowing services to run as needed as would be typical in steel or concrete buildings with dropped
ceilings. While this may simplify servicing, it potentially is less desirable in wood buildings where there
is often a desire to see the beauty of the wood structure. Dropped ceiling solutions do offer increased
acoustic performance benefits as well.
2.2.2
Structural Mass Timber Walls
In Europe, early mass timber panel projects routed services directly into the wood of the panels to allow
the wood to be the finished surface of walls in the interior rooms. While this is possible it is generally
understood to be a significantly more expensive and difficult to coordinate solution for larger buildings in
North America. Instead most structural walls are furred out with additional light steel (and sometimes
wood in lower buildings) framing and drywall creating service space for electrical, mechanical, plumbing
and fire suppression.
The addition of furred walls with drywall over the mass timber will generally also help with acoustic and
fire performance between spaces.
A design team may want to expose some structural walls as a feature of their design. This would
typically be achieved by locating services in adjacent walls to eliminate the need for integrating systems
in the panel itself. Integrating systems into a structural panel may diminish the fire and acoustic
performance characteristics of the panel.
2.2.3
Floor Assemblies
Typical approaches to floors in tall wood buildings has to date been based on a nominal concrete topping
over a mass timber panel structure, or a hybrid concrete- mass timber panel structure or a hybrid precast
concrete- glulam beam structure. In each of these cases the addition of a concrete topping helps the
acoustic performance of the floor assembly and provides space for the integration of wiring and radiant
heating and cooling systems. Once in place, however, these systems are difficult to service or access.
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In Waugh Thistleton’s CLT platform framed, Whitmore Road Project and in the system developed by
MGA and Equilibrium for WIDC, concrete toppings have been eliminated in lieu of a “dry construction”
solution composed of built up layers that meet the acoustic performance criteria. In the case of the WIDC
project, the dry construction approach and the coffered cross section of the mass timber panel structure
create an accessible mono-directional raceway for altering wiring or ducting. The raceway on top of the
staggered structural panels is deep enough for ductwork and piping as may be needed.
Figure 11 Services Integration: WIDC Approach (Michael Green Architecture)
2.2.4
Mechanical/Plumbing Systems
Beyond the standard plumbing detailing requirements for fire penetrations, seismic bracing and acoustic
isolation there are some conditions unique to wood buildings with respect to plumbing that should be
considered. In exposed wood structure solutions, systems will hang below the floor assembly and be
exposed or will be integrated into the structural solution itself as is seen in WIDC. An integrated system
will need to resolve the routing of ductwork, piping and electrical conduit etc. in all directions. WIDC and
CREE LCT are directional structural systems where systems run easily in one direction but are more
challenged in the other. The resolution of system access to the corners of these single directional
structural layouts can be particularly.
In both exposed and concealed structures, water leaks and condensation on plumbing or other mechanical
system pipes and internal rainwater leaders is a significant concern for visual and potentially structural
damage to the wood structure. Insulating of pipes, provision of drip pans, gaskets and other measures
should be taken to limit the risk of damage that is often difficult to access and repair structurally or
visually.
Leaks or overflows from washrooms or kitchen fixtures on the floor(s) above can also cause significant
visual damage to the ceiling below. Designers may consider a “bathtub” membrane under bathroom and
kitchen flooring and floor drains as a way to mitigate this risk.
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Long-term leaks in concealed locations can become larger risks to the building. These leaks can cause
structural damage or impact indoor air quality with the introduction of mould. Ensuring appropriate
access to visually inspect plumbing from time to time is recommended.
2.2.5
Electrical Systems
One of the unique challenges for electrical systems is the ability to route conduit and locate fixtures
within the solid mass timber panels. Often this issue has been solved by dropped ceilings and furred out
walls with secondary walls of gypsum. This typical solution simplifies electrical conduit and fixturing
and is similar to how a concrete building would be coordinated. But this approach also means the wood
structure is fully concealed.
An alternative is seen in CREE’s LCT and WIDC where service channels are designed into the floor
structure/ ceiling assemblies. These channels simplify wiring to fixtures and can be covered with any
number of architectural finishes. In the case of WIDC additional service channels were designed on top
of the structural floor assembly for flexible wiring of office spaces in a single directional partially raised
floor assembly. A full raised floor might also be considered for wiring from the floor. Conduit and
fixturing within concrete toppings might also be considered if concrete toppings are to be employed.
For walls it is possible to route conduit into the mass timber panels as is done in some European
applications. This solution requires careful coordination and limits future renovations to the system. It is
also a costly solution currently in Canada. If the intent is to expose some mass timber walls it is common
to see raceways integrated into baseboard conditions and wider doorframes for locating wall switches etc.
This solution was illustrated in the Case for Tall Wood Buildings and is shown below where FFTT is
described further.
2.2.6
Fire Suppression Systems
All high buildings in Canada require a sprinkler and standpipe system. Consideration of integration of
sprinklers and related piping is necessary. Where timber is encapsulated, no further provisions other than
that normally provided in a high building are required. However, consideration of the placement of
sprinklers should be made in developing the structural system. Where there is exposed timber, there may
be requirements for additional sprinklers in void spaces and other typically un-sprinklered spaces. The
provision of an on-site water supply tank in addition to the normal City supply is recommended in
Chapter 5.
When the intent is to conceal sprinkler systems within the structural floor (and therefore ceiling) assembly
of an exposed wood structure, the structural approach and dimensions will need to be considered with the
routing, size and sloping of the sprinkler system.
As an example in the case of the WIDC project, the design team staggered the structural CLT floor panels
with voids between for the routing of sprinklers. The voids needed to consider the depth necessary for
sloping of the piping and the vertical drops of the heads themselves. The success of the system required
the deeper voids on the underside (ceiling) rather than the floor side of the assembly.
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2.3
Important Considerations
2.3.1
Acoustics
2.3.1.1
Types of Sound
2.3.1.1.1 Flanking Sound
Flanking noise refers to sound vibrations that are transmitted through an assembly by moving across its
top, bottom or sides and into an adjoining space.
A flanking path transmits sound through connections other than the common partition between two
spaces. Sound can travel considerable distances in a structure because of flanking noise re-radiating from
space to space. Flanking noise is difficult to control because of the low frequency of the sound waves and
the way in which it is transmitted. Typical flanking paths include open plenums that are over walls and
through suspended ceilings, common ductwork, adjacent exterior windows, common floor heaters, open
vents and under doors. The sensitivity to details and materials in a structure will determine the effect of
flanking noise, which is almost impossible to avoid.
2.3.1.2
Measuring Sound
The passage of sound between units of a residential or commercial building, as well as from the outside
in, plays a large role in the comfort level (and general happiness) of its occupants. There are two ways to
measure the passage of sound: Sound Transmission Class (STC) and Impact Insulation Class (IIC). An
overview with some specific details is provided here. A more technical discussion of the issues and
design parameters is presented in Section 4.4.
2.3.1.2.1 Sound Transmission Class
Sound transmission is defined as sound waves hitting one side of a partition, causing the face of the
partition to vibrate. This re-radiates as sound on the other side.
Sound transmission class or STC, is a numerical rating assigned to a wall or floor assembly, used to
describe how well it transmits sound. STC classifies the average noise reduction in decibels for sounds
that pass through an assembly. A high STC rating for an assembly implies good sound attenuation
characteristics. For example, loud or amplified speech and loud music would still be audible with an
assembly that has an STC rating of 45. In an assembly with a rating of STC 60, loud music would be
inaudible except for very strong bass notes (Canadian Mortgage and Housing Corporation 2009).
The STC rating ignores low-frequency sound transmission below 125 Hz, which is often associated with
mechanical systems, transportation noise, and amplified music. Low-frequency sounds can be a major
cause for complaint in multi-family construction. A heavier assembly with the same STC as a lighter
assembly may often outperform the lighter assembly at low frequencies.
2.3.1.2.2 Impact Insulation Class
Impact sound is caused by a direct contact or impact on a floor or wall that vibrates the partition. This
sound is then radiated in the cavity of the assembly, which can then be transmitted into a space as sound.
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The standard test for impact sound results in a rating called “impact insulation class” (IIC). The standard
test method uses a tapping machine that consists of a motor and turning shaft that lifts and drops five steel
hammers on the floor a total of 10 times per second. Sound pressure levels are measured in the room
below at specific frequencies.
IIC increases as the impact sound insulation improves. The building code does not outline acceptable IIC
ratings for walls or floors but recommends an IIC of 55. In practice, this is deemed largely ineffective and
levels of IIC 70 are necessary for residential applications.
2.3.1.3
Design Considerations
2.3.1.3.1 Mass
The weight or thickness of a partition is one of the major factors in its ability to block sound. Mass is
commonly added to existing walls by adding additional layers of gypsum. When the mass of a barrier is
doubled, the STC rating increases by approximately 5 dB, which is clearly noticeable. The denser a
product the better its sound transmission performance will be. (Canadian Mortgage and Housing
Corporation 2009)
2.3.1.3.2 Discontinuity
An air space within a partition or floor assembly can also help to increase sound isolation. When sound
vibrations are allowed to move from one wall face to another through a solid internal element, the STC
rating significantly decreases. The airspace can be increased or added to a partition by using components
such as resilient channels and layers of gypsum board. An airspace of 1 ½” will improve the STC by
approximately 3 dB. An air space of 3” will improve the STC by approximately 6 dB. An airspace of 6”
will improve the STC by approximately 8 dB. (Canadian Mortgage and Housing Corporation 2009)
There are several ways to create discontinuity in wall partitions in a mass timber building. Additional
framing and gypsum board with an airspace furred out on one or both sides of the panels is a common
solution. This also creates space for other building systems to run without impeding the structural
elements. A furred out solution may also be more costly than a partially exposed or exposed mass timber
panel system. Exposing the mass timber panels has been done in some buildings where two panels were
used and separated with airspace between. This has performed well in the residential application for
Waugh Thistleton’s Whitmore Road project as an example. In each instance flanking sound should be
considered in the partition design with transmission through the structure itself being difficult to mitigate.
2.3.1.3.3 Resilient Connections
Fastening horizontal resilient channels to the structural members of an assembly are common approaches
used to break the sound transmission path. Resilient channels installed on both sides of a wall may be
beneficial where flanking sound can enter the wall framing from above or below. The position and
location of resilient channels are important because, if installed incorrectly, they can actually decrease the
STC rating. Resilient channels should be oriented with their bottom flange attached to the wall stud
framing.
2.3.1.3.4 Sound Absorbing Materials
Sound-absorbing material can be installed in a wall cavity or floor to reduce sound transmission between
spaces. Sound-absorbing materials are usually porous foams or fibrous layers so that sound can easily
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pass through them. Examples of sound-absorbing materials are mineral wool, glass fibre, cellulose fibre,
open cell foams, and acoustical tiles. These materials convert sound vibrations into heat, as sound
repeatedly reflects from the surfaces of an enclosed space, passes through the sound-absorbing material
many times, and decreases with each pass.
2.3.1.3.5 Assembly Components
A sound rating depends on, and is affected by, the components in any wall or floor assembly. The
construction details play a large role in this, from materials and thickness in the layers (gypsum board or
sound absorption material) to spacing of studs and resilient channels in a wall assembly. In a floor
assembly, the same principals apply where finishing, topping, sub-floor, ceiling boards, sound-absorbing
materials, space between layers, and the size and spacing of joists and resilient channels all affect sound
ratings. An ideal assembly to control sound transmission would include an airtight construction
(especially at penetrations), two layers that are not connected at any point by a solid material, the heaviest
or most dense material that would be practical, and the deepest cavity that is practical filled with a soundabsorbing material.
•
•
•
•
•
Floors and Ceilings:
Concrete topping
Hang drywall ceiling on resilient hangers
Rubber or other underlayments below floor finishes
Staggered floor (i.e. at WIDC)
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Figure 12 WIDC’s “Slab” Assemblies (Michael Green Architecture)
2.3.2
Energy Efficiency
It is estimated that more than 40% of all energy use is consumed within buildings. In Canada, the
majority of energy in buildings is consumed for space conditioning, including for heating, air
conditioning, tempering ventilation air, and for fan and pump power to distribute heating and cooling
throughout the building. In the design of energy efficient buildings, it is important to consider energy
from a whole building perspective. An energy efficient building enclosure, employing strategies such as
well insulated assemblies, air tight construction, thermal mass, and passive solar design, can significantly
reduce the need for mechanical energy consumption for heating and cooling. Efficient mechanical
systems provide lower energy means of delivering heating and cooling to a building. As energy
requirements are reduced, renewable energy systems become increasingly practical and cost effective and
contribute to sustainable, self-sufficient buildings. Though all of these systems help reduce the energy
footprint of a building, starting with an energy efficient enclosure to reduce space-conditioning energy is
key to designing energy efficient buildings and is a primary focus of this guide.
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Tall wood buildings can be designed with energy efficient enclosures. Well-insulated assemblies, airtight
construction, and thermally massive assemblies are all desirable features that can be part of an energy
efficient tall wood building. Minimum thermal insulation requirements vary by climate zone across
Canada and are based on space-conditioning needs. Buildings in colder climate zones such as the
Northern Territories generally require more insulation than those in temperate climate zones such as
coastal British Columbia. As the cost of energy increases, higher R-value targets in all climate zones
become economically justifiable. There are, however, depreciating energy savings and returns on the
super-insulation of wall and roof assemblies, and consideration for a whole-building systems design
approach is most appropriate. Tall wood buildings will have different optimal insulation levels than a
low-rise wood-frame house. High performance house targets are generally R-40 walls, R-60 roofs, and R20 below grade. For taller buildings with a lower surface-to-volume ratio and greater window areas, these
targets may be in the range of R-20 for walls and roofs.
Whole-building energy efficiency takes into consideration the thermal loss or gain through all of the
building-enclosure components, which impacts the mechanical and electrical systems in the building that
deliver heating and cooling to compensate for these losses or gains. Heat loss or gain can occur through
all parts of the building enclosure, including the above-grade and below-grade walls, roofs, decks,
balconies, floors, windows, doors, skylights, and all the interfaces and details in between. Windows have
perhaps the largest thermal impact on the overall thermal performance of the building enclosure. Window
components, because of their relatively low thermal resistance compared to insulated walls and roofs, can
be considered as large thermal bridges within the building enclosure. Other components that can
significantly affect the thermal performance of a tall wood building include un-insulated floor edge details
and uninsulated structural columns. Heat loss through all of these components needs to be considered in
order to design an energy efficient building enclosure.
The thermal mass of the building enclosure elements, as well as that of the interior floors and walls, can
act to improve the energy efficiency of buildings by storing and releasing energy during different periods
in the day or night. For example, during heating periods, thermal massive assemblies with exterior
insulation can store heat from the sun during the day, and release it at night when temperatures cool. This
acts to reduce peak utility loads by shifting the time and intensity at which they occur, reduce the
building’s overall energy use and peak demand, and improve occupant comfort. The actual benefits of
thermal mass within a building will vary with climate and solar radiation, building type and internal heat
gains, building geometry and orientation, and the actual amount and location of thermal mass used, but it
is a common strategy in energy-efficient buildings. Thermal mass is typically associated with concrete or
masonry buildings; however, heavy timber framing, such as CLT panels, does have considerable thermal
mass, which will have whole-building energy-efficiency benefits.
Passive solar design strategies incorporate windows and exterior shading to maximize solar heat gain
during heating periods, while also providing shading during cooling periods to prevent overheating and
reduce air conditioning energy. South-facing windows with fixed or operable exterior shades, or
landscape features to provide summer shading, contribute to reducing the need for heating and cooling in
a building. Passive solar design is commonly used in houses with fixed overhangs to shade windows, but
can also be incorporated into tall wood buildings. Windows with high solar heat gain can provide passive
solar heating in the winter, while architectural features can be designed to provide shading during cooling
periods. When using passive solar design and high solar heat gain windows, it is very important to ensure
adequate exterior shading in the summer and swing seasons in order to prevent overheating and increased
cooling energy. Passive solar design requires consideration of the geographic location and climate,
including solar radiation, solar angles, and heating and cooling degree-days.
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Energy efficient buildings require a whole building design approach. An energy efficient enclosure should
be a primary consideration, incorporating strategies such as well-insulated assemblies, airtight
construction, thermal mass, and passive solar design to reduce the need for mechanical heating and
cooling. Tall wood buildings can incorporate high R-value assemblies, minimal thermal bridging, airtight
construction and thermal mass with exterior insulation to provide the groundwork for a high performance
energy efficient building.
2.3.3
Architectural Finishing
The following aspects should be taken into consideration when providing for architectural finishing:
•
•
•
•
•
•
•
•
Exposed or concealed wood structure, including columns
Humidity, risk of water damage
Shrinkage, creep, and other changes over time
Material selection
Grades/Finishes of mass timber panels and other wood products
Protection during construction from weather and damage
Flame spread requirements and potential treatments
Quantity of exposed wood area per codes and program use
Wood finishes exposed in exit lobbies, exit corridors, etc. may require coatings to reduce flame spread
Consider species being used in exposed structures and how other wood finishes (millwork, etc.) and
species coordinate/harmonize.
2.3.4
Constructability
2.3.5
Costing
2.3.5.1
Cost Implications of Different Assemblies and Comparison to Traditional Assemblies
Mass timber buildings enjoy a distinct schedule advantage over cast-in-place concrete or steel-concrete
composite structures. On-site erection is faster, due in part to the elimination of temporary shoring after
installation. In addition, pre-drilling and coring at the factory accelerate the installation of building
services fixtures and finishes. The schedule advantage may be somewhat reduced when compared to
structural steel and pre-cast concrete buildings due to the challenges related to fire stopping and joint
sealing for acoustic separating between walls and floors.
Within the industry, expectations are that as design and construction of mass timber buildings advance,
there will be a significant improvement in cost savings. Gains will come primarily from off-site
prefabrication of sections, the use of larger panels and from faster installation as companies gain
experience and develop systems that improve panel placement and fastening techniques.
2.3.5.2
Costs of Deconstruction, Salvaging, Recycling, Re-use and Waste Disposal
Costs associated with the deconstruction of mass timber buildings would be similar to those of structural
steel and pre-cast concrete structures. Costs are anticipated to be less when compared to cast-in-place
concrete buildings primarily due to the ease of removal of panel sections. The material can also be re-used
and re-worked into various sizes for other building or non-building applications (e.g. furniture, wood
flooring) , thereby reducing the requirements for disposal.
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2.4
Structural Capacity for Alterations
Timber is a material that is inherently easy to work with and modify with light and simple tools. Small
openings can often be accommodated in mass timber elements such as posts, beams, and solid wood
panels without the need for scanning or reinforcing. Should reinforcing be required, easy-to-install selftapping screws, now available in a wide range of lengths and sizes, can be specified.
Larger openings can also often be accommodated in solid timber panel walls, since the panel itself is
usually much stronger than the connections between the panels, which typically govern the design.
Should larger openings be required at a panel joint, additional fasteners can be provided to replace those
removed. Reinforcing members, if necessary, can usually be secured with simple site-installed
connectors.
Unless specifically designed for that purpose, few structures are truly demountable and re-useable. This
said, with few exceptions, timber connections are often easily dismantled, particularly in the case of solid
panel construction where self-tapping screws make up the majority of the connectors. Likewise, it is easy
to add to a timber structure, thanks to the use of light tools and simple, yet versatile site-installed
connection options. Unlike light framing however, which is notoriously easy to alter due to the small
scale of its components, mass timber construction and de-construction does require heavy lifting
equipment.
2.5
Building Code Compliance
2.5.1
History of the National Building Code of Canada
The National Building Code of Canada (NBCC) is a model building code that sets the standard for
building construction in Canada. When adopted by a province or territory, it becomes a regulation in
effect in the region.
The NBCC is intended to represent a consensus reached by the public regarding the minimum level of
safety required in buildings. It has traditionally been “prescriptive” in that Code provisions are directly
stated in the regulation. While the NBCC is revised in each Code change cycle, some of the fundamental
provisions such as building height and building area remain much the same as in its first 1941 edition.
These provisions are historic in nature and do not necessarily reflect modern engineering practice and
construction technologies.
2.5.2
Objective Approach to Building Code Compliance
In 2005, the Canadian Commission on Building and Fire Codes (CCBFC) publish the National Building
Code of Canada as an objective-based Code (NRCC, 2010; 2010). The benefit of the objective-based
Code is that for the first time, specific Code objectives and functional statements are available, allowing
practitioners, builders and Code regulators alike to understand the intent of the NBCC and its application
as well as to develop alternatives to the limited solutions provided.
The objective-based Code allows one to comply with the NBCC through “acceptable solutions” which are
the prescriptive provisions found in Division B of the NBCC or through “alternative solutions” that
demonstrate an equivalent level of performance to the Acceptable Solution in the areas identified by the
objectives (Figure 8). As stipulated by Buchanan et al. (2006), prescriptive Codes are more concerned
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with how the building is built, rather than how it will actually behave. The main advantage, to some
extent, of complying with the prescriptive provisions is that it is easier and faster for designers and
authorities having jurisdiction to develop, apply, review and approve a design. However, it also presumes
that there is only one way of providing a given level of fire safety in a building (Hadjisophocleous,
Benichou, & Tamin, 1998).
Unlike performance-based codes, the objective-based code does not provide specific performance levels.
Instead, it provides objectives that explain the intent behind the prescriptive provisions. Under this
framework, the acceptable solutions in Division B demonstrate the minimum acceptable level of
performance for the specific objectives attributed to the acceptable solutions.
Building Code
Compliance
Objectives and
Functional Statements
Acceptable Solutions
(Division B)
Alternative Solution
• Meet the objectives and functional
statements
• Provide the same level of
performance relative to objectives
and functional statements
• Deemed-to-satisfy solutions
• Establish level of performance
Figure 13 Summary of the two compliance paths in the NBCC
To demonstrate NBCC compliance using an alternative solution, one must carry out a qualitative or
qualitative fire risk assessment to establish the level of risk associated with the Division B solution, then
carry out the same assessment for the alternative solution, so that the level of performance between the
two designs can be compared. If it is shown in this comparative risk analysis that the alternative solution
provides at least the same level of performance as the Division B provision, then the alternative solution
can be accepted as also complying with the building code.
2.5.2.1
Acceptance by Authorities Having Jurisdiction
An alternative solution requires agreement by the authority that the solution provides the requisite level of
performance, although the process for review varies by jurisdiction.
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An alternative solution for a tall timber building is inherently complex and it may be appropriate for the
applicant and the authority to agree to delegate the review process to third-party or peer reviewers with
qualifications in timber engineering and fire science (or structural engineering in the case of a structural
solution). It is recommended that the review process and selection of peer reviewers be agreed upon very
early in the process and that reviewers and proponents establish a good dialogue on the project.
Furthermore, it is recommended that for an effective peer review process, peer reviewers be tasked with
assisting in finding a solution rather than just identifying errors and omissions. Further guidance or peer
review in the case of alternative solutions to meet fire requirements is found in the SFPE Guide.
http://www.sfpe.org/Portals/sfpepub/docs/pdfs/technical-resources/Peer_Review_Guidelines_2009.pdf
Experience has shown that for complex alternative solutions it is important for the proponent and
authority to meet early and often and that an effective dialogue between the applicant, the authority and
peer reviewers, if any, is essential for all parties to be satisfied with the outcome.
2.5.2.2
Objectives and Functional Statements
The NBCC objectives and functional statements attributed to a particular provision identify the risk areas
that the NBCC is addressing in that provision. Risks that are not addressed by the objectives are outside
the NBCC framework and are therefore not considered (i.e. the risk of failure due to terrorist attack is
currently not a risk area recognized by the NBCC). For compactness, the following discussion will
outline the process for meeting fire requirements. A similar approach should be follow for meeting the
other fundamental requirements of the NBCC.
The fire safety provisions set forth in the NBCC interrelate to four main objectives. They describe, in very
broad and qualitative terms, the overall goals that the NBCC's provisions are intended to achieve, namely:
•
•
•
•
OS – Safety;
OH – Health;
OA – Accessibility for persons with disabilities, and;
OP – Fire and structural protection of buildings.
The objectives describe undesirable situations and their consequences, which the NBCC aims to prevent
occurring in buildings. Each objective is further refined with has sub-objectives which can be found in
Parts 2 and 3 of from Division A of the NBCC. The NBCC recognizes it cannot entirely prevent all
undesirable events from happening or eliminate all risks. Therefore, its objectives are to “limit the
probability” of “unacceptable risk”. It is thus assumed, within the NBCC, that an undesirable situation
may occur and means shall be provided to limit its consequences. Moreover, an “acceptable risk” is the
risk remaining once compliance with the NBCC prescriptive solutions has been achieved (NRCC, 2010).
Each provision (i.e. acceptable solution) prescribed in Division B of the NBCC is linked to one or more
objectives and sub-objectives and one or more functional statements. A functional statement describes a
function of the building a particular requirement helps achieve. They are more detailed than the objectives
and, similarly, are entirely qualitative. Examples of functional statements that relate to the provisions that
can be found in Part 3 of Division B of the NBCC are:
•
•
•
•
F01 – to minimize the risk of accidental ignition;
F02 – to limit the severity and effects of fire or explosions;
F03 – to retard the effects of fire on areas beyond its point of origin;
F04 – to retard failure or collapse due to the effects of fire;
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•
•
F05 – to retard the effects of fire on emergency egress facilities;
F10 – to facilitate the timely movement of persons to a safe place in an emergency
Additional information on objectives and functional statements can be found respectively in Parts 2 and 3
of Division A of the NBCC.
2.5.2.3
Level of Performance
In the objective-based NBCC, the performance targets for the NBCC provisions are implicit in the
provisions themselves; the performance attained by the acceptable solutions in Division B constitutes the
minimum level of performance required. For example, Sentence 3.4.2.5.(1) requires that the maximum
travel distance to an exit in a sprinklered office (Group D) floor area be 45 m. The objective and
functional statement attributed to Sentence 3.4.2.5.(1) is [F10-OS3.7], which is to facilitate the timely
movement of persons to a safe place in an emergency in order to limit the risk of injury due to persons
being delayed in or impeded from moving to a safe place during an emergency. The performance target is
the measure of time for occupants to reach an exit within the 45 m maximum distance relative to the onset
of unsafe conditions. If an alternative solution is proposed, one would need to demonstrate that the
resultant travel distance to exit meets or exceeds the performance attained by the 45 m travel distance
scenario with respect to [F10-OS3.7], assuming all other factors remain unchanged.
2.5.2.4
Fire Implications
The fundamental approach to fire safety in this report is to achieve a code-conforming tall wood building
that it will conform to all of the provisions of the acceptable solutions of Division B for a high-rise
building, with the one exception of being constructed of a combustible material as laid out in Chapter 5.
The Division B solutions provide for 2-hour fire resistance ratings of floors, structure, and exits. The
means to conform to all fire resistance and fire rating requirements currently exists and is further outlined
in Chapter 5.
2.5.2.4.1 Exposed Mass Timber
Unlike light frame construction, mass timber can provide a high level of fire resistance, as during a fire
wood will char at a predictable rate, and the wood beneath the char layer is not significantly affected by
fire or heat. Therefore a sacrificial layer of wood can be used to protect the required minimum sized
structural elements. Depending on the type of wood, char rates vary from 0.5 to 1.2 mm per minute for
standard mass timber construction. This results in a sacrificial thickness of 60 to 144mm at the outer layer
of the timber element. Steel connections between wood/timber elements can similarly be protected by–
either by recessing connections into wood or covering with sufficient sacrificial wood material to provide
protection.
This approach enables mass timber buildings to provide the required fire resistance rating, however it
does not isolate the mass timber from the fire and the mass timber will contribute to both the intensity and
duration of the fire. As discussed further in Chapter 5, it is probable that exposing mass timber in areas of
the building where exposed timber finishes are already permitted can be demonstrated to provide the level
of performance required by the Code.
It is, of course, desirable to expose a significant amount of the timber, and in many cases it is more
practical to expose the timber within void spaces. Development of an alternative solution for a fully
exposed timber building was however beyond the resources and time available during the development of
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this guide; however with further analysis it may be feasible to demonstrate that a fully exposed mass
timber approach can provide the required level of performance.
2.5.2.4.2 Encapsulation
An alternate approach is to protect all wood members. At the simplest level, if all mass timber members
are encapsulated (wrapped in an acceptable non-combustible material, such that they will neither be
exposed to or contribute to a fire), then it is relatively easy to demonstrate the equivalent performance of a
mass timber building to a permitted non-combustible building. It is significant that the early British
examples of tall timber buildings were fully encapsulated, and as experience and comfort has developed,
later buildings have had larger quantities of mass timber exposed.
In order to fully encapsulate all mass timber elements so that they are completely protected from fire and
do not char or contribute to fire would require an inordinant level of protection, likely four layers of
gypsum wallboard. It is believed that a lesser level of encapsulation, allowing some charring of timber
should be acceptable as discussed further in Chapter 5.
2.5.2.4.3 Recommended Approach to Fire Protection
The approach recommended is to start with the building as fully encapsulated, then work systematically
through the building to establish which elements can be exposed without reducing the level of
performance below that which is required by the Code. An understanding of the basics of tall building fire
safety, how wood burns, and how fire and smoke is transmitted through the building is essential to this
analysis. The current code provisions in Division B permit exposed wood linings for walls and floors, and
construction of interior partitions of solid lumber within fully sprinkler protected buildings, and this
provides justification for exposing the timber structure in these and similar locations. Special
considerations will be required for all shafts and concealed spaces and, it is likely appropriate to consider
encapsulation of all combustible members in exit, elevator, and other vertical shafts, unless more detailed
analysis and compensating measures are taken.
2.5.2.4.4 Exterior Fire Spread
Exterior fire spread via windows and exposed cladding requires similar cladding to that provided in noncombustible buildings. Currently Division B provides for a performance test of fire spread up the exterior
walls, related to the ULC S134 test. Whether combustible or non-combustible, cladding systems must all
meet this level of performance. However, where there are wood structures that may be exposed to an
exterior fire, it will be necessary to either isolate this from the exterior fire, or incorporate these elements
in the exterior cladding test.
2.5.2.4.5 Additional Considerations
Due to unknown responses to fire in tall wood buildings, and given that much of the provisions of the
code are based on the assumption that structural elements and concealed spaces (both horizontal and
vertical) are non-combustible, it is appropriate to review the performance of the building relative to first
principals, including:
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•
•
•
•
•
Response to real fires, as opposed to standard fires
Potential smoke movement through the buildings
Occupant ability to evacuate
Firefighter safety
Response to disaster events, such as earthquakes or another event that may disable water supplies
and inhibit firefighter response.
A review of these issues is found in Chapter 5.
2.5.3
Alternative Solutions That May Be Required
In addition to the alternate solutions required to address combustibility, it is probable that alternate
solutions will be required to address other design details where accepted solutions are not already
available.
Additional areas that may require alternative solutions include:
• Protection of combustible concealed spaces
• Fire stopping
• Mechanical and Sprinkler flexible joints
• The behaviour of mass timber panel shear-walls and their connections.
• Size effects in mass timber panel construction
• Use of low pressure adhesives in mass timber panel assemblies
• The behaviour of connections in mass timber panel assemblies
• Pre-fabrication and erection considerations
• Weather protection
From a structural perspective, the complexity of an alternate solution application for a tall wood building
would in large part reside in assessing the performance of a timber based lateral load resisting system. For
this reason, the use of a hybrid structure consisting of a concrete or structural steel lateral load resisting
system and a timber gravity resisting system would significantly simplify the design and approval
process, and help move a project forward in a jurisdictions that may be less favourable to innovation. This
may prove necessary in the event where scheduling may limit the design and R & D process or funding
may not be sufficient to cover the full scope of an all timber structural alternate solution application
process.
In either case, it is expected that a structural alternate solution for a tall wood structure would include a
considerable amount of non-traditional modelling and analysis, and a full independent peer review
verification of the design and construction process.
As with any building, whether a high-rise building or not, there will be elements that are not directly
conforming with the solutions found in Division B, which can be appropriately addressed with alternate
solutions.
2.6
Examples of Tall Wood Building System Solutions
There are a growing number of systems for tall wood structures. Each has benefits and drawbacks that
should be weighed for a particular project and each offers designers an opportunity to adapt, evolve, and
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improve upon these systems or introduce new systems for building tall in wood. The Guide makes no
recommendation of the appropriateness of any one system; rather it is the intent to discuss a wide range of
approaches, including all wood, wood-concrete hybrid, and wood-steel hybrid systems.
In the remainder of this Chapter, several systems will be discussed in detail to highlight system
integration concepts. See Section 4.1 for an overview of these and other wood and wood-hybrid system,
2.6.1
All-Wood Systems
The ability for any single material to solve all structural issues is relatively unlikely in a tall wood
building. In general steel, concrete, and possibly other structural materials, such as fibreglass and
aluminium, may be employed in the finished building. All wood systems typically use steel connection
details and concrete foundation systems but otherwise are predominantly wood for the vertical and lateral
load bearing systems.
2.6.1.1
FFTT
FFTT is an “all-wood” and wood-steel system introduced by Michael Green and Eric Karsh in 2008. The
FFTT system is adaptable to a variety of building types, scales, and locations. The general principal is the
use of mass timber panels “tilted up” as a balloon frame walls and columns and central core with either
wood or imbedded steel ledgers and beams that receive wood floor slabs. Green and Karsh’s “The Case
for Tall Wood Buildings” conceptualized FFTT at heights up to 30 storeys in a Vancouver high seismic
context. In recent years, FFTT has been adapted to several variations unique to different applications and
different types of Mass Timber Panels being considered.
There are several different approaches to the floor structure itself that have been developed by MGA and
Equilibrium. Each variation offers a different benefit from cost effectiveness to acoustic performance,
from constructability and prefabrication to systems integration and optimization.
2.6.1.1.1 Structure
FFTT is a tilt-up structural system that effectively balloon-frames mass timber panels in a simple, cost
effective manner. Designed for stability in high seismic environments, FFTT uses a “Strong-Column
Weak-Beam” approach, in which energy is dissipated through yielding of the beams rather than through
the columns. The main structure of this system is comprised of engineered wood columns and mass
timber panels, used for floors, walls, and the building core. Above 12 storeys, steel beams and ledger
beams are integrated into the mass timber panels that support the floors, which provides for the “WeakBeam” solution and for additional flexibility in the system to achieve greater heights. These structural
elements can be organized in a number of ways, including the following configurations, in order to
accommodate a variety of performance criteria (Figures 09-12).
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The original FFTT approach (left) was a simple flat slab
and beam system without consideration of systems
integration. This configuration is economical and best
used with drop ceilings to conceal exposed building
systems.
Figure 14
FFTT
Green Architecture)
Structural
Configuration
(Michael
This floor configuration was first developed for the
North Vancouver City Hall project to allow for services
to be run below the mass timber panels with high
acoustic performance and two directional systems
integration.
Figure 15
North Vancouver City Hall
Configuration (Michael Green Architecture)
Structural
In MGA and Equilibrium’s WIDC floor system, the
floor/ceiling panels are staggered and beams are added
between columns for support. This provides the same
structural effect as a thicker slab, while saving in
material. In addition, the staggering provides acoustic
performance benefits and allows services to be run in the
channels above and below the floor.
Figure 16
WIDC Structural Configuration (Michael
Green Architecture)
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MGA “W” and “V” is a simplified floor construction
method for preassembled ‘w’ or ‘v’ beams of Mass
Timber. By creating triangular box beams with increased
depth, less material is needed than in flat floor slab
solutions. The depth of the triangular boxes matches the
depth of the main beam line. The triangular forms allow
for the integration of services in the coffers above and
below as long as the structure is compartmentalized for
fire and sealed. The system was developed for ease of
lifting and assembling the prefabricated floors increasing
site efficiency and speed of erection.
Figure 17
“W”
Floor
System
(Michael
Green
Architecture)
2.6.1.1.2 Integration of Services
At a building scale, services are integrated in a similar manner to a typical concrete building:
continuously through vertical rated shafts and locally through fire rated vertical and horizontal
penetrations. Within each unit or suite, integration can be handled in one of the following three ways,
depending on the method of fire separation employed and the desired interior finish.
1. CNC or route out chases within the mass timber panels to receive all services.
This method is popular in Europe, but requires a high level of pre-construction coordination that is not
typical of North American construction practice. Furthermore, this approach offers no flexibility during
construction.
2. Provide non-combustible chases or cavities to run services outside of the fire protection layer.
This method, used with the encapsulation approach to fire separation, is the most flexible approach and is
most akin to current North American construction practice (Figure 18).
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Figure 18 Services Integration: Encapsulation Approach (The Case for Tall Wood Buildings, Michael Green)
3. Provide a zone of services along the floor perimeter in corridors and at doorways to run services and
outlets.
This methodrequires some pre-construction coordination, but retains flexibility during the construction
phase (Figure 19). This option could also utilize a sprinklered cavity at the ceiling level, which could be
localized if services are grouped together.
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Figure 19 Services Integration (The Case for Tall Wood Buildings, Michael Green)
2.6.1.1.3 Constructability
One of the primary advantages that FFTT construction shares with other tall wood systems is the
extensive level of design and fabrication completed off site, minimizing on-site errors. In using mass
timber panels, the number of trades on site at any one time can be reduced in comparison to concrete
construction, ultimately producing cost savings. The tilt-up method of construction used to assemble these
panels allows for fast erection. This timesaving advantage serves to further drive down the cost of
assembly, increasing the cost competitiveness of these wood solutions.
There are several factors that will need to be taken into account when planning for the constructability of
an FFTT system including:
•
•
•
•
Site location, size, and characteristics
Panel size (dictated by manufacturers’ pressing capabilities and transportation limitations)
Availability of adequate access routes from storage to site
The availability of one or more on-site tower cranes at the building site.
2.6.1.1.4 Flexibility
Engineering to date with the FFTT system indicates a great deal of flexibility in tower planning and
facade design, with some decrease in flexibility once the system is utilized in applications above 20
storeys. Flexibility in tower planning is important for a number of reasons:
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•
•
•
2.6.1.2
An open plan, where there are no load-bearing interior partitions, allows for a variety of uses
including office or residential. This allows for future non-structural modifications as uses and
tenants change.
Developers typically look to this flexibility in the structural system to ensure they can manipulate
the solution to meet their market goals.
The exterior character and massing of the building are important to adjust to the specifics of a
given site. Setback requirements, view corridors, sunlight and shadow conditions, climactic and
cultural conditions, neighbourhood context, and architectural expression must all be considered.
Platform Approach: Stadthaus
One example of an “all wood” platform-framed approach can be seen in Waugh Thistleton’s Stadthaus
project, a nine-storey residential tower in East London, constructed entirely of CLT from the first floor
upwards. At the time of construction, there were no existing precedents for this scheme, as building code
regulations in Europe had prevented prior development of wood buildings of this height. The construction
methods pioneered through this building are now being added to UK Building Regulations in annexe
form.
2.6.1.2.1 Structure
Stadthaus “is the first [building] of this height to construct load bearing walls and floor slabs, as well as
stair and [elevator] cores, entirely from [mass timber panels]” and, as a result, the structure stores 186
tonnes of carbon for its lifetime. (Waugh Thistleton) The foundation and first floor utilize standard
concrete construction, with CLT being used for the structure above the first floor. The core panels are
balloon-framed and the floor and wall panels are installed systematically one floor at a time, resulting in a
highly durable cellular structure (Figures 17-18). Steel brackets are used to secure the wall panels to the
ceiling/floor panels, and are installed quickly and easily with hand tools and screws. In this example, once
the panels were in place, they were furred out and encapsulated with gypsum board (Figure 19).
Architect Andrew Waugh worked closely with KLH from Austria throughout the design process in order
to integrate the structural technology used in this building without sacrificing important principles of
design.
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Figure 20 CLT Panel Structure (Waugh Thistleton)
Figure 21 CLT Panel Structure (Waugh Thistleton)
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Figure 22 Encapsulated CLT Panel Structure (Waugh Thistleton)
2.6.1.2.2 Integration of Services
Once the CLT panels are in place, services are integrated in much the same way as in standard steel and
concrete construction and are concealed behind gypsum board. However, there is one important
difference: speed and ease of installation. In a concrete frame structure, service elements would have had
to be fastened into the concrete, which can be a difficult and time consuming process. With a CLT
structure, these elements are quickly and easily secured to the CLT, using simple power tools. Figure 17
above provides one clear example, in which the ties for the fire suppression system are screwed directly
into the wood using a power drill.
The prefabricated CLT panels for the platform-based approach are craned into position on site,
dramatically reducing construction times. In comparison to the “seventy-two weeks programmed for a
concrete frame design, Stadthaus took forty-nine weeks to complete. The timber structure itself was
constructed in just twenty-seven days by four men, each working a three-day week” (Waugh et al, 2009).
As an all-wood system, the Stadthaus project provides a certain level of flexibility in that a portion of the
structural panels could conceivably be demounted and reused in future building projects. However, it is
unlikely that the floor plan of the Stadthaus building itself could be reconfigured for uses other than its
intended programming as a residential building. This limitation results from the use of interior load
bearing walls on each floor, which reduces flexibility in the floor plan for some program types.
The Stadthaus approach has been replicated with several projects in the UK, Australia and now Italy. The
platform framed approach is simple, efficient and effective at lower heights and depending on seismic and
wind load conditions.
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2.6.2
Wood-Concrete Hybrid Systems
2.6.2.1
CREE (Creative Resource and Energy Efficiency)
CREE by Rhomberg is a wood-concrete hybrid system that provides for a 90% improved carbon footprint
over steel and concrete structures, for buildings up to 30 storeys (100m). In this approach, the building’s
structure and services are integrated in a modular system, with all components (columns, slabs, core, and
façade elements) prefabricated off-site at an industrial scale.
2.6.2.1.1 Structure
In a CREE building, the basement and ground floors are reinforced concrete and, above the ground floor,
the structure is comprised of unenclosed double glulam columns and glulam-concrete hybrid floor slabs
(Figure 23). The wood-concrete hybrid slabs offer multiple benefits, such as providing for a long span
(<9.45m), meeting code requirements for fire separation between floors, improved acoustic performance,
and allowing for service integration between beams (Figure 24). Simple mortise and tenon joints are used
at the connection between the double columns and slabs to prevent separation under lateral forces (Figure
22).
In addition to these structural elements, one or more vertical circulation cores serve to further stiffen the
building’s structure. These cores can be constructed of wood, reinforced concrete, or steel, depending on
regional building regulations and desired performance criteria. The final element of the system is woodframed exterior walls, which are attached to the double columns (Figure 26). These versatile panels can
be outfitted as desired, providing for the aesthetic and performance-based individualization of the facade.
A variety of options can be incorporated, such as the use of a single or double façade, solar screening
elements, photovoltaics, living wall systems, manual vents for natural ventilation, and a number of
diverse aesthetic configurations, among others.
Through utilizing a wood hybrid structural strategy, the CREE approach is 30% lighter than reinforced
concrete structures, allowing for a minimized foundation and smaller dead loads overall.
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Figure 23
Column and Wood-Concrete Hybrid Slab Structure (CREE)
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Figure 24
Service Integration Between Beams (CREE)
Figure 25
Column to Slab Connection (CREE)
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Figure 26 Façade Panels (CREE)
2.6.2.1.2 Integration of Building Services
The CREE system integrates a number of building services efficiently by providing space within the floor
slabs to run service components such as mechanical, electrical, plumbing, and fire protection elements.
This space can be left open, or covered with a panel to provide a flush ceiling. In order to maximize the
sustainability performance of the structure, Rhomberg has accounted for the ability to integrate renewable
energy sources, such as geothermal or solar thermal systems, among others, as well as systems for more
stringent performance criteria such as low-energy, passive house, or plus-energy standards. As the various
elements are situated within the slab structure, thinner floor plates and reduced floor-to-floor heights are
achievable.
The CREE system successfully integrates an exposed wood structure with coffers for building services.
The primary structural elements of the CREE system are prefabricated in the shop, and lifted into place on
site with a crane (Figures 24-25). Simple connection details are carefully designed, minimizing the
number of complex details that need to be executed on site. For example, as illustrated in Figure 28, as the
floor slab is lifted into place, a round mortise in the slab slides over a tenon integrated into the wall
panels. The detailing of connections such as these, in addition to prefabrication, provides for an efficient
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construction process, proceeding at a rate of approximately 1 storey per day with a crew of 5 people. This
was proven in 2012, when the 8 storey (27m) prototype LifeCycle Tower One building, designed by
Hermann Kaufmann, was constructed in 8 days.
Figure 27
Fabrication of Façade Panels (CREE)
Figure 28
Floor Slabs Lifted Into Place (CREE)
CREE’s primary structural system provides for an open floor plan that can be reconfigured for a variety
of uses during its lifecycle to accommodate commercial, institutional, or residential spaces. This
flexibility can be maintained throughout the building’s lifespan if non-load bearing walls are integrated in
a way in which they can later be removed without causing damage and if services are carefully planned
and integrated from the outset.
2.6.2.2
Concrete Jointed Timber Frame Solution
This conceptual solution for a 42-storey tower was introduced in May of 2013 by Skidmore, Owings and
Merrill LLP in their “Timber Tower Research Project” report.
2.6.2.2.1 Structure
SOM’s hybrid structure, “The Concrete Jointed Timber Frame consists of solid mass timber products
connected with steel rebar reinforcement through concrete joints. Mass timber products are used for the
primary structural elements such as [CLT] floors and shear walls, and [glulam] columns. Steel rebar
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reinforcement is connected to the primary structural elements by drilling holes in the timber and epoxy
bonding reinforcement in the hole. The connection of timber member to timber member is done via lap
splicing reinforcement through the concrete joints. The result is a band of concrete at the perimeter of the
building and bands of concrete at all wall/floor intersections. Supplementary reinforcement is provided in
the concrete perimeter beams to achieve long spans as well as the concrete link beams which couple the
behaviour of individual panels. Additional structural steel elements are used at the join locations to
connect the primary timber members during erection and prior to concreting the joints. The system is
approximately 80% timber and 20% concrete by volume for a typical floor…[and] approximately 70%
timber and 30% concrete by volume when the…substructure and foundation, [consisting of belled
caissons], are considered.” (Figure 29)
Figure 29 Concrete Jointed Timber Frame (SOM)
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Figure 30
Concrete Jointed Timber Frame (SOM)
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2.6.2.2.2 Service Integration
Services are integrated into the Concrete Jointed Timber Frame system in much the same was as in a
standard steel or concrete structure. “Primary mechanical and plumbing systems are routed vertically
within the units and distributed on a floor by floor basis,” (SOM) and it is expected that penetrations will
be required for plumbing. It is recommended that wherever possible penetrations are routed “through the
concrete connecting bands in the shear walls, away from the boundary elements.” (SOM) This is because
the concrete bands can be reinforced as required. The remaining services (electrical, telecom, and data)
are routed through the core and distributed to the units.
The SOM system assumes a dropped ceiling approach to concealing of the building services.
SOM describes that the structural system can be “constructed similar to a structural steel building with
metal deck slabs in terms of erection and sequencing of trades. The vertical column and wall elements are
connected to the corresponding vertical elements on the stories above and below with structural steel end
fittings. This allows the erection of the timber elements to proceed up the building without immediate
concreting of the joints. The formwork for the concrete joints would be supported on the vertical structure
so that re-shoring is not required. The lower portions of the spandrel beams were also designed as precast
concrete in order to avoid re-shoring of concrete elements.” (SOM)
The SOM study focused on a residential structure, making use of the interior shear wall approach. While
this is a practical application in the residential context, there is not as much inherent flexibility in the floor
plan, which may present challenges for other building programs requiring greater flexibility.
2.7
References
CREE by Rhomburg. Cree by Rhomburg. http://www.creebyrhomberg.com/en/.
Timber Tower Research Project. SOM, LLP, 2013. https://www.som.com/publication/timber-towerresearch-project.
Murray Grove. Waugh Thistleton Architects.
http://www.waughthistleton.com/project.php?name=murray
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CHAPTER 3 Sustainability
Lead Author:
Co-Authors:
Peer Reviewers:
Michael Green
Helen Goodland, Dave Ricketts, Angela Lai
Kathy Wardle, Peter Moonen, Jennifer O’Connor, Adam Robertson
Abstract
The UN defines sustainability as the ability to meet the ecological, social, and economic “needs of the
present without compromising the ability of future generations to meet their own needs." Within this
context, this Chapter discusses the factors that contribute to the overall sustainability of tall wood
structures, from the point of harvest to end of life, ultimately demonstrating that tall wood systems offer a
number of important environmental advantages. Wood structures achieve this by utilizing sustainably
harvested and renewable wood resources, enabling them to sequester carbon throughout their life cycle
and avoid additional emissions during manufacturing and construction, thus significantly reduce the
embodied energy and waste footprints of a given structure. When properly designed and maintained,
wood structures are durable. At the end of the building’s service life or when it is repurposed, wood
components have the potential to be reused, down-cycled, or recycled with a minimum additional
expenditure of energy.
Building off of these principles, the second half of the chapter focuses on how to measure and evaluate
the environmental impacts of the building over its lifespan. A special emphasis is placed on the sciencebased life cycle assessment (LCA) methodology, followed by a discussion of the various green building
certification systems and other tools applicable to tall wood structures.
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3.1
Introduction
3.1.1
Defining Sustainability
Although the parameters of sustainability are widely debated, two common definitions prevail in modern
society. The first describes sustainability through the lens of development, defining it as the ability to
meet “the needs of the present without compromising the ability of future generations to meet their own
needs” (United Nations, “Our Common Future” 15). The second definition, formulated by the Enquete
Commission of the German Bundestag, provides a more nuanced outlook, describing sustainability as the
“lasting forward-looking development of all economic, ecological, and social aspects of human
existence,” recognizing the “three pillars of sustainability” (qtd. in Ebert, Ebig, Hauser).
Although there are numerous considerations, components, and indicators for sustainability within each of
these three broad categories, this chapter of The Guide is written with a focus on the environmental
aspects that are integral to the performance of tall wood building systems.
3.1.2
Sustainability and Tall Wood Building Systems
0 of The Guide seeks to answer a number of important questions regarding sustainability and tall wood
building systems, including:
•
•
•
•
•
•
•
•
•
What environmental benefits do wood products and building systems provide?
Can wood products be sourced without negatively impacting forest health and are there enough
sustainably managed forests in North America to support a growing industry?
Are wood building systems as durable and long lasting as steel and concrete systems?
Are wood building products regionally available?
What is the environmental impact of bringing the wood products to site?
How much waste is generated from wood building systems during the construction process?
Can structural wood products be re-used? Can they be recycled at the end of their useful lifespan?
What impact do these materials have on human health and well-being?
How can environmental performance be measured and evaluated?
Throughout the remainder of 0, we will address each of these questions in more detail to provide design
teams with the foundation of information and guidance needed to arrive at a thoughtful balance between
material efficiency and environmental performance within the context of their particular project.
Ultimately, this chapter will demonstrate that, when compared to steel and concrete structural systems
(See Figure 31), tall wood building systems offer a number of important environmental advantages; from
utilizing sustainable and renewable resources, to reducing the energy, carbon, and waste footprints of a
given structure, to their capacity for durability, longevity and recyclability at end of life. This discussion
is augmented with an overview of the tools available for evaluating and certifying the sustainability of tall
wood building systems (See Section 3.8), with an emphasis on the science-based life cycle assessment
methodology.
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Figure 31 Environmental Impact of Structural Typologies
Source: Canadian Wood Council; Data compiled using the ATHENA EcoCalculator with a data set for Toronto, Ontario
3.2
Material Sourcing and Forest Impact
As tall wood building systems gain in popularity, one concern that arises is: “Will the corresponding
increase in demand for wood resources have a negative impact on the health of forest ecosystems and
ultimately lead to forest depletion?” In the North American context, where stringent laws and sustainable
forest management practices restrict harvesting levels and ensure that forest values are protected, the
simple answer is no. Below, we will discuss the ability of the Canadian Forest to accommodate this
increase and how design teams can ensure that the wood products they are specifying come from
sustainable sources.
3.2.1
Sustainable Forest Management
3.2.1.1
Defining Sustainable Forest Management
The Canadian Council of Forest Ministers defines sustainable forest management as “management that
maintains and enhances the long-term health of forest ecosystems for the benefit of all living things, while
providing environmental, economic, social, and cultural opportunities for present and future generations.”
This definition serves as the foundation for forest policies in Canada, with the underlying goal being to
achieve a balance between the demands placed on our forests and the maintenance of forest health and
diversity.
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Regardless of forest type or jurisdiction, sustainable forest management typically upholds the following
seven principles:
•
•
•
•
•
•
•
Conserve bio-diversity.
Maintain the productive capacity of forest ecosystems
Maintain the vitality and health of forest ecosystems.
Conserve and maintain soil and water resources.
Maintain the forest contribution to global carbon cycles.
Maintain and enhance long term multiple socio-economic benefits to meet the needs of societies.
Provide legal, institutional, and economic framework for forest conservation and sustainable
management.
Building off of these broader principles, regionally applicable standards for sustainable forest
management are codified in order to address, among others, the composition of species; the density,
distribution, age, and height of the regenerating trees; and the distribution of forest types and age classes
across the landscape.
3.2.1.2
Sustainable Forest Management in Canada
In Canada, there are 397.3 million hectares of forest and other land with tree cover, accounting for 53.8
percent of the total surface area. The majority of this forested area is publicly owned, with “77 percent
under provincial and territorial jurisdiction, and 16 percent under federal jurisdiction” (Natural Resources
Canada 9). This ownership status is accompanied by stringent and strictly enforced legislation, governing
areas such as land use, forest management practices, the designation of protected areas, licensing,
allocation of wood, public consultations, and Aboriginal participation, among others. This legislation sets
the standard for sustainable forest management practices in Canada.
The strictness of Canada’s national standard was recently highlighted in an independent study by the
Finnish research company Indufor Oy, which compared forest legislation schemes in jurisdictions around
the world and concluded that Canada (along with Australia) had the most demanding schemes of those
studied. Under this legislation, less than 0.2 percent of the Canadian forest is harvested annually and
harvested areas are required to be promptly replanted (Natural Resources Canada 9). In fact, there are
fewer trees harvested for wood and paper products each year in Canada than are affected by forest fires
and insect defoliation (the more recent being the Mountain Pine Beetle infestation). For example, in 2010,
688,000 hectares of forest were harvested and replanted; a relatively small number in comparison to the
3.2 million hectares lost to forest fires and the 12.7 million hectares affected by the Pine Beetle (Natural
Resources Canada 27-29).
Because the Canadian forest is protected by such strict environmental laws, it is the recommendation of
this guide that design teams source wood products for their buildings nationally, as they can be assured
that the wood has been grown, extracted, and replenished under sustainable forest management practices.
As an additional measure of assurance, teams can choose to specify products that are certified by one of
the three forest certification systems operating in Canada, discussed below.
3.2.2
Forest Certification
Since forest certification programs were introduced in Canada in the mid-1990s, they have become a
widely respected means of demonstrating that Canadian forest companies meet high standards of
sustainable forest management, complementing the nation’s already stringent laws and regulations.
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Under the forest certification process, independent third party auditors are able to evaluate, measure, and
certify the sustainability of the forest management practices and forest products associated with a
particular organization. A variety of factors are taken into account during audit, including forest
inventory, management, silvicultural and harvesting practices, road construction and other related
activities, and the environmental, social, and economic impacts of forest activities. Ultimately, this
evaluation process results in a written statement attesting to the origin of the wood material and its
qualification as a sustainably harvested product.
There are more than 50 independent forest certification systems worldwide, addressing a variety of forest
types and tenures. Despite their differences, credible forest certification systems typically uphold the
following requirements, each of which can be broken down into a variety of criteria and indicators:
•
•
•
•
•
•
•
•
Protect biodiversity, species at risk, and wildlife habitat.
Protect water quality and other resources.
Ensure sustainable harvest levels.
Ensure prompt regeneration.
Involve multiple stakeholders in the standards development process.
Obtain third-party certification performed by accredited certification bodies.
Make certification audit summaries publicly available.
Provide a complaints and appeals process.
In Canada, there are three nationally recognized certification systems: the CSA SFM, FSC, and SFI.
Together, these systems account for approximately 150.6 million hectares, or 38 percent, of the nation’s
total forest cover (Natural Resources Canada 9).
3.2.2.1
Canadian Standards Association’s Sustainable Forest Management Standards (CSA SFM)
There are two standards under the CSA: CAN/CSA Z809 (applicable to any defined forest area) and
CAN/CSA Z804 (applicable to woodlots and other small area forests), both of which are endorsed by the
Programme for the Endorsement of Forest Certification Schemes (PEFC). Approximately 45 million
hectares (or 11 percent) of the Canadian Forest are currently certified under the CSA SFM standards
(Certification Canada).
3.2.2.2
Forest Stewardship Council (FSC)
The FSC is the fastest growing global forest certification program, accounting for over 90 million
hectares of certified forestland across 78 countries. Approximately 54 million hectares (or 14 percent) of
the Canadian Forest are certified under the FSC (Certification Canada).
3.2.2.3
Sustainable Forestry Initiative (SFI)
Approximately 57.5 million hectares (or 14.5 percent) of the Canadian Forest are covered under the
global SFI 2010-2014 Standard (Certification Canada).
When specifying certified wood products, it is important for design teams to note that certain products
may not be available under all three certification systems. Thus, it may be necessary for design teams to
exercise some flexibility in working amongst the three systems. If pursuing green building certification
system credits, design teams should consult the appropriate guidelines to best determine which forest
certification systems and products can be used and how this can best be achieved in the context of their
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specific project. An overview of the credits to date related to certified wood products is provided in
Section 3.8.2
3.2.3
Carbon Storage and Savings in Emissions
In addition to utilizing sustainably managed renewable resources, tall wood building systems store, or
sequester, carbon within their wood mass during their service life. This storage occurs during the lifecycle
of the tree when carbon dioxide is removed from the atmosphere through photosynthesis and broken
down into oxygen, which is released, and carbon, which is stored within the tree’s biomass. This wood
material is “composed of about 50 percent carbon by dry weight” (Sathre and O’Connor 4) and stores
between “1 to 1.6 tonnes of carbon per cubic meter” (qtd. in Green and Karsh). At the end of the tree’s
life, this is released back into the atmosphere, through decay, forest fires, and insect outbreaks, thus
completing the natural carbon cycle.
When a tree is manufactured into a wood product the carbon remains sequestered within the wood
biomass until it is returned to the atmosphere at the end of the product’s life cycle. As such, design teams
should note that wood products are not considered a permanent mechanism for removing greenhouse
gases, but rather serve to prolong the release of the carbon. While this carbon neutrality helps to mitigate
negative climate change impacts in the present, another important impact of tall wood building systems is
revealed when considering wood as a substitute for greenhouse gas emitting materials, such as steel and
concrete. Recent studies show that this substitution, equivalent to avoided fossil emissions, is more
significant than the carbon sequestering benefit of wood materials alone (Sathre and O’Connor).
Design teams can calculate the carbon sequestering potential, as well as emissions avoided through wood
substitution, through utilizing tools such as the Carbon Calculator (See Section 3.1.2). For example,
Michael Green Architecture recently calculated this potential for a 20-storey mass timber structure and
compared this with the carbon footprint of a concrete structure of the same height. This study
demonstrated that the wood structure has the potential to sequester approximately 3,141 tonnes of CO2. In
contrast, the concrete structure emits 1,215 tonnes of CO2 in its manufacturing, a net difference of 4,356
tonnes, or the equivalent of keeping approximately 900 cars off the road for one year.
In order to maintain the carbon-storing capacity of the wood material, design teams should understand
and account for the factors that influence this capacity. These include the type of product used, the tree
species, harvest practices, service life, and the fate at end-of-life, among others. Design teams must
account for these factors early in the design process to ensure that the product is properly managed at the
various stages of its lifecycle and that the carbon is kept in service as long as possible.
3.2.4
Sourcing Regionally Available Materials
Design teams should specify regionally available wood materials wherever possible, as this further
contributes to an optimized environmental building performance. Sourcing local materials has the
following advantages:
•
•
•
•
Regional materials are context appropriate and tend to be more durable within the local climate.
Sourcing regional materials provides savings in transportation costs, both economically and in
terms of environmental footprint.
Sourcing regional materials contributes to local economic growth and stability.
Green building certification systems often award credits for regionally sourced materials. These
systems typically define local or regional materials as those that are extracted, harvested, and
manufactured within 800 km of the project site (or 2,400 km if shipped by rail or water).
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3.3
Transportation of Materials
Prior to specifying wood products, design teams will need to consider the distance and mode of
transportation required for a given product, tracing the route from the point of harvest to the point of
manufacture, to the construction site. In most cases, especially for regionally sourced materials,
transportation will be a relatively small portion of a building’s total embodied energy and carbon
footprint.
Despite this small proportion, design teams should make every effort to select the least energy and carbon
intensive mode(s) of transportation. As demonstrated by the figure below, long haul aircraft transport
represents the highest energy impact in megajoules per ton of material per mile, while rail and ocean
shipping represents the lower end of the spectrum. Design teams can utilize life cycle assessment to
evaluate and compare available options for transportation from initial logging through construction on
site, to ensure that the overall impact of transportation is minimized within the context of their particular
project.
Figure 32 Transportation Impact
Source: Canon Design, with Data from Argonne National Laboratory
3.4
Construction Waste and By-product Use
30 to 40 percent of all waste that ends up in Canadian landfills comes from the construction and
demolition of buildings (Naturally Wood, “Construction Waste Management” 2). One benefit provided by
tall wood building systems is that material flows can be controlled to minimize this waste, as well as
effluent and pollution. An initial source of waste savings can be achieved through prefabricating certain
structural elements, carefully planning mass timber panel sizing and layouts to maximize material
efficiency, and utilizing CNC machinery to make necessary cuts. The exacting nature of this process
means that there is minimal site cutting of materials required, greatly reducing waste on site. To address
any off cuts and sawdust created during prefabrication, many Canadian mass timber panel producers have
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invested in biomass energy solutions that reclaim and utilize wood waste to generate the energy used in
their manufacturing facilities.
For the waste produced on site, it is important that design teams have a waste management plan in place
prior to construction. Discussing this with the contractor early on will ensure that a system has been
designated for reclaiming waste materials, keeping the wood waste clean, and distributing them to the
proper reuse, recycling, or waste diversion location. In addition to the environmental benefits provided by
waste management planning, green building certification systems often award credits for successfully
diverting construction waste from local landfills. Finally, to reduce waste at the end of the building’s
lifespan, design teams should ensure that wood products are installed in a way that they can be demounted
and re-used or recycled.
3.5
Durability and Longevity
Durability and longevity are critical components of the overall environmental performance and
sustainability of a building. As an increasing proportion of a structure’s environmental impact is
represented by its embodied energy and greenhouse gas emissions, it is clear through life cycle
assessment that the longer a building remains in service, the smaller this impact becomes. This is
especially true of wood structures, which not only have a lower embodied impact than steel and concrete
structures, but also sequester carbon throughout their service life.
As with any building, the durability of tall wood structures is a function of the quality of building
materials used, the way the materials are connected and detailed, the environment the building is in, the
proper protection of the materials from the elements, and the ease of maintenance over time. In addition
to these common factors, tall wood building designers must also consider material properties unique to
wood, including moisture content and humidity effects, exposure to UV light, shrinkage, creep, and the
natural movement of the materials, all of which can have an impact on the structure’s long-term
performance. If these factors are taken into consideration early in the process and agents of deterioration
are managed through design and maintenance, it is reasonable to expect engineered wood structures to
achieve 120 year life spans and longer (Lenz, Schreiber, and Stark).
The ability to achieve this lifespan is illustrated by several recent studies. Kenneth Koo’s report on the
mid-rise post and beam wood structures built throughout Canada between 1849 and 1950 demonstrated
that several of these structures remain in use today. Of the buildings surveyed, the oldest was constructed
in 1859, still standing 154 years after its construction (Koo 9). The longevity of historical tall wood
structures is further reinforced by examples standing around the world, discussed in greater detail in
Section 0. A second study, executed by the Athena Institute, examined the average service life of
demolished North American buildings. This study found that, of the wood building structures surveyed,
the majority were aged 75 years or older, indicating that “wood structural systems are fully capable of
meeting longevity expectations.” (O’Connor 3) The study goes on to explain that the buildings surveyed
were typically demolished because of area redevelopment, lack of maintenance, and because the building
was no longer suitable for its intended use (O’Connor 4). As the causes for demolition of these structures
largely did not correlate with the physical state of the structure itself, this study highlights the potential for
a longer average service life of wood buildings, especially in cases where they are designed to
accommodate changing needs.
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3.6
Re-use and End of Life
Structural wood products have an inherently long lifespan and, in an optimal scenario, the wood products
used in a building can be demounted at the end of that building’s life and re-used in another project.
However, providing for an extended product life requires planning early in the design process. To ensure
that the integrity of the material is maintained and that, ultimately, the wood can be recycled, design
teams should take the following into consideration:
•
•
•
•
•
•
The choice to treat wood products must be weighed against a variety of considerations, including
the requirements of authorities having jurisdiction. In some cases, the use of treated wood or
naturally durable wood may be required to meet service life expectations.
To minimize the need for wood treatments wherever possible, design teams should ensure that the
building envelope provides optimal weather protection and that future maintenance can be easily
performed.
The re-use of treated wood in secondary applications is very common but design teams and
building owners should be aware that there are regulations prohibiting the use of wood treated
with certain preservatives in residential applications (See Section 3.7.1.3).
Most treated wood used in buildings is currently not recyclable. Technologies are under
development to address this and it is reasonable to expect that this will change in the near future.
Nevertheless, if additional fire protection, moisture protection, or protection from wooddestroying organisms is needed, design teams should do their best to target where treated wood
must be used and specify wood treatments that will have a minimum impact on the end of life of
the wood product.
Efforts should be made to utilize mechanical fastening systems to connect wood products over
irreversible chemical bonding systems so that the materials can be easily separated and re-used in
other projects.
In order to ensure that high value products do not end up in a landfill, design teams should
consider developing a plan that identifies the building products to be re-used over time and
describes their subsequent destination.
Through performing life cycle assessment and ensuring that the potential lifespan of wood material is
optimized, design teams can ensure that the carbon sequestered in the wood remains in storage and that
the maximum environmental benefits of the material are realized.
3.7
Impact on Human Health, Well-Being, and Comfort
As with any building, design teams for tall wood buildings should consider and address human health,
well-being, and comfort. There are several aspects that will need to be considered, including acoustic
comfort (See Section 0), thermal comfort (See 0), and indoor air quality and toxicity, discussed below.
3.7.1
Indoor Air Quality and Toxicity
Indoor air quality, an important component of human health and well-being, is predominantly impacted
by building materials and other indoor elements that release gases and other dangerous particles into the
air. Each of the major structural types (steel, concrete, and wood) can have negative impacts that must be
considered and mitigated wherever possible. For example, steel structural systems typically require
primers and treatments for fire and rust protection, while concrete systems often require formwork release
agents, curing compounds, and coatings for traffic and to protect reinforcing steel. While providing
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benefits that improve the service life of these structures, each of these, among others, has negative
tradeoffs. This is also true of wood building systems.
Material toxicity and off-gassing is one major contributor that negatively affects indoor air quality and
design teams should make every effort to ensure that the materials specified are safe for inhabitants. The
wood used in solid and engineered structural wood products is inherently benign and hypoallergenic.
However, there are a variety of additives that may come in contact with the wood material that must be
considered by the design team, including adhesives, treatments for protection against wood destroying
organisms and wood rot, and treatments for fire protection, discussed below.
3.7.1.1
Structural Adhesives
Structural adhesives are used in engineered wood products for lamination purposes and to transfer stresses
between adjoining wood fibres. Typically, the selection, application, and curing of adhesives, as well as
testing for VOC emissions, reliability of bond, and performance under various environmental factors, is
controlled at the point of manufacture. As a result, the adhesives used for wood products in Canada can
differ both by product and manufacturer and may include the following, among others: phenol
formaldehyde (PF), resorcinol formaldehyde, phenol resorcinol formaldehyde (PRF), polymeric
diphenylmethane diisocyanate (polymeric MDI), emulsion polymer isocyanate, polyurethane/emulsion
polymer, polyurethane polymer, polyvinyl acetates (PVA), and melamine formaldehyde (See Figure 33,
below).
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Figure 33 Adhesives Used in Wood Products
Source: Adhesives Awareness Guide.
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Each of these adhesives provides important performance benefits, as well as negative impacts on indoor
air quality and human health. As such, it is important that design teams consult with manufacturers and
available resources such as MSDS and EPD sheets prior to specifying a given wood product in order to
understand these nuances. Generally speaking, structural adhesives are known to off-gas during the gluesetting period and, as noted by Frihart and Hunt, “uncured adhesives can be harmful at high
concentrations or with chronic exposure and require safety pre-cautions. [Once cured, these] adhesives
are usually safe for human contact, [with the exception of] urea-formaldehyde, which can cause low
concentrations of formaldehyde gas from bonded wood products, especially under hot, moist conditions”
(10-14).
3.7.1.1.1 Formaldehydes
Many structural wood products use formaldehyde-based adhesives. Although formaldehyde is a naturally
occurring chemical, it is also well known as an irritant and carcinogen (International Agency for Research
on Cancer). Different formaldehyde-based products have different levels of chemical stability that either
reduce (high stability) or increase (low stability) their emissions of VOCs under different environmental
conditions. For example, in contrast to the more volatile urea formaldehyde (which are not used for glued
wood products with a structural rating), phenol formaldehyde, resorcinol formaldehyde, phenol resorcinol
formaldehyde, and melamine formaldehyde polymers “do not chemically break down in service; thus no
detectable formaldehyde is released” (Frihart and Hunt).
Due to regulations mandating lower formaldehyde emissions, new adhesive formulations have been and
are being developed to significantly reduce levels of formaldehyde emissions, both during manufacturing
and in bonded wood products. Examples of these include ultra-low emitting formaldehyde (ULEF) and
“no added formaldehyde” (NAF) adhesives such as polyvinyl acetate, isocyanate, and soy adhesives.
3.7.1.1.2 Adhesives and Fire Performance
Adhesives vary in their performance during fire, with some upholding their bonding capacity at higher
temperature limits than others. Design teams should consult with manufacturers and other available
resources to understand how the adhesives used in the specified products will perform under fire, both in
terms of harmful off-gassing and delamination potential, and to ensure that the adhesive used
complements the building’s overall fire performance criteria.
3.7.1.2
Treatments for Wood-Destroying Organisms and Wood Rot
There are a variety of treatments that can be used to prevent the occurrence of wood-destroying organisms
and wood rot. Of these, treatments typically referred to as “heavy-duty wood preservatives,” such as
Chromated Copper Arsenate, Creosote, Pentachlorophenol, and Ammoniacal Copper Zinc Arsenate, are
considered toxic and are not permitted for residential use by Canada’s Pest Management Regulatory
Agency (PMRA). However, there are a variety of safer, water-based alternatives currently available,
including the following that are registered with PMRA for use in residential applications (Wood
Preservation Canada). Prior to selecting a wood treatment, teams should consult available resources, such
as EPD and MSDS sheets, which provide additional detail on the human and environmental impacts of
these products. Although these are safer than the heavy-duty wood treatments, some negative impacts
may still be cited in some cases.
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3.7.1.2.1 Alkaline Copper Quaternary (ACQ)
Alkaline Copper Quaternary is a water-based wood preservative that may be used inside residences,
provided that all sawdust and construction debris are removed after construction. ACQ treated wood can
be painted or stained and, if necessary, water repellent coatings can be applied to improve weathering
performance. Design teams should note that ACQ is corrosive to metal.
3.7.1.2.2 Copper Azole (CA)
Copper Azole is a wood preservative that prevents fungal decay and insect attack. It is used to treat a
variety of softwood species, including Southern Pine and Douglas Fir, and, when applied, leaves the
wood a greenish brown colour with little or no odour. The newer CA product, CA-B, is comprised of
copper (96.1percent) and azole as tebuconazole (3.9 percent). Unlike other water-based treatments,
Copper Azole can be used in ground contact and fresh water and marine applications, in addition to
aboveground contact. Design teams should note that Copper Azole is corrosive to metal.
3.7.1.2.3 Micronized Copper Azole (MCA)
In comparison to Copper Azole (CA), wood treated with micronized copper is less corrosive to metal and
is lighter in colour.
3.7.1.2.4 Borates
Borates (Disodium Octoborate Tetrahydrate) are a non-volatile wood preservative with a near-neutral pH.
According to the US Environmental Protection Agency, “borate preservatives are low toxicity alternatives
[that protect against] decay, fungi, [wood-boring insects], and termites. Borates are naturally occurring
minerals [and] consequently, they have marginal environmental impact.” They are colourless, odourless,
non-off-gassing, non-corrosive, and there is no acquired resistance among target organisms.
3.7.1.2.5 Wolman AG
Wolman AG wood preservatives is a non-metallic pressure treatment, containing active fungicides and
termiticides.
In addition to the above products approved for residential use by the PMRA, the following treatments are
also considered safer alternatives to heavy-duty wood preservatives by the US EPA.
3.7.1.2.6 Cyproconazole
Cyproconazole (Formula 360 SL) is comprised of cyproconazole and didecyldimethylammonium chloride
(DDAC) and is designed to prevent fungal decay in above-ground applications. It is a clear to slightly
yellow liquid with no odour and can be applied as a surface treatment or by pressure impregnation. Where
there is a potential for insect attacks, a registered insecticide should be applied as well.
3.7.1.2.7 Propiconazole
Like Cyproconazole, Priopiconazole (Formula 100 SL) prevents fungal decay in aboveground
applications. It is a clear to reddish brown liquid with a slight oily odour and can be applied as a surface
treatment or by pressure impregnation. Where there is a potential for insect attack, a registered insecticide
should also be applied.
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It is important for design teams to note that water-based treatments are not suitable for applications
exposed to direct wetting (unless noted otherwise) and will leach out of the wood if exposed to water. As
such, they are designed to provide an additional level of protection, but are not substitutes for a properly
designed and detailed building envelope.
3.7.1.3
Fire Treatments
Intumescent coatings can be applied to wood products to provide additional protection against fire and
flame spread. These treatments may allow wood to be used in a wider context, but may also negatively
impact the materials ability to be reused or recycled in the future.
3.7.1.4
Ventilation and Air Tightness
Inadequate ventilation in any building can increase indoor air pollutants and toxins by not allowing
enough outside air to enter, dilute emissions within the interior, and carry air pollutants out of the home. It
is important for design teams to take this into consideration when designing the building envelope, as well
as when integrating mechanical systems that control airflow. For more information on these topics, see 0:
The Building as a System and 6: Building Enclosure Design.
3.8
Tools to Measure, Evaluate, and Certify Sustainability
3.8.1
Life Cycle Assessment
The construction industry is a significant consumer of natural resources and the pace of consumption is
reaching unprecedented levels. The extraction of raw materials used in the manufacturing of building
products can result in significant environmental impacts and the manufacturing processes are often very
energy intensive. To date, green building design has focused heavily on minimizing the ongoing impacts
of building operations, including energy use, water use, and maintenance impacts. Yet, a successful green
building strategy should also address the upstream environmental burdens of the materials and products
with which a building has been built. This is particularly important when considering the environmental
benefits of wood products, which can be significant, but are difficult to fully understand without taking a
holistic “life-cycle” approach.
A product or material life cycle is defined as the “consecutive and interlinked stages of a product system,
from raw material acquisition or generation from natural resources to final disposal” (ISO 14040:2006). It
is generally accepted that Life Cycle Assessment (LCA) is the best way to objectively evaluate the full
life-cycle impacts of a product or design and to understand tradeoffs in terms of measurable
environmental effects. LCA also allows designers to compare products, assemblies and whole building
designs completely, fairly and impartially across a wide range of environmental measures (see Figure 34.
Driven by increasingly stringent building codes and the popularity of green building rating systems,
designers in many jurisdictions are starting to pay greater attention to the provenance of materials, how
they were manufactured, and what happens to them at the end of their service life. However, as the
operational impacts of buildings decrease, the “embodied” impacts of materials comprise a
correspondingly larger proportion of the overall environmental “footprint of the building. Further,
operationally energy efficient buildings may have thicker walls, insulation, triple glazed windows, etc. all
of which can, in fact, add to the embodied impacts.
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Because building projects are complex, regionally specific, and usually in place for a long time, it is
important that a science-based methodology such as LCA is used to quantify the embodied environmental
impacts of materials. In particular, LCA can assist designers in assessing the benefits of tall wood
buildings and gauging the extent to which they contribute towards a project’s overall sustainability
objectives.
Figure 34 The Life-Cycle Approach
Source: NaturallyWood, “Building Green with Wood”
3.8.1.1
The Benefits of Life Cycle Assessment
Despite the fact that there is growing awareness of the embodied impacts of building materials, LCA has
yet to gain mainstream market traction. Therefore, it is likely that, within conventional practice, the
majority of material choices and budgets do not factor in all the impacts of manufacturing, transportation,
maintenance and disposal, and the true costs of production, including environmental and social costs, are
either hidden or subsidized. However, the situation is changing. Rating systems, such as LEED, are
starting to include LCA. This means that the efforts by some manufacturers to incorporate efficient
“closed-loop” production processes, such as using mill waste for fuel in lumber drying kilns, may soon be
considered during the materials selection process. Because LCA can analyze all of the various impacts of
materials, the environmental benefits of wood (such as the capacity of trees to absorb and store carbon)
can be factored against the impacts associated with drying, processing and transporting wood products.
In fact, calculations of LCA impacts have shown clear environmental advantages for wood, because of its
low embodied energy, and because its carbon storage properties actually give wood a positive
environmental impact (Athena Institute and FPInnovations; Naturally Wood). In several studies, the life
cycle assessment of wood has demonstrated a better overall environmental impact than either steel or
concrete, with wood showing the least impact on energy, climate, and air pollution. For example, studies
from the UK have established that (Edinburgh Centre for Carbon Management):
•
•
•
Using wood instead of other building materials saves on average 0.9 tonnes of carbon dioxide per
cubic metre
3 Mt of CO2 can be saved by using timber framing over a 20 Mt CO2 footprint of a typical 3
bedroom detached house (in the UK).
Increasing wood content, including softwood cladding, can reduce the footprint to 2.4 tonnes – a
total reduction of 17.6 tonnes CO2.
In the US, the Consortium for Research on Renewable Industrial Materials (CORRIM) found wood to
have the lowest environmental impact compared to concrete and steel, and that the global warming
potential of the steel-frame home was 26 percent higher than the wood-frame home, and 31 percent
higher for the concrete-frame home than the wood frame house. It is likely that tall wood buildings with
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large amounts of wood from sustainable sources will potentially be able to contribute significantly to the
reduction of a project’s overall carbon footprint.
As LCA becomes mainstream, it is
reasonable to expect that buildings
will soon be designed with total
combined operating and embodied
environmental impact limits in mind
(such as carbon, energy, water and
more). LCA can be used to inform an
optimal suite of measures tailored for
a specific building in a particular
location based on internationally
accepted standards. LCA not only
provides a more complete account of a
project’s environmental performance
but it also allows comparison of the
relative merits and impacts of
alternative building designs. For
example, not only can different
materials and structural systems be
compared impartially (steel, concrete, wood) but also the operational performance benefits provided by
the structure (such as thermal mass) may be considered and potentially “traded off” against the more
intensive embodied impacts. Inevitably, low carbon materials such as wood may afford the designer more
choice when it comes to the selection of building systems.
Example:
A 2013 study by Intep LLC and Brantwood Consulting for
the City of Vancouver, Forestry Innovation Investment Ltd,
and The University of British Columbia showed that the
estimated average embodied GHG emission intensity
(measured on a unit of gross floor area basis) in a typical
multi-unit residential building constructed to current
standards in southwest British Columbia is approximately
7.0kg CO2 eq./(m2 yr). Currently, these emissions are about
25% of the operating emissions (about 28.0kg CO2 eq. / (m2
yr), assuming a 60-year life of the building.
However, when the same building is designed to achieve an
operational C02 emissions target of 5.0kg CO2 eq./(m2 yr),
which meets Vancouver’s 2020 “carbon neutral” goal, the
average embodied emissions of 7.5kg CO2 eq./(m2 yr) now
exceed the operational emissions.
Overall, the key benefit of LCA is that it is a transparent, internationally recognized methodology that can
provide measurable, comparable and fair outputs. Improved understanding of the life cycle impacts of
material choices in buildings can also guide capital planning for renovations and retrofits. Making
informed choices about the environmental footprint of products, weighed against intended service life and
anticipated replacement, can protect asset value and future-proof investments.
3.8.1.2
Life Cycle Assessment Tools and Approaches
There are three basic options for bringing LCA into building design decisions: at the product specification
level, the assembly level or the whole building level. Environmental Product Declarations (EPDs) are
developed by product manufacturers in compliance with international standards in order to describe the
impacts of the product consistently (ISO 14025). An EPD includes information about the environmental
impacts associated with a product, such as raw material acquisition; energy use and efficiency; content of
materials and chemical substances; emissions to air, soil, and water; and waste generation. EPDs can be
used by designers on a product-by-product basis to compare a range of environmental impacts. A number
of online product directories include information from EPDs (such as BRE Green Guide:
www.bre.co.uk/greenguide) or directly host the EPDs themselves (for example: www.greenwizard.com).
Comparison of products and materials using EPD data is also possible using the BEES free software tool
developed
by
the
US
National
Institute
of
Standards
and
Technology
(www.nist.gov/el/economics/BEESSoftware.cfm).
Performing LCA for a complete building is becoming increasingly straightforward and, with the support
of increasingly sophisticated software tools, whole-building LCA can be employed as a design tool is
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similar to the use of energy, ventilation/air flow, and other models and simulations. A whole-building
LCA model can be used throughout the design process to allow designers to explore trade-offs and refine
a design for optimum LCA performance, ideally leading to a final design with a lighter footprint.
LCA is a very powerful methodology and there are proprietary software tools capable of assessing a large
number of environmental impacts (biodiversity, water quality, and so on). Similar to energy modelling,
LCA is a specialized analytical field typically practiced by experts. For complex projects, design teams
wishing to do LCA may need to hire LCA professionals, who have access to advanced LCA tools and
LCA databases. It is important to note that LCA data are constantly being incorporated into life-cycle
inventories around the world, and expertise is necessary to understand the status, quality and currency of
the information available. However, many design projects can be served by the free software tool, the
Athena Impact Estimator for Buildings (www.athenasmi.org). This LCA tool was developed to make
LCA accessible to building designers and does not require specialized expertise. The Impact Estimator is
the only North American LCA software tool specifically created for use by practitioners in the
construction sector.
3.8.2
Green Building Certification Systems
3.8.2.1
Leadership in Energy and Environmental Design (LEED)
Organization: Canada Green Building Council
Website: www.cagbc.org
The LEED Green Building Rating System, developed by the US Green Building Council, is a third-party
certification program for the design, construction, and operation of high-performance green buildings. It
is currently the most popular rating system for larger projects in North America and has been adapted and
licensed by the Canada Green Building Council to specifically address the Canadian climate, construction
practices, and regulations. Credits and prerequisites for LEED Canada NC 2009 are organized into the
following seven categories:
•
•
•
•
•
•
•
Sustainable Sites
Water Efficiency
Energy & Atmosphere
Materials & Resources
Indoor Environmental Quality
Innovation in Design
Regional Priority
Certification is on a scale, ranging from Certified, Silver, and Gold, to Platinum at the highest level, and
is based on the total points achieved. The following credits apply to tall wood building systems and
should be taken into consideration by design teams seeking LEED certification:
•
MRc2: Construction Waste Management (1-2 points)
This credit is designed to divert construction and demolition debris from incineration and landfills
and to redirect recyclable resources back to the manufacturing process and reusable materials to
appropriate reclamation sites. Construction waste management and by-product use for tall wood
buildings is discussed in more detail in Section 3.4, above.
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•
MRc3: Materials Reuse (1-2 points)
This credit is designed to encourage reuse of building materials in order to reduce waste and
demand for virgin materials. This credit can be achieved through using salvaged, refurbished, or
reused materials, which must constitute at least 5 or 10 percent of the total value of materials for a
given project.
It is feasible that design teams could achieve this credit through re-using mass timber elements
that have been reclaimed from another building, provided that the previous structure was
designed to be demounted and the elements have been appropriately protected.
•
MRc5: Regional Materials (2 points)
For this credit, designed to increase demand for regional materials and reduce the environmental
impacts associated with transportation, points are achieved through using materials or products
that have been extracted, harvested or recovered, and manufactured within a specified distance of
the project site. These materials must comprise 10-20 percent of the total material value of the
project.
The difficulty or ease of achieving points for wood products will ultimately depend on the
proximity of the site to the forest of origin and point of manufacture. Other materials used in the
project, such as asphalt, concrete (used for foundations, exterior paving, etc.), granular fill, and
gravels, among others, may be significant contributors in achieving this credit.
•
MRc6: Rapidly Renewable Materials (1 point)
This credit is designed to reduce the depletion of finite and long-cycle materials by replacing
them with rapidly renewable materials. Points for this credit are achieved by using rapidly
renewable materials for 2.5 percent of the total value of all building materials used in the project,
with the LEED definition for rapidly renewable being a 10 year harvest cycle or shorter.
Given this definition of rapidly renewable, many Canadian wood products will not currently fall
within the LEED requirements for this credit. For example, Laminated Strand Lumber (see
Section 4.1 for additional information) is one of the quickest renewed mass timber product
available, using hardwoods ranging from approximately 10 to 20 years of age. Although these
products fall outside of the LEED definition, as the market develops further, products within the
required harvest cycle may become more available in the future.
•
MRc7: Certified Wood (1 point)
This credit requires that a minimum of 50 percent (based on cost) of wood-based materials and
products used for a given project are certified by FSC. To achieve this credit, it is recommended
that design teams work with the mass timber suppliers early in the process to source FSCcertified wood products.
Some design teams may find that this credit presents certain limitations, as not all structural wood
products are certified under the FSC, but may instead be certified under one of the other two
certification systems operating in Canada (CSA and SFI). Public Works and Government
Services Canada views this credit as too restrictive and has instructed all project managers to
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focus on other credit areas in the case that this credit poses restrictions in specifying the most
sustainable materials available for the project.
•
IDc1: Innovation in Design (1-5 points for New Construction)
This credit provides tall wood building design teams with an opportunity to pursue points for
items not accounted for in the typical rating system. For example, teams may choose to pursue
points for environmental performance aspects, such as carbon sequestration or design for
disassembly, among others.
•
There are a number of credits under “Indoor Environmental Quality” that wood products will
ultimately play a factor in. For example:
EQc4.1- EQc4.4: Low Emitting Materials (Up to 4 points)
These credits address indoor air contaminant emissions (such as VOCs) from building materials.
In order to specify wood products, adhesives, and treatments that meet these standards, design
teams should consult with manufacturers, as well as MSDS and EPD sheets, early in the design
process to understand the contents of a given product and ensure that the product will not have a
negative impact on indoor environmental quality. For more information on Indoor Air Quality
and tall wood building products, see Section 3.7.1.1, above.
Design teams should note that the above credits are for the Canada NC 2009 version of LEED. LEED v4,
currently being drafted, will include additional credits relevant to tall wood building systems. For
example, credits will be given for the completion of a whole-building Life Cycle Assessment, as well as
for material ingredient reporting. It is recommended that design teams consult with Canada Green
Building Council about these and other applicable credits if seeking certification under v4.
3.8.2.2
The Living Building Challenge
Organization: International Living Future Institute
Website: www.ilbi.org/lbc
The Living Building Challenge, developed by the International Living Future Institute, is a third party
audited system that is one of the most rigorous and challenging systems in the market place. To date, it is
predominantly active in Canada and in the United States and seeks “to define the most advanced measure
of sustainability in the built environment possible today and acts to diminish the gap between current
limits and ideal solutions” (Living Building Challenge).
The Living Building Challenge is not a credit-based program. Rather, there are 20 imperatives in the
following 7 petals:
•
•
•
•
•
•
•
Site
Water
Energy
Health
Materials
Equity
Beauty
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All wood used in a Living Building Challenge project must be either FSC certified, be salvaged, or
harvested on-site. According to the Living Building Challenge, pine beetle-killed wood can qualify as
salvaged material if the timber supplier can document that sustainable forestry management practices
have been applied to the harvesting of this resource.
In addition, practitioners must pay particular attention to the Materials Red List to ensure that these
substances are not used in the manufacturing and installation of wood based products in a building. It is
important to note that the Living Building Challenge currently states that a temporary exception is made
for glulam beams using phenol formaldehyde adhesives. At the present, there is no requirement to
complete a Life Cycle Assessment or address embodied energy and greenhouse gas emissions of the
materials. For more information on the Living Building Challenge, please see: http://livingfuture.org/sites/default/files/LBC/LBC_Documents/LBC%202_1%2012-0501.pdf
3.8.2.3
BuiltGreen High Density Program
Organization: BuiltGreen Canada
Website: www.builtgreencanada.ca
Within the BuiltGreen program, points fall under the following seven categories:
•
•
•
•
•
•
•
Envelope and Energy Systems
Materials and Methods
Indoor Air Quality
Ventilation
Waste Management
Water Conservation
Business Practice
Tall wood building systems used in residential and mixed-used applications can achieve certification
under the BuiltGreen program within the “High Density” category, provided that they meet the following
criteria:
•
•
In mixed-use applications, the residential area must account for at least 50%.
The building must be greater than 600 square metres in footprint, or 32 dwelling units.
The level of certification under this program depends on the number of points achieved, ranging from
Bronze, Silver, and Gold, to Platinum at the highest level. Credits related to tall wood building systems
fall into several areas, including, among others, the use of: engineered wood products; advanced framing
techniques to reduce the use of dimension lumber, and strategies to encourage the use of certified wood
(from the FSC, SFI, or CSA), recycled materials, and low-emitting materials. More information about the
BuiltGreen High Density program, as well as a complete checklist for potential point areas is available at:
http://www.builtgreencanada.ca/high-density
3.8.2.4
Green Globes Design for New Buildings
Organization: Operated by ECD Energy and Environment Canada ltd.
Website: www.greenglobes.com
Green Globes is an online rating tool developed in Canada, originally as an adaptation of the British
BREEAM system. Organizations involved in the development of Green Globes over time have included
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the Canadian Standards Association, the Athena Institute, the Building Owners and Manufacturers
Association of Canada, and several departments of the Government of Canada.
The Green Globes program is a questionnaire-based tool that consisting of approximately 150 questions,
which take an estimated 2 to 3 hours to answer. The reader should note that this process is not third-party
audited. There are seven primary areas of assessment:
•
•
•
•
•
•
•
Project Management
Site
Energy
Water
Resources
Emissions, Effluents, and Other Impacts
Indoor Environment
In the Green Globes rating system, buildings are given one to five globes, depending on performance
levels. A minimum of 35 percent of the 1000 points need to be reached in order to achieve the lowest
certification level. Credits are given for selecting materials with the lowest life cycle environmental and
embodied energy burden, and the program encourages the completion of a whole building Life Cycle
Assessment. It also recognizes the use of wood products that originate from certified and sustainable
sources (certified by the CSA, FSC, or SFI) and which avoid the use of tropical hardwoods. For more
information
on
Green
Globes
Design
for
New
Buildings,
see:
http://www.greenglobes.com/design/Green_Globes_Design_Summary.pdf
3.8.3
The Carbon Calculator
Organization: WoodWorks
Website: www.woodworks.org/design-tools/online-calculators
The Carbon Calculator was developed by WoodWorks, a cooperative venture of North American wood
associations, research organizations, government agencies, and other funding partners established to
provide free technical support and education resources related to the design of larger wood buildings. This
free tool, available online, calculates the amount of carbon sequestered by a wood building, which can be
used by design teams to help quantify the environmental benefit of a wood structure.
3.8.4
The Wood Calculator
The FPI Wood Calculator Tool is a spreadsheet that enables design teams to calculate the percentage of
wood content within a given building.
3.8.5
Environmental Product Declarations
An Environmental Product Declaration (EPD) is a document that conveys the environmental data
associated with a particular product, primarily derived from life cycle assessment. Already in use for over
a decade in Europe and Asia, this communication tool is now gaining popularity in North America. EPDs
currently exist for a number of wood products and can be provided by individual manufacturers, or found
through the American Wood Council (located here: www.awc.org/greenbuilding/epd.html) and
FPInnovations (located here: www.forintek.ca/public/Eng/E5-Pub_software/5a.fact_sheets.html).
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3.9
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Emery, John A. “Structural Wood Panels and Formaldehyde.” APA The Engineered Wood Association,
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Sathre, R. and J. O’Connor. A Synthesis of Research on Wood Products and Greenhouse Gas Impacts, 2nd
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(CORRIM).
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https://www.som.com/publication/timber-tower-research-project. 1-72.
Wood and Human Health. FP Innovations. Wood and Human Health Series: Issue 1.
Vacca, Phil. LP® SolidStart® Engineered Wood Products and Formaldehyde Emissions. 2009. 1-2
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Structural and Serviceability
CHAPTER 4 Structural and Serviceability
Lead Author:
Co-Authors:
Peer Reviewer:
Eric Karsh
Marjan Popovski, Mohammad Mohammad
Sylvain Gagnon
The four Sections in Structural and Serviceability Chapter provide an outline for how tall wood structural
systems can be designed beyond the scope of current Canadian codes. Tall wood currently falls outside of
the NBCC specified acceptable structural systems, and thus needs to be designed under the “alternative
solutions” provisions. The following four sections detail how a structural alternative solution can be
developed for a tall wood building. This Chapter covers important and unique aspects of wood design
compared to other materials, and it reviews ways in which mid- and high-rise wood design differs from
low-rise design.
Chapter 4 is not prescriptive. Instead it follows performance-based principles that provide several possible
methods of design and analysis. It is still up to the structural engineer to ensure that the chosen solution is
acceptable. Chapter 4 covers necessary topics that span the entire structural design process. From
conceptual design (Section 0), to gravity and lateral design and analysis (Sections 0 and 0), to
serviceability design (Section 0), this Chapter is intended to give the engineer direction. Instead of
providing a step-by-step building design method, Chapter 4 raises important issues, highlights relevant
research, and presents the designer with possible options. For instance, in Section 0, Considerations for
Conceptual Design, a discussion of several built and proposed tall timber and timber-hybrid buildings is
given. These case studies are meant to provide the designer with insight into what has been built, what
can be built, and the materials that are available. Section 0 is a good starting point in developing a
building system concept, with many resources that can be used to obtain further information on any one
building or system.
Tall timber buildings may require structural systems that have not been used before. In this case Section
0, Development of Input Data for Connections and Assemblies, and Section 0, Advanced Analysis and
Testing of Systems for Design, can be used together to obtain input data from testing to apply in analysis
of building systems for gravity and lateral loads. For novel connections and systems, Section 0 provides a
detailed guide to testing requirements and analysis methods to provide useful data for building analysis
(Section 0). Section 0 is a valuable resource for timber connection design concepts that are relevant to all
wood structures, but especially important for tall buildings. For instance, the effects of shrinkage and
creep may be tolerated in a 2-storey building, but need to be tightly controlled in a 20 storey building.
These Sections make up a holistic structural and serviceability picture, but coordination with additional
chapters is crucial. Just as in any building project, the structural and serviceability should be coordinated
with other disciplines, in the context of this Guide; the four Sections of this Chapter should be used with
all other chapters to ensure proper coordination is achieved. For instance, the fire rating of a building
system may depend on the type of connections used and whether they are exposed. Should
instrumentation and monitoring of a building be specified, the work should be coordinated with the
structural designers. And the type of structure chosen can dictate the construction methods used.
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Recommendations for Conceptual Design
4.1
Lead Authors:
Co-Authors:
Peer Reviewers:
Eric Karsh and Ilana Danzig
Kevin Below, Sylvain Gagnon, Michael Green,
Leung Chow, Michael McNaught, Grant Newfield
Abstract
For centuries, wood has been used as the material of choice for a variety of small and large structures. In
the last century, with the increase of concrete and steel research and construction, and with growing
concerns over fire and durability, wood has fallen out of common use for tall buildings. This trend is
beginning to reverse.
In the last decade the world has seen a resurgence of wood-based systems and concepts that will pave the
way for tall wood buildings in Canada. This chapter reviews a list of case studies demonstrating tall wood
building concepts that have been built to date, or are currently under construction or consideration. Over
the last decade, many innovative 8 to 10-storey buildings, as well as other innovative structures, have
been erected, demonstrating that wood is a viable building material for tall buildings.
Focusing on structural considerations, the case studies provide a brief overview of available products,
connections, and systems that can be used to build tall wood structures. Moving beyond what has been
done, this chapter also explores what can be done, with a discussion first of materials and products that
are available, then of possible and proposed tall wood systems that have been developed by engineers,
architects, and researchers. This chapter summarizes a number of viable structural building systems,
useful for the conceptual design stage of a tall wood building.
4.1.1
Introduction
There are many existing examples of timber mid-rise industrial, office, and residential buildings from the
early 20th century all around the world; however it is relatively recent that the international architecture,
engineering, and construction industries have once again started using wood in their multi-storey building
concepts. In the course of the last decade, numerous tall buildings have been erected worldwide using
wood as the primary structural material and several others are either under construction or in design stage.
In the early 1900’s in Canada and elsewhere, tall timber structures were built using a “brick and beam”
structural system. Figure 31 shows an 8-storey office building that was built in Toronto in the 1920s. As
was common at the time, the exterior walls were constructed with bricks or masonry, while a heavy
timber post and beam structure was used for the interior. Figure 36 shows a 9-storey building that was
built in Vancouver in 1905 and is still in use.
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Figure 35 8-storey brick-and-beam building built in Toronto in the 1920s
Figure 36 9-storey brick-and-beam building built in Vancouver in 1905
A decline in the construction of tall timber-based buildings was observed over the second half of the 20th
century due to the technological advancements in alternative construction materials such as steel and
concrete, and the desire by both developers and designers to build taller buildings. During this time, fire
science and fire safety engineering had not yet been developed and the building codes response to fire
loss at the time were simple rules that limited building heights and areas depending on whether a
combustible material was used or not. For smaller construction, light framing, also known as stick
framing (Figure 37, centre), was and still is used as an efficient option that takes advantage of the
availability of renewable softwood lumber. However, platform light-framed systems have reached their
practical limits at around 6 storeys high.
Recent development and advancements of innovative engineered wood-based products and systems, in
addition to the introduction of objective- and performance-based building codes, have contributed
significantly to reviving the interest in using wood-based products in mid- and high-rise construction. A
new generation of engineered wood-based products has been developed which provides designers and
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engineers with alternative materials and systems of comparable performance and enhanced environmental
attributes. Advancements in fire safety and protection engineering, building science, and structural
engineering analysis have also benefited wood construction.
Figure 37 Post and Beam (Left), Light Framing (Centre), Solid Construction (Right)
4.1.2
Tall Wood Case Studies
4.1.2.1
Historical Case Studies
Wood has long been a natural building material of choice due to its strength, low weight, and availability.
Timber structures from ancient times far exceed current limitations set by building codes worldwide.
Timber pagodas that are the equivalent of 10 storeys high built in China and in Japan over 1000 years ago
are still standing, despite exposure to seismic events, strong winds, and high moisture environments.
Churches built in the last millennium demonstrate timber’s ability to withstand the centuries. In Canada,
timber was used with bricks from the mid 1800’s to the early 1900’s to build large warehouse-type
structures that are still standing today.
4.1.2.1.1 Ancient Pagodas
Tall timber pagodas were developed in China, and were based on similar Buddhist monuments built out
of stone in India. The Yingxian Pagoda (Figure 38, right) is believed to be the oldest surviving large
wood building in China. Built around 1056 A.D., it is 67m high including a 10m tall steeple, which makes
this pagoda one of the tallest wood structures in the world. The pagoda takes advantage of the high
compressive strength of wood in a post and beam arrangement. The Yingxian Pagoda is supported by
octagonal rings of columns, with a base dimension of 35m. The timber columns and beams are intricately
connected with a system that allows movement under lateral loads (Lam, He, & Yao, 2008). Timber
elements used in such buildings were usually taken from very dense and durable hardwood species.
Timber pagodas came to Japan via China in the 6th century. Horyu-Ji (Figure 38, left) is a 5-storey
wooden pagoda in Nara, Japan, that is 32.5m high. This temple was built around A.D. 711, and is
considered the oldest pagoda that is still standing. The storeys are not connected to one another through
columns or ties, nor are the first storey columns tied to the foundations. Under seismic loads, storeys can
move independently providing the system with friction damping and base isolation (Nakahara, Hisatoku,
Nagase, & Takahashi, 2000).
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Figure 38 Horyu-Ji Temple Pagoda (left) (Nakahara, Hisatoku, Nagase, & Takahashi, 2000),
Yingxian Pagoda (right) (Lam, He, & Yao, 2008).
4.1.2.1.2 Churches and Monasteries
Urnes Stavkirke is a timber medieval church built in Norway in 1130. The structure consists of timber
circular columns, cubic capitals, and semicircular arches (UNESCO, n.d.). The Barsana Monastery
(Figure 39, right) was built in 1720 in Romania and is 56m high. The heavy oak beams and posts are
supported on a foundation of stone blocks (Green, 2012).
Figure 39 Urnes Stavkirke (left) (UNESCO, n.d.), Barsana Monastery (Green, 2012)
4.1.2.1.3 Early Post and Beam Timber Structures
In Canada, tall wood-masonry hybrid structures, known as “brick and beam” buildings, were built for
factories, plants, mills, and warehouses between 1850 and 1940. These structures were typically made up
of unreinforced brick exterior walls with heavy timber beams and posts supporting the interior structure.
These buildings were up to 9 storeys tall, some with very large floor spaces. Examples can be found
throughout Canada, including Toronto, Kitchener, Ottawa, Montreal, and Vancouver.
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The Leckie Building (Figure 40), built in Vancouver in 1908, is a six-storey example of this type of
structure. Toronto has a minimum of 19 tall wood buildings that are 7 or 8 stories tall, and Vancouver has
6 of these buildings that are 7 to 9 stories tall. Tall brick and beam buildings in both cities reach a
maximum height of 30m (Koo, 2013).
Figure 40 Leckie Building in Vancouver, BC
The Eslov building in Sweden is a 10-storey post and beam warehouse that was built in 1918 (Figure 41).
This building was a granary until 1980, when it was renovated and converted to residential. Of the 9 wood
warehouses built in Sweden during World War One, 3 are still preserved today. The Eslov building, at
31m high, is Sweden’s tallest wood residential building (Bengtsson, 2012).
Figure 41 The Eslov Building in Sweden Built in 1918 (Source: to be determined)
4.1.2.2
Modern Case Studies
Modern examples of mid-rise and tall wood or wood hybrid buildings are showing up all over the world.
Europe, North America, New Zealand, Australia, and Asia are starting to build tall timber structures, as
the advantages of building with wood are understood and the engineering and architectural challenges are
met. Several case studies are explored in the following section, including light framed mid-rises, mass
timber structures, hybrid timber structures, and timber on concrete podiums.
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4.1.2.2.1 Light Framing
Over the last century, many countries, including Canada, limited the height of wood frame construction to
3 or 4 stories. In 2009, the province of British Columbia revised the building code limit for residential
buildings from 4 to 6 stories. Around this time, a full scale mid-rise wood frame building was tested on
the Miki, Japan shake table under seismic loading, and the performance was satisfactory. The Association
of Professional Engineers and Geoscientists of British Columbia (APEGBC) released a technical practice
bulletin on 5- and 6- storey timber structures. This bulletin covers aspects of structural and seismic
design, fire safety, and building envelope. (APEGBC, 2009).
In May 2013, the same exercise took place in the province of Québec, when the Wood Charter was
announced. Created to grow the wood construction industry in Québec, the Charter supports the use of
wood in publicly financed projects and allows for the construction of 5- and 6-storey wooden residential
buildings that meet the requirements of the Régie du Bâtiment du Québec (RBQ) using design guidelines
produced by FPInnovations experts. This move positioned Québec alongside British Columbia as a North
American leader in the use of wood as a structural building material.
In 2009 in Seattle, Washington, a 5-storey plus mezzanine timber structure was built on top of a two
storey concrete podium. Marselle, a residential building shown in Figure 42, was cited as an example in
the 2009 change to the British Columbia Building Code for the techniques used in its construction. The
wood-framed building was estimated to cost 30% less than a similar concrete building, and was erected
much faster than either a steel or concrete building would have been. Marselle reaches 26m high at the
mezzanine and is fully sprinklered. Walls, plates, and some joists were built with dimensional lumber,
and engineered wood products were also used including wood I-joists, parallel strand lumber (PSL) and
laminated veneer lumber (LVL) beams. Wall panels were prefabricated off site to save time and because
of limited space. A continuous rod tie-down system with a device to compensate for shrinkage was used
to limit deflection and avoid wall separation (WoodWorks, 2011).
Figure 42 Marselle in Seattle, Washington (courtesy of Matt Todd Photography)
4.1.2.2.2 Tall Timber Structures with Mass Timber
Showcased in this category are structures that use wood as the primary structural material above the
foundation. In each case study, mass timber panel elements from Structural Composite Lumber (SCL) or
CLT are used to provide strength, stiffness, and better fire performance to the structure.
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A bell tower built for the Myers Memorial United Methodist Church in Gastonia, North Carolina in 2010
was the first non-residential structure to be built out of CLT in the United States (Figure 43). The tower
reaches 23.8m in height and the structure is entirely built from CLT panels with the exception of a 1meter high concrete foundation at the base (WoodWorks, 2010a).
Figure 43 Gastonia Bell Tower during Construction (courtesy of WoodWorks)
Bridport is an 8-storey residential building in the London borough of Hackney, and it was completed in
2011 (Figure 44). With the exception of the foundation, the structure of this platform-framed building is
built entirely out of CLT, including the elevator shaft. The CLT structure was designed in part to
accommodate a large Victorian storm sewer that was found underneath the site. A heavier reinforced
concrete structure would have required more extensive foundations to bridge the sewer. Bridport was
erected in only 12 weeks on site, less than the typical construction time using traditional building
materials (Willmott Dixon, 2011), (Lehmann, 2012).
Figure 44 Bridport House (Lehmann, 2012)
A German company, TimberTower, designed and built a 100m tall timber wind turbine in Germany in
2012 (Figure 45). The octagonal structure consists of a timber post and beam falsework skeleton with
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CLT panels as the skin and the support structure. The tower is supported on a concrete base that is 21m in
diameter. The tower diameter is about 7m at the base and 2.4m at the top. CLT panels were bonded
together with steel connectors slotted into panels and secured with adhesive (HSK system). The Timber
Tower is among the tallest modern timber structures worldwide, and can provide valuable insight into the
dynamic behaviour of tall wood structures (Schäfer, 2012).
Figure 45 Timber Tower (courtesy of TimberTower GmbH)
In Via Cenni, Milan, Italy, a social housing project was built in 2012 that included four 27m tall 9-storey
timber towers (Figure 46). In each building, a concrete basement and foundation supports 9 storeys of
timber structure, with CLT panels providing the floors, walls, and stair and elevator cores. CLT partition
walls are included as part of the structural system (Wood Solutions Auckland, n.d.).
Figure 46 Via Cenni 9 Storey Buildings (courtesy of Prof. Arch. Fabrizio Rossi Prodi)
In Norway in 2013, the Studentenwohnheim (student dorm) was built, a two building student housing
complex in the University of Life Sciences in the municipality of Ås (Figure 47). These are 8-storey
timber buildings with CLT as the primary structural material for gravity and lateral load resisting systems,
and glulam used for the cantilevered balconies. Wood was chosen for a lower environmental impact and
for rapid construction time. Connections were primarily achieved with specialty self-tapping wood screws
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and steel plates, which were used to anchor the building to the foundation to resist the high overturning
loads (Hausegger, 2013a).
Figure 47 The Studentenwohnheim in Norway (Copyright Raimund Baumgartner GmbH
(Hausegger, 2013a))
The Wood Innovation and Design Centre (WIDC) is a 6-storey building in Prince George, BC, that will
reach 30m high, which will make it one of the tallest modern wood buildings worldwide (Figure 48). This
building, currently under construction, contains glulam columns and beams, and CLT floors and
shearwalls. Additional elements include SCL products: laminated veneer lumber (LVL) wind columns
supporting an LVL canopy, and parallel strand lumber (PSL) transfer beams at the second level.
Figure 48 Rendering of WIDC (courtesy of MGA)
Shiang-Yang Woodtek is an office building under construction in Taiwan (Figure 49). This building is 5storeys high and reaches 20m above the concrete base at the high roof. It is built with CLT as the primary
floor and wall structure, and is supported on a concrete base. Shiang-Yang is in a high seismic zone, so a
dynamic analysis was undertaken to determine demands on this CLT structure. Steel hold-downs and
straps are used with self-tapping screws to secure panels between storeys, and self-tapping screws at step
joints provide panel-to-panel shear connections. At high bearing points, CLT panels are locally reinforced
with self-tapping screws for improved shear and tension perpendicular to grain capacities. The use of
self-tapping screws in this manner is discussed in Section 4.2.
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Figure 49 Model of Shiang-Yang Woodtek Office Building (courtesy of Equilibrium Consulting)
District 03 is a 4-storey and 6-storey CLT residential complex that is currently under construction in
Québec City. It is being built with a mix of glulam beams, glulam columns, and CLT panel elements at
the floors and some walls (Figure 50).
Figure 50 District 03 residential building, Québec City (Source: to be determined)
4.1.2.2.3 Hybrid Structures
Many engineering challenges in tall timber construction can be overcome by combining different
structural materials in hybrid buildings. Ranging from a timber diaphragm in an otherwise steel building,
to timber buildings with a concrete core for lateral resistance and egress routes, hybrid structures are
designed to take advantage of the best properties of each material. In a timber-concrete hybrid, the timber
is lightweight and excellent in compression parallel to grain, while the reinforced concrete can provide
lateral anchorage to the foundation and mass to resist overturning due to wind loading. Concrete can also
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provide a non-combustible core which may be required by some building codes. Steel can be
advantageous when used with timber for its high tensile capacity. The following case studies explore a
range of possibilities for hybrid timber structures.
Scotia Place, a 12-storey studio apartment building in Auckland, New Zealand that was built in 2000, is
an early example of a hybrid timber-steel structure (Figure 51). The building is pile-supported with a
concrete basement, concrete transfer slab at the ground level, and structural steel superstructure with
timber diaphragms. The diaphragms consist of glued 30mm by 65mm laminates prefabricated into
1200mm wide planks. The floor spans in one direction only with steel beams supporting the ends of the
planks. The planks are secured to beams with wood screws, transferring the lateral load into the steel
beams. Wood diaphragms reduced the building weight enough that wind governed the design of the
lateral system rather than seismic forces which would have otherwise governed. A nonlinear analysis of
the behaviour of the floor system demonstrated that the diaphragm was rigid relative to the building.
Additional damping was added to the system via elastomeric sealants in order to reduce floor
accelerations from wind loading (Moore, 2000).
Figure 51 Scotia Place (Moore, 2000)
Designed under the 2005 NBCC “alternative solutions” clause, a six-storey Québec City hybrid timberconcrete building reaching a height of 22m was built in 2010 (Figure 52). The gravity system consists of
continuous glulam posts and beams (balloon-frame construction), while the lateral system is made up of
89mm deep glulam diaphragms, concrete cores, and concrete shearwalls. This building was designed with
a ductility factor, Rd of 1.5 and an overstrength factor, Ro of 1.3. With a building mass at just over half of
the mass of a similar building in concrete, vibration control under wind was the limiting factor in the
design. Dowelled connections such as threaded bolts, nails, and self-drilling-screws were used, and were
generally concealed for fire resistance purposes. In addition, this building has sprinklers in case of fire.
The building has been instrumented to monitor any differential movement between the glulam and the
concrete elements (Gagnon & Rivest, n.d.)
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Figure 52 6-Storey Hybrid Québec City Building
An eight-storey timber hybrid building was built in Germany in 2011 in the town of Bad Aibling (Figure
53). This building is a mixed use office and residential tower with non-bearing partitions designed to be
easily moveable to allow for flexibility in the floor plan. The primary structure is CLT floors and walls,
with a concrete stairwell to meet fire regulations.
Figure 53 8 Storey Timber Building in Bad Aibling (courtesy of Woodworks)
The Earth Sciences Building (ESB) at the University of British Columbia is a 5-storey building built in
2012 with a timber academic wing and a concrete laboratory wing (Figure 54). In addition to the heavy
timber post and beam style construction, this building exhibits many innovative timber elements that can
be expanded to tall timber design. The floors are wood-concrete composite, with 89mm thick laminated
strand lumber (LSL) panels topped with insulation and 100mm of reinforced concrete. The wood and
concrete are secured together with Holz-Beton-Verbund (HBV) perforated steel connectors slotted into
the wood panels and cast into the concrete. The roof was built using the first CLT panels produced in
British Columbia. Glulam chevron braces are included as part of the seismic force resisting system, with
ductile connections by means of steel knife plates and pins. The second floor contains a full storey steel
and glulam hybrid transfer truss spanning over the first floor’s lecture theatres. The “free floating”
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staircase in the main atrium is a timber-concrete-composite system that relies on glued-in steel HSK
connectors for rigid moment connections (Canadian Wood Council, n.d.).
Figure 54 UBC Earth Sciences Building (courtesy of Equilibrium Consulting)
Lifecycle Tower One (LCT One) is an 8-storey timber-concrete hybrid building erected in 2012 (Figure
55). The Austrian firm CREE GmbH developed a unique concept that can be expanded to wood buildings
up to 30 storeys tall, with LCT One as the prototype. The floor slab is a timber-concrete-composite that
can span up to 9m for office spaces, which permits flexibility in building use. The slab and glulam
columns were assembled off-site into floor panels that included the windows and insulation. The premanufacturing of the panels took place while the concrete foundation and concrete core were cast. Once
the panels arrived on site the building was rapidly erected, with as much as one storey erected each day.
The floor slab allowed mechanical, electrical, and plumbing services to run between glulam beams,
avoiding the need for dropped ceilings. The glulam columns and beams were oversized to account for the
charring rate (see Chapter 5 for guidance on this), and fire tests were performed on the panel systems.
Though it was technically possible to design the core to be timber instead of concrete, the latter was
chosen to expedite permitting. (Zangerl & Tahan, 2012).
Figure 55 LCT One Tower by CREE (courtesy of CREE by Rhomberg)
CREE’s Illwerke Zentrum Montafon (IZM) is another prototype of CREE’s hybrid timber-concrete
building system (Figure 56). With slightly less than 10,000m2 of floor space, IZM is among the biggest
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timber office buildings in Europe. IZM is 5 storeys high with a total height of 21m, and the base
dimensions are 16m by 120m long. This building was completed in 2013 and uses CREE’s modular
construction system.
Figure 56 CREE's IZM Building in Austria (courtesy of CREE by Rhomberg)
Built in 2013, the Pyramidenkogal (lookout tower) in Carinthia, Austria, is a hybrid steel and timber
structure that is the tallest of its kind in the world (Figure 57). The highest lookout platform is 71m above
the ground (approximately 20 storeys high), with an antenna reaching 100m high. The curved glulam
columns provide vertical and lateral support, with maximum individual glulam column lengths of 27m.
The columns are supported in their weak direction by steel HSS rectangular tubes forming elliptical steel
rings and diagonal struts. Adhesive is used to bond the steel to the wood and to seal the joint. The
structure supports a glass elevator as well as a coiled slide with an incline of 25 degrees that can reach
speeds of up to 30 km per hour (Hauseggar, 2013b) (Pyramidenkogel, 2013).
Figure 57 Carinthia Lookout Tower, Austria (courtesy of Marcus Fischer, Rubner Holzbau GmbH)
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4.1.2.2.4 Structures on a Single Storey Concrete Podium
Timber structures on a concrete podium are a common occurrence in timber construction. A concrete first
storey can open up the bottom level for storefronts and larger, open lobbies, and can be required to meet
occupancy separation and fire code requirements. Particularly in the case of residential buildings, which
can have many structural partition walls, the concrete second floor provides a transfer slab to the more
widely spaced structural supports below. The timber structure supported by a concrete podium can be
light-framing, hybrid, or mass timber construction.
Encouraged by a national policy promoting the use of wood countrywide, the city of Växjö in Sweden
developed the project “Välle Broar,” which was a sector of the city devoted entirely to wood. As part of
this project, four 8-storey buildings were built, called the Limnologen project (Figure 58). The project
was started in 2006 and construction was completed in 2008. In each building, the first floor is concrete
and there are 7 timber storeys above. With the exception of the concrete first storey, the vertical and
lateral load carrying systems are made up of glulam beams and CLT panels. The floor is a composite
CLT-glulam system (Figure 58, right) in which CLT slabs make up the top flange, glulam beams on edge
provide the web, and additional glulam beams on flat make up the bottom flange. Components are glued
and screwed together. CLT wall panels are connected to one another with plywood strips screwed to each
panel. With low seismic loads, wind forces governed the lateral design of the system. The lateral load
resisting system consists of CLT panels, both interior partition walls and exterior load bearing walls,
secured to the foundation with steel rods that extended the entire height of the building (Gagnon & Hu,
2007).
Figure 58 One of the 8 storey Limnologen buildings in construction
The Stadhaus building, also known as Murray Grove, is a 9-storey residential building that was built in
London in 2009 (Figure 59). The entire building is platform framed CLT with the exception of the ground
floor which is reinforced concrete. The dividing walls, core walls, and floors are CLT. Both vertical and
lateral load resisting systems are provided by the CLT walls, floors, and core. Including the partition
walls in the structural system created an egg crate structure which provides additional redundancy
necessary for robustness. To accommodate changes during procurement, the foundation is reinforced
concrete bored cast-in-place piles designed for the full weight of a concrete 9-storey building.
Connections between panels include self-tapping screws and wood screws in combination with steel
brackets or plates. Due to timber’s low strength perpendicular to grain, nails or screws were used to
locally reinforce the panels at highly loaded areas. The designers expect that the long term movement due
to creep and shrinkage of the timber will be minimal (Techniker, 2010).
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Figure 59 Stadhaus Building (Lehmann, 2012)
Forté is a 10-storey residential building with 9 storeys of timber construction on a single storey concrete
podium (Figure 60). The building was completed in 2012. Forté was the first CLT building in Australia,
and the CLT structure allowed for very rapid construction time and reduced building weight. Cost savings
were achieved with smaller foundations and less time spent in construction, given that construction was
completed in 8 months (Lehmann, 2012).
Figure 60 10-Storey Forté Building (courtesy of Lend Lease)
The Bullitt Center in Seattle, Washington, is a 6 storey hybrid timber steel building with 4 storeys of
timber and steel structure supported on two storeys of concrete (Figure 61). This building was designed to
meet the very stringent energy requirements of the Living Building Challenge, and 100% of the wood was
certified to Forest Stewardship Council standards. Its gravity frame is primarily heavy timber with
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Douglas-fir glulam beams and columns, and the floors are nailed edge laminated 2x4’s with plywood
sheathing and concrete topping. The lateral load resisting system consists of steel cross braces. To avoid
the cumulative effects of shrinkage at each floor, columns at each level are separated by a steel HSS post
which the floor beams connect into, in a modified platform framing approach (Figure 61, middle). Within
this detail, any perpendicular-to-grain (or cross-grain) shrinkage of the floor beams will not affect the
overall building shrinkage. Steel bucket assemblies were also used with ¼” self-drilling screws (SDS) for
rapid erection. The building was completed in 2013 (WoodWorks, 2013).
Figure 61 Bullitt Center (left, middle: courtesy of John Stamets, right: courtesy of Ben Benschneider)
4.1.3
Conceptual Structural Systems
Many different systems are possible for mid- and high-rise timber structures, as can be seen by the many
configurations that have already been built. Additional systems have been proposed and studied but have
not yet been implemented. Below is a summary of structural materials used in modern timber mid- and
high-rise construction, as well as some of the systems that have been proposed for tall timber building
design.
4.1.3.1
Structural Materials
To create tall timber structures in the early 1900’s, designers relied upon solid timber members cut from
large trees for post and beam construction. Low-rise timber structures were built in the last century using
light frame construction, with walls and floors built of boards or wood-based panels and smaller
dimensional lumber. Today, light framing is still the most economical solution for low-rise and some
mid-rise buildings, but for tall timber structures light framing is limiting, and very large and long sawn
timber elements are no longer easily available. Solid timber construction using engineered wood products,
composites, and hybrid systems are preferred for tall wood buildings. Most of these timber products make
efficient use of the commercial wood resource, by laminating small dimensional lumber, strands, or plies
of wood into larger and longer wood members. Because the wood components are smaller and need to be
glued, they start with a lower initial moisture content, which helps to minimize shrinkage and cracking
due to drying.
Glued-laminated timber, or glulam, is an engineered wood product that was first introduced in the 1900’s,
and is manufactured by gluing together smaller laminates to form a larger and ultimately more efficient
structural element (Figure 62). Typically, 38mm thick plies are glued and pressed together to form simple
structural beams and columns, or more complex arches and members curved along two axes. The strength
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of the section can be engineered based on the grade of each laminate and where it is placed along the
member depth (WoodWorks, n.d.).
Another category of proprietary engineered wood products are called Structural Composite Lumber
(SCL). There are several general types of SCL. While they may appear different on the surface, they all
have design properties that are available from the manufacturers.
Figure 62 Glulam beam (left), Glulam Columns in Prince George Airport Expansion (right, courtesy
of MGA)
Parallel Strand Lumber (PSL) is a class of SCL that uses strands of high quality veneers taken from small
trees and bonded together under pressure (Figure 63). PSL is made with a moisture content of 11% or
less, and is less prone than other wood products to shrinkage effects such as warping, cupping, bowing, or
splitting. The shreds of veneer combined with the adhesive are extruded into billets that can be used for
beam, column, and truss type structural members (Canadian Wood Council, 2005).
Figure 63 Parallel Strand Lumber Beam
Solid wood panels can form part of a building’s gravity and lateral load resisting systems. Mass timber
panels are becoming a popular material choice for their relatively light weight, high strength and stiffness,
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aesthetic appeal, great fire performance, and high level of prefabrication. Panel elements are typically prefabricated with CNC (Computer Numerical Control) machines, so assembly on site is simple and
efficient. Panels can be used in composite with concrete, steel, or on their own.
Mass timber panels can be made in a number of ways. Gluing, nailing, or dowelling layers of laminates
together produces different types of laminated timber products. Solid panels can also be produced by edge
laminating plies with glue, nails, or dowels. Finally, panels can be produced with SCL instead of
laminations of dimensional lumber.
Cross laminated timber (CLT) is an extension of the same concept as glulam to produce panel type
elements that are used as slabs, walls, beams, and columns (Figure 64). CLT was first developed in
Austria and Germany in the mid 1990’s. CLT panels consist of 3 or more layers of boards with the grain
of adjacent layers at right angles to one another (layers are stacked crosswise). Cross lamination improves
dimensional stability of the panels, reducing shrinkage that frequently occurs with timber elements. The
combined bearing perpendicular to grain of CLT is 30 to 50% higher than that of dimensional lumber
(Gagnon & Pirvu, 2011). Glued CLT panels are now available from Canadian suppliers in sizes up to
400mm thick, 3m wide and 18m long.
Figure 64 CLT Panels (left), CLT framing of UBC Okanagan Wellness Centre (right) (Photos
courtesy of McFarland Marceau Architects)
Laminated Veneer Lumber (LVL) is a class of SCL that is made of several layers of wood veneers glued
together, is a similar manner to plywood (Figure 65). Unlike plywood, the grain of the veneers are usually
aligned in the same direction, rather than alternating directions. Parallel laminations improve the
predictability and performance of the material compared to cross laminations. With the grain running in
the long direction, LVL is strong when edge loaded and face loaded, making it ideal for slabs, beams, and
walls. LVL is initially fabricated into large billets and then cut into members (Canadian Wood Council,
2005). Alternatively, crosswise LVL is also available (X-LVL). Unlike typical LVL, X-LVL is
manufactured with some veneers placed crosswise to improve the behavior and stability in two directions.
This product can be used as large panels for tilt-up construction or as large panels for cladding and
roofing.
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Figure 65 Laminated Veneer Lumber
Laminated Strand Lumber (LSL), another class of SCL, allows much more of a log to be used than the
other methods requiring bigger laminates or solid members (Figure 66). Wood from fast growing and
small trees that otherwise would not be usable for structural materials is cut into thin strands that are
oriented parallel to the length, mixed with adhesive, and pressed into mats using a steam injection press.
LSL and LVL are typically produced in Canada in billets up to 89mm thick, 2.44m wide and 19.5m long
(Canadian Wood Council, 2005).
Figure 66 Laminated Strand Lumber
CLT, LSL, and LVL can all be used to create structural wood panels that can be integral parts of tall
wood structures. However the material differences will impact the detailing of the design. Bearing
capacity of panel ends will be lower for CLT panels than for LVL or LSL because CLT panels have plies
that will have their grain perpendicular to the direction of load. LVL and LSL currently have higher
published shear capacities than lumber; however CLT panels can be produced in thicker panels with the
addition of plies (Green, 2012).
Table 1 compares different timber products and their design properties. The table shows a sample of each
product, though the engineered wood products have a range of capacities depending on the grade and the
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supplier. The LVL, LSL, and PSL capacities are from a Canadian producer and listed below are the
published Canadian limit states values. CLT may use visual grade and machine stress-rated dimensional
lumber, and under axial loading some plies will be aligned with the load and some plies will be
perpendicular to load.
Table 1
Comparison of Different Wood Products
Compr. Stress
Parallel to
Grain, fc
(MPa)
Compr. Stress
Perpendicular
to Grain, fcp
(MPa)
Longitudinal
Shear, fv
(MPa)
Bending at
Extreme Fibre,
fb (MPa)
Dim Lumber
No.1/No.2
stud, SPF
Dim Lumber
SS, D.Fir.
Glulam
Douglas Fir
24f-E
LVL
2.0E
LSL
1.55E
PSL D.Fir
2.2E
CLT Wall
5-Ply
11.5
19.0
30.2
35.2
22.5
31.9
11.6 (long)
1
5.4 (trans)
5.3
7.0
7
9.4 (beam)
6.9 (plank)
10.0 (beam)
6.1 (plank)
9.4 (beam)
6.0 (plank)
5.3
1.5
1.9
2
3.7 (beam)
1.8 (plank)
5.2 (beam)
2.0 (plank)
3.7 (beam)
2.7 (plank)
2.2
11.8
16.5
30.6
37.6 (beam)
37.6 (plank)
29.6 (beam)
33.3 (plank)
37.0 (beam)
35.7 (plank)
N/A
2
3
4
1
The longitudinal plies are assumed to have fc of 19.3MPa (MSR lumber), and the transverse plies are assumed to have fc of
9.0MPa (No.3 / stud). For longitudinal stress, only the longitudinal plies are expected to contribute, and vice versa, so the overall
compressive capacity is reduced based on the number of plies in the direction of loading.
2
Out of plane
3
Based on a shear capacity of 5.5MPa resisted by only the longitudinal layers
4
The plies have different fb depending on the orientation, so the wall bending capacity must be calculated as a composite section
Table 2 compares timber, concrete, and steel properties. The materials are compared on the basis of yield
stress and modulus of elasticity alone, which can be misleading. For instance, in compression, steel will
rarely meet its yield stress and will most likely fail in buckling. Its relatively high strength to weight ratio
is valid in tension but not in compression. Thus, when compared to timber, steel can provide higher
tensile capacity relative to its weight, and timber may be advantageous in compression. Conversely,
concrete and timber have similar compressive capacities if the wood is loaded parallel to grain. However,
if the timber needs to be loaded perpendicular to grain, concrete can be advantageous. Using these
fundamental material principles, hybrid building systems and composites can be developed to make the
best use of each material.
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Table 2
Comparison of Different Materials
Compr. Stress
Parallel to Grain
(MPa)
Tension Stress
Parallel to Grain
(MPa)
Modulus of Elasticity
(MPa)
Density
(kg/m3)
Compr. Strength to
Weight Ratio
4.1.3.2
Dim Lumber No.1/No.2
stud, SPF
Glulam Douglas Fir
24f-E
Concrete
f’c=45MPa
Steel
fy=350MPa
11.5
30.2
45
350 (though stability will
likely govern)
5.5
20.4
4
(rebar carries tension)
350
9,500
12,800
31,000
200,000
420
490
2,400
7,850
0.027
0.062
0.019
0.045
(tensile strength to weight
ratio)
Gravity Load Systems
Timber’s high compressive strength-to-weight ratio renders it an excellent choice for load-bearing
columns and walls. However, long term shortening due to shrinkage and creep cannot be ignored, and in
particular differential shortening should be considered. In light-framed structures with platform framing
and solid sawn lumber joists, shrinkage due to moisture loss can be fairly significant, as much as 20mm
each storey (APEGBC, 2009). Initial solid sawn lumber moisture content is typically from 15 to 19%, and
stabilizes at 6 to 10%. Engineered wood products tend to use drier wood, with a moisture content of 6 to
10%, resulting in less shrinkage. CLT panels will experience slightly higher shrinkage than LVL or LSL
panels due to the solid wood elements contained within, however the cross lamination process stabilizes
the panels and limits in-plane shrinkage of walls. Platform framing, common among the taller timber
structures that have been built to date, can cause accumulative shrinkage over the building height due to
shrinkage perpendicular to grain of the floor and wall plate elements. Conversely structures with balloon
framing, where the columns and walls are not interrupted by floors, experience very little shrinkage over
the building height. Green and Karsh’s FFTT system is expected to have shrinkage on par with shrinkage
due to creep in a concrete building (Green, 2012). Shrinkage is further discussed in Section 0 of this
Guide.
Although long-term creep in perpendicular to grain under sustained loads is not yet fully understood so
that it can be predicted accurately, differential vertical movement can be managed through proper
detailing, use of materials, and sequencing of construction. This is particularly important in tall wood
buildings. Similar challenges arise when addressing the differential shortening between concrete and
timber or steel and timber in hybrid buildings. Some new buildings, such as the Québec City 6-storey
hybrid, have been instrumented to learn about the differential shortening that takes place after
construction and to monitor shrinkage. In Sweden, one of the Limnologen 8-storey buildings was
instrumented for relative displacement of each storey. Deformations were in line with expected creep
deformation in the first year, and reduced significantly after that. The Limnologen buildings were
platform framed (Figure 67). The total shortening that was measured was 18mm over the height of the
building, with most of the shortening coming from the CLT floor slabs (Serrano, 2009). Creep is
discussed further in Section 0 of this Guide.
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Figure 67 Limnologen Building Vertical Deformation (left) and Platform Framing (right) (Serrano,
2009)
The 6-storey building that was built in Québec City uses balloon-type construction in which vertical
column elements are stacked to bear on each other without intermediate wood elements (i.e. beams). In
other words, the beams were not placed on top of posts, but rather they attached to the side of posts to
avoid excessive deformations due to the cumulative effects of shrinkage of the beams in the direction
perpendicular to grain. Therefore, the dominant shrinkage in the building is shrinkage in the direction
parallel to the grain. Because timber elements often arrive to a site at higher moisture content than what
they would typically experience in-service, the structure was instrumented using potentiometers,
temperature (T), and relative humidity (RH) probes and devices to monitor differential movement
between the wood structure and the concrete core. Monitoring data from the various potentiometers and
RH and T probes installed at the different locations within the building collected over approximately 18
months of continuous monitoring were analyzed. Analysis of collected monitoring data at one particular
location indicated that the maximum cumulative movement recorded over a year was in the order of
9mm. This movement includes movement due to shrinkage, joint gaps and other construction tolerances.
Results also showed that overall movement had stabilized after 8 months of construction and 4 months of
occupancy. Moreover, most of the movement took place during the first 4 months which corresponded
with the various snow falls and the accumulation of snow on the roof. Following that period, the building
was occupied, which added more live and dead loads to the building resulting in more movement
recorded. Some localized changes in the recorded movement were evident but they were mostly
associated with causal events such as snow falls and mobile equipment used during the construction
period. Building instrumentation and monitoring is further discussed in Chapter 9 of this Guide.
Concrete topping is typically not connected to the wood floor structure in a conventional building. When
connected, the composite action between the concrete and timber layers can be highly effective. Timberconcrete-composite (TCC) typically refers to a floor system in which a reinforced concrete layer is
connected to timber joists or a timber slab with shear connectors. With sufficient shear connection
between layers, the concrete and timber will work together to resist bending stresses and deflections,
resulting in an efficient system that can span longer than timber alone, and is lighter in weight than
concrete alone. In addition to the engineering advantage, construction can be simplified because the wood
can provide the concrete form. The 50-100mm thick reinforced concrete layer improves insulation,
reduces vibrations improving acoustic separation, and provides the diaphragm. Insulating material can be
included between the wood and the concrete. Vibrations and acoustics are further discussed in Section 0.
In TCC systems, the shear connection between the timber elements and the concrete topping is critical.
The connection must be stiff, strong, and should load the wood in compression parallel to the grain and
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avoid any compression in the wood perpendicular to the grain. These connections can be mechanical,
such as screws, or shear keys, or rely on adhesives in the case of glued-in connectors. Combining a TCC
system with timber post-tensioning could further increase spans, improving building layout flexibility
(Crews, John, Gerber, Buchanan, Smith, & Pampanin, 2010).
An example of TCC can be found in the UBC Earth Sciences Building. 89mm thick LSL panels were
topped with foamed board insulation and 100mm of reinforced concrete (Figure 68). The shear
connection between LSL and concrete was provided by proprietary Holz-Beton-Verbund, or HBV
connectors. The connectors are perforated steel plates that are epoxied into the timber, pass through the
insulation, and are cast into the concrete. The assembly is 50% lighter than a solid concrete floor,
allowing spans as long as 6.7m.
Figure 68 HBV Connectors for TCC Slab of UBC Earth Sciences Building (courtesy of Equilibrium
Consulting)
4.1.3.3
Lateral Loads and Complete Building Systems
The design of timber high-rise buildings will typically be heavily influenced by the effect of lateral loads
in combination with gravity loads. Be it wind or seismic loading, the lateral stiffness, lateral strength, or
resistance to overturning can be the driving force behind the structural system that is chosen. Replacing
concrete with timber can significantly reduce the mass of a building, reducing the seismic load. Often at
heights greater than about 10 storeys, even in high seismic zones, lateral deflection due to wind will be
the governing load case in a timber structure. A number of building systems have been proposed for tall
wood structures. Among these, Green and Karsh’s FFTT, CREE’s Life Cycle Tower, CEI and Read Jones
Christofferson’s timber concrete hybrid, and Skidmore, Owings, and Merrill’s (SOM) Concrete Jointed
Timber Frame are promising options for timber high rises that have been elaborately explored and will be
discussed further in this section. These are all holistic systems that consider building aesthetics,
construction methods, economy of materials, gravity loads, and lateral loads.
In a comparison of material stiffness, SOM found that for a timber, concrete, and steel column all
designed to resist the same load, concrete experienced the least vertical deformation, and timber and steel
experienced similar vertical deformations. The stiffness of concrete is approximately 3 times that of
timber, meaning to achieve the same stiffness, 2 to 3 times more material is required for timber than for
concrete (Skidmore Owings & Merrill, 2013a). Even with triple the material, the mass of the timber
elements would still be less than that of concrete. Alternatively, for lateral force resisting systems,
advantageous arrangements of shear walls or elements could also increase the system stiffness.
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Mass panel elements like CLT, LVL, and LSL are strong and stiff under lateral loads, while deformation,
drift, and ductility tends to be as a result of connections within the building system. Extensive testing in
Canada as well as other countries is ongoing to study the behaviour of CLT and other panel elements
under seismic loads (Figure 69). Preliminary tests by the FPInnovations found that with standard hold
downs and L-shaped angle brackets at the base, CLT panels demonstrate good hysteretic behaviour. They
recommend preliminary R values of Rd=2.0 and Ro=1.5 for single panel assemblies with standard light
framing anchorage. Additional ductile link beams could further improve the behaviour, and approach the
response of moderately ductile or ductile steel moment frame systems (Gagnon & Pirvu, 2011).
Figure 69 FPInnovations Testing
Some of the timber mid-rise structures that have been built in the last decade rely on CLT panels for the
lateral force resisting system. The Limnologen buildings in Sweden, Stadhaus (Murray Grove) and
Bridport House in London all use CLT panels for shearwalls and do not contain additional concrete or
steel lateral force resisting systems.
Lateral resistance of timber buildings can also be achieved without using panel elements. The UBC Earth
Sciences Building uses glulam chevron braces with ductile steel connections between elements (Figure
70, left). Moment frames in timber can be used with many different kinds of rigid connections. The UBC
Bioenergy Research & Demonstration Facility building uses the Bertsche connection system for moment
connections between timber elements (Figure 70, right).
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Figure 70 Ductile Steel Connection at Chevron Brace (left, photo courtesy of Equilibrium
Consulting), UBC Bioenergy Research & Demonstration Facility Moment Frames (right, photo
courtesy of Don Erhardt)
In addition to pure timber systems, many variations on timber-steel and timber-concrete hybrids are
possible to expand the possibilities for timber structures to meet the aesthetic, health/safety, and
functional requirements.
The most common type of tall timber hybrid structure built to date is a building with a cast-in-place
concrete core that resists the lateral load, with the timber structure carrying the remainder of the gravity
load and diaphragm loads. Many of the advantages of timber construction can be achieved using this
method. In order to avoid platform framing and allow for balloon framing, slip forming the concrete
elements ahead of the floors is preferred. On the LCT One project, for instance, the concrete core was cast
while the panels were being prefabricated. Examples of timber-concrete hybrid buildings include the sixstorey hybrid building in Québec City built in 2010, where the stair and elevator cores were concrete, as
was an additional shear wall, with the rest of the structure built from post and beam style timber (Figure
71). Wind-induced vibrations governed the lateral loads for this building.
Figure 71 Six-Storey Québec City Hybrid Building (courtesy of Nordic Engineered Wood)
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The company CREE has developed a timber-concrete hybrid concept called LifeCycle Tower (LCT) with
potential heights of up to 30 storeys and floor spans of up to 9.4m (Figure 72). LCT involves the use of
glulam beams and columns as the primary building material, with concrete used in prefabricated concretetimber composite deck elements. Connections between the deck and glulam columns are achieved with a
mortise joint, forming a frame. Glulam columns are paired for fire resistance and to create modular
frames to enable rapid erection. All components are prefabricated to support modular installation. LCT
does not contain structural dividing walls in order to allow maximum flexibility in the building space over
its lifetime. The core can be designed with wood panel elements; however, concrete and steel are also
options (Professner & Mathis, 2012), (CREE by Rhomberg, 2012).
Figure 72 CREE’s LCT Panelized System (CREE by Rhomberg, 2012)
CEI Architecture along with Read Jones Christofferson developed a proposal for a 40-storey timberconcrete hybrid building. The proposed building has a footprint of 46m by 27m, a concrete core, and
wood-concrete hybrid floor panels, similar to CREE’s system, that span 9m. Mass timber trusses at the
building perimeter are at every second floor, and span an entire floor, with the top and bottom chords
supporting the hybrid floor panels. Concrete piers are positioned to support the wood trusses which
cantilever past the piers (Figure 73). The concrete provides the lateral load resisting system, while the
timber provides the gravity support at each level (Bevanda, 2012).
Figure 73 CEI's Wood Concrete Hybrid System (Bevanda, 2012)
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Hybrid timber-steel buildings are another option for timber high-rises. Steel elements can be designed to
provide ductility, high tensile capacity, and predictability where required, while timber is used for weight
reduction and high bearing capacity parallel to grain. Steel and timber working together is not new; many
timber connections rely on steel to transfer loads, be it from plates, bolts, screws, etc. Although steeltimber hybrid structures are relatively uncommon in Canada, such systems may provide an excellent
model for tall timber hybrid buildings. Timber-steel hybrid vertical and lateral elements can increase load
carrying capacities without increasing cross sections (Stiemer, 2012) (Khorasani, 2011). Scotia Place in
New Zealand is an example of steel timber hybrid (Figure 74, middle). Linea Nova in Holland was an
existing 4 storey concrete building with 16 storeys of hybrid-wood steel structure that were added on top.
Limited by the soil capacity and the existing structure’s capacity, the high-rise portion uses steel posts and
beams with conventional timber joisted floors (Figure 74, right).
Figure 74 Steel-timber hybrid in Spain (left), Scotia Place in New Zealand (middle), Linea Nova in
Holland (right)
Finding the Forest Through the Trees, or FFTT (Green, 2012), is a timber-steel hybrid concept for highrises up to 30 storeys in height in high seismic regions (Figure 75). FFTT relies on a “strong column –
weak beam” approach to building design. Mass timber panels, such as CLT, LVL, or LSL create the
strong vertical structure, lateral shear walls, and slabs. Ductile steel beams bolted to the mass timber
panels provide a weak link element and deliver ductility to the system. Concrete is required for basements
and foundations, but concrete is not structurally necessary elsewhere in the building. This system’s
framing is based on balloon framing, and is adaptable for use in residential and office buildings. This
system was introduced in the peer reviewed Tall Wood Report that was published in 2012, and has not yet
been used in design.
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Figure 75 30 Storey FFTT Rendering (courtesy of MGA)
The architectural and engineering firm Skidmore, Owings & Merrill (SOM) proposed a new system for a
30-storey timber composite high-rise (Figure 76). SOM published a report in 2013 detailing their
proposed system: Concrete Jointed Timber Frame. This system uses mass timber for the main structural
elements and introduces reinforced concrete at the joints. This system eliminates the cumulative
deformation that can occur due to compression perpendicular to grain in timber (see additional discussion
on this topic in Section 4.2). The result is a system with concrete perimeter beams at each level and
concrete bands at all wall-floor connections. The perimeter beams are reinforced to allow fixity for long
spans, and some are strengthened provide link beams between timber shearwall elements to encourage
coupling. The timber panels are connected to each other and to the concrete link beams with steel rebar
that is either epoxy bonded into the timber or cast into the concrete. Structural steel members are used at
joints during erection and before the concrete bands are in place. Including concrete substructure and
foundations, the structure is approximately 30% concrete and 70% timber by volume. The lateral load
resisting system is provided by solid timber panel (CLT or other) shearwalls around the core and at the
building perimeter, with adjacent walls coupled by concrete link beams. The foundation and first level
above grade is reinforced concrete, with the timber structure starting at level 2. The system that SOM
proposed can be adapted to a variety of building layouts and uses (Skidmore Owings & Merrill, 2013a).
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Figure 76 SOM's Concrete Jointed Timber Frame System (Skidmore Owings & Merrill, 2013b)
Many of the built or proposed lateral force resisting systems for tall timber structures rely on traditional
elements, albeit with non-traditional materials, such as bracing, shearwalls, and moment frames. There are
many ways in which alternative lateral force and lateral deformation resisting systems can be used in tall
wood construction. Recently the research focus of seismic resisting systems has been on highperformance structures that are easily repairable and allow for immediate occupancy after an earthquake
event. Features such as ductile replaceable fuses and self-centering systems are highly advantageous. Life
Safety is required as a minimum, but the costs of repairing or rebuilding a damaged building after an
earthquake can be very high, and a system designed for rapid repairs followed by return to use will make
a big difference on a community after a major seismic event.
Researchers at the University of Canterbury in New Zealand and elsewhere have studied post-tensioned
timber self-centering systems over the last several years. These systems are based on research that has
been done on pre-cast post-tensioned concrete walls. Using post-tensioning with timber can help reduce
the potentially brittle failure of timber connections. Based on traditional moment frames or wall elements,
these systems are unique in the post seismic response and reduction in residual drift. Such a system could
also be used to reduce vibrations due to wind. In Figure 77, post tensioning is used in glulam or LVL
frames, and in Figure 78, LVL shearwalls are linked together with U-shaped steel plates to provide
coupling, energy dissipaters, and sacrificial fuses. The shearwalls also contain post-tensioning elements.
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Using rocking and ductile connections that rely on post-tensioning can minimize damage to the structure
by focusing damage on easily repairable or replaceable components. Further, systems that are selfcentering improve a building’s capacity for immediate occupancy after an earthquake event. This system
has been used for a 3 storey structure, and further analysis has demonstrated that the system can be easily
adapted to buildings up to 6 storeys in height (Holden, Devereux, Haydon, Buchanan, & Pampanin,
2012). Concepts from this system could be adapted to timber high-rise construction.
Figure 77 Timber Frame self centering system (Newcombe, Pampanin, Buchanan, & Palermo, 2008)
Figure 78 Timber Coupled Shearwalls (Holden, Devereux, Haydon, Buchanan, & Pampanin, 2012)
There are many additional innovative lateral force resisting systems that are possible for tall wood and
wood-hybrid buildings. One of the major drawbacks of using fully timber systems is the lack of
experience as to what systems will not only perform well under the anticipated occupancy and
environmental loading, but can be accurately costed and built to the specifications. Although this Guide
provides some of this information to facilitate developing the case for a tall wood building, much can be
achieved by combining wood with non-wood systems in a hybrid system. For example, in order to ensure
predictable behaviour, tall wood buildings can contain hybrid lateral force resisting systems or fully steel
or concrete systems. Advantages of using wood will still be achieved in a hybrid building. Many of the
case studies demonstrate a timber gravity resisting system with concrete cores or shearwalls to resist
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lateral loads; however cast-in-place concrete can interfere with the erection sequence used for timber
construction unless the concrete is cast ahead of the timber construction. An alternative to concrete
hybrids is a timber gravity resisting system with steel elements to resist lateral loads. These steel elements
can include traditional braced frames or moment frames, or more innovative steel systems which include
steel plate shearwalls, buckling restrained braces (BRB), or link beams as in the case of FFTT. Using
steel-timber hybrids or pre-cast concrete-timber hybrids allows more efficient construction sequencing
than cast-in-place concrete.
Recently research has been done on concrete or steel frames in-filled with solid timber (CLT or LVL)
panels. Such a system effectively allows partition walls to add lateral stiffness to an otherwise moment
frame system. In these systems, connections between the infill wall and the frame are of utmost
importance, and can dictate the amount of ductility that can be achieved or determine when the wood
walls contribute to the overall stiffness of the building. The infill wall can be designed to significantly
increase the strength and the stiffness of the moment frame system. Since steel moment frames are
frequently governed by deflection, this increase in stiffness can be highly beneficial (Dickof, Stiemer, &
Tesfamariam, 2012), (Leijten, Jorissen, & Hoenderkamp, 2011).
Other systems that can improve the lateral behaviour of a building can include viscous damping, as used
in Vancouver’s stadium, BC Place, and base isolation, both inspired from concrete and steel research.
These options can be used to reduce floor accelerations, or reduce drift to improve performance under
lateral loads for timber high-rise buildings. These options are further discussed in Section 0 of this Guide.
4.1.3.4
Tall Buildings on a Podium
Many residential structures require a first storey that can accommodate commercial clients, requiring
open spaces, or to meet occupancy separation and fire code requirements. To accommodate transfer
beams and open areas, the lower floor of a tall timber structure can be reinforced concrete or steel. Many
of the case studies already built, and some of the proposed systems for 30 storey high-rises use a
reinforced concrete podium at the first storey. Significant overall building weight reduction can still be
achieved in a podium-type hybrid building.
Additional advantages to podium-style construction are that the reinforced concrete or steel podium can
support the high construction loads or can protect the building from any potential unwanted impact loads
from the streets. A concrete lower level will also be advantageous for building sites on a floodplain.
Here, because soil conditions are likely to be poor, the lower overall building mass may reduce the
foundation costs. In high wind areas or high seismic zones, a concrete podium provides the mass to
which the timber structure can be anchored to resist overturning.
A potential disadvantage in a timber building on a concrete podium, particularly a multi-storey podium, is
that the seismic demand on the timber superstructure may be much higher due to the high weight of the
concrete storeys and lateral force transfer up the building. The connection between the timber and
concrete portions, as well as the relative stiffness between the two, will dictate the amount of force
transfer that takes place.
The transfer between the timber structure and the concrete base, be it a low podium or foundation, can be
challenging. However in the case of the podium, for seismic loads the connection should either be
designed to provide ductility to the system, or be designed according to capacity design principles, to
either meet the capacity of the system above or be designed for an appropriate overstrength factor
(WoodWorks, 2010b). The use of post-installed anchors requires good coordination between trades,
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particularly if the transfer slab contains post-tensioning. Any drilling needs to avoid the rebar and posttensioning in the slab.
4.1.4
Practical Guidelines for Given Heights
Below are sample guidelines for the design of mid-rise to high-rise timber structures. These are
suggestions based on some of the systems that have been built, and/or researched. For buildings under
10 storeys high, the guidelines are based on buildings that have been erected worldwide. Above
10 storeys, the guidelines are based on FFTT concept and on SOM’s prototype Concrete Jointed Timber
Frame (CJTF) concept. Additional systems are possible for buildings taller than 10 storeys: the CEI
concept was proposed for a 40 storey tower, while the CREE system can be adapted for buildings up to 30
storeys tall. Furthermore, with concrete-timber or steel-timber hybrids, buildings can have concrete or
steel lateral systems that have good design and construction information, in addition to timber elements
for the gravity system.
The guidelines below are based on a compilation of the studies that have been done for FFTT and for
CJTF systems (Green, 2012) (Skidmore Owings & Merrill, 2013a), and are presented here to provide an
example of the type of structure possible, and some of the structural considerations for a structure given
its height. The guidelines should be viewed within the context of the rest of the case studies and concepts
presented within this chapter. Additional resources on any one system should be pursued for further
details.
Buildings of 6 storeys or fewer (under 20m high):
•
•
•
•
•
Currently the British Columbia Building code allows wood framed 6 storey residential buildings,
according to the Technical Practice Bulletin published by APEGBC
Québec allows wood framed 6 storey residential buildings following prescriptive directives and
design guidelines developed by FPInnovations
Can be built with light framing (stick framing, prefabricated wood I-joists) or mass timber
framing (glulam post and beam, mass panels, etc.)
The designer should consider the cumulative effects of shrinkage, particularly with light framing
and platform framing
Lateral resistance can be achieved with stud shear walls, mass panels, bracing, moment frames,
etc.
Building between 6 and 10 stories (under 30m high):
•
•
•
•
There are several examples of these worldwide, with aspect ratios from 1 to 3
Overturning due to wind or seismic loads may be an important consideration in design
FFTT prototype: Can be achieved with a structural core and glulam columns around the perimeter
CJTF prototype: Link beams between shearwalls are not required, minimal differential shortening
is expected, minimal uplift due to wind is expected, lateral drift due to wind is minimal.
Buildings 20 stories (under 60m high):
•
•
The foundation should be designed to resist overturning, as overturning will mostly likely govern
the design
Even in seismic zones, vibration due to wind loads will typically govern the design of the lateral
load resisting system
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•
•
FFTT prototype: Can be achieved with a structural core (wood, concrete, or steel) and additional
interior or exterior shear walls that are linked with steel beams. Building stiffness tends to govern
design over ductility
CJTF prototype: Link beams between walls are not strictly required for this height, but they will
reduce uplift due to overturning moment and reduce the building period.
Buildings 30 stories (100m high), aspect ratio over 2:
•
•
•
4.1.5
Extensive testing and analysis may be required (see Sections 4.2, 4.3)
FFTT: Can be achieved with a structural core and additional exterior shear walls and steel link
beams
CJTF: Link beams between walls are required.
Foundations for Tall Wood Buildings
One of the main structural advantages of timber buildings when compared to other materials is reduction
in weight. The weight reduction may translate to reduced foundation costs. However in high seismic
zones or areas with high wind loads, lower mass could mean less resistance to overturning forces, and
overturning can be the limiting factor for the foundation, as was the case for the Scotia Place building. On
the other hand, Bridport House in London had a foundation which needed to bridge an underground storm
sewer, and the lighter CLT building significantly reduced the demand on the foundation. SOM found in
their study that foundations for their prototypical building required 65% of the material as for the concrete
benchmark building. The materials needed would have been 55% if not for high overturning moments due
to wind loads (Skidmore Owings & Merrill, 2013a).
The designer can take certain steps to improve a building’s overturning resistance, either by reducing
uplift forces or improving resistance to uplift. Shearwalls or lateral resisting elements that are closer to the
perimeter of the building will result in a building with a larger moment arm, better able to handle
overturning moment and reduce uplift. Carefully planning the load path in order to maximize the amount
of gravity load supported by the lateral load resisting system will also reduce uplift forces (Skidmore
Owings & Merrill, 2013a). Transferring the tension forces from the timber to the foundation needs to be
considered, and may be efficiently achieved by using steel tension elements at building extremities. Using
tension piles, caissons, or soil anchors will help handle uplift once the load is transferred to the
foundation.
4.1.6
Conclusion
There are many examples of how buildings up to 10 storeys tall have been achieved to date, and there are
many proposed tall wood building system concepts. Further research on timber materials, connections,
and building systems will open up more possibilities to what can be achieved with timber in tall building
construction. The first demonstration buildings will provide valuable information and experience for
future projects and research. As discussed in Chapter 9, it is important to make plans to gather this
information. As more tall wood buildings will be realized, broader spectrum of engineers, architects,
contractors, and building officials will learn more about the systems that work well and how to best
design tall wood buildings.
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4.1.7
References
APEGBC. (2009). APEGBC Technical and Practice Bulletin: Structural, Fire Protection and Building
Envolope Professional Engineering Services for 5 and 6 Storey Wood Frame Residential Building
Projects (Mid-Rise Buildings).
Bengtsson, A. (2012). Lagerhuset, Eslöv. Flickr. Retrieved August
http://www.flickr.com/photos/barracuda666/6730996939/in/photostream/
6,
2013,
from
Bevanda, N. (2012). Wood, Water and Air. Architect Corner August 2012.
Canadian Wood Council. (2005). Wood Design Manual 2005. Ottawa (ON): Canadian Standards
Association.
Canadian Wood Council. (n.d.). Innovating with Wood: A Case Study Showcasing Four Demonstration
Projects.
CREE by Rhomberg. (2012). CREE - The Natural Change in Urban Architecture: LifeCycle Tower.
Crews, K., John, S., Gerber, C., Buchanan, A., Smith, T., & Pampanin, S. (2010). Innovative Engineered
Timber Building Systems for Non-residential Applications, Utilising Timber Concrete Composite
Flooring Capable of Spanning up to 8 to 10m. Melbourne, Australia: Forest & Wood Products
Australia. Retrieved from www.fwpa.com.au
Dickof, C., Stiemer, S. F., & Tesfamariam, S. (2012). Wood-Steel Hybrid Seismic Force Resisting
Systems: Seismic Ductility. World Conference on Timber Engineering. Auckland, New Zealand.
Gagnon, S., & Hu, L. (2007). Trip Report: Sweden, Norway and France. November 1-11, 2007. Quebec,
PQ: Forintek Canada Corp.
Gagnon, S., & Pirvu, C. (2011). CLT Handbook - Canadian Edition. FPInnovations.
Gagnon, S., & Rivest, S. (n.d.). A Case Study of a 6-Storey Hybrid Wood-Concrete Office Building in
Québec. 1st International Structures on Structures and Architecture. Guimarães, Portugal.
Green, M. (2012). Tall Wood: The Case for Tall Wood Buildings.
Hauseggar, G. (2013b). Aussichtsturm Pyramidenkogel Kärnten. pro:Holz. Retrieved June 20, 2013, from
http://www.proholz.at/kommunalbauten/aussichtsturm-pyramidenkogel-kaernten/
Hausegger, G. (2013a). Studentenwohnheim, Ås, Norwegen. pro:Holz. Retrieved July 17, 2013, from
http://www.proholz.at/haeuser/studentenwohnheim-aas-norwegen/
Holden, T., Devereux, C., Haydon, S., Buchanan, A., & Pampanin, S. (2012). Innovative Structural
Design of a Three Storey Post-Tensioned Timber Building. 2012 World Conference on Timber
Engineering, 9, pp. 323-330. Auckland, New Zealand.
Khorasani, Y. (2011). Feasibility Study of Hybrid Wood Steel Structures. University of British Columbia.
Koo, K. (2013). A Study on Historical Tall-Wood Buildings in Toronto and Vancouver. FPInnovations.
Lam, F., He, M., & Yao, C. (2008). Example of Traditional Tall Timber Buildings in China – the
Yingxian Pagoda. Structural Engineering International, 2, 126-129.
Lehmann, S. (2012). Sustainable Construction for Urban Infill Development Using Engineered Massive
Wood Panel Systems. Sustainability, 4(12), 2707-2742.
Leijten, A. J., Jorissen, A. J., & Hoenderkamp, J. C. (2011). Lateral Stiffness of Timber Frames with CLT
Infill Panels. 2011 International Conference in Advances and Trends in Engineering Materials and
their Applications. Montreal, Canada.
Moore, M. (2000). Scotia Place–12 Story Apartment Building. A Case Study of High-Rise Construction
using Wood and Steel. New Zealand Timber Design Journal, 10(1), 5-12.
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Nakahara, K., Hisatoku, T., Nagase, T., & Takahashi, Y. (2000). Earthquake Response of Ancient FiveStory Pagoda Structure of Horyu-Ji Temple in Japan. 12th World Conference on Earthquake
Engineering, (pp. 1-6). New Zealand.
Newcombe, M. P., Pampanin, S., Buchanan, A., & Palermo, A. (2008). Section Analysis and Cyclic
Behavior of Post-Tensioned Jointed Ductile Connections for Multi-Story Timber Buildings. Journal
of Earthquake Engineering, 12(S1), 83-110.
Professner, H., & Mathis, C. (2012). LifeCycle Tower – High-Rise Buildings in Timber. ASCE 2012
Structures Congress, (pp. 1980-1990). Chicago, USA.
Pyramidenkogel. (2013). Kärnten
http://www.pyramidenkogel.info/
Pyramidenkogel.
Retrieved
June
20,
2013,
from
Schäfer, W. (2012). Vorzeigeprojekt in der letzten Phase. 18-23.
Serrano, E. (2009). Documentation of the Limnologen Project. (56).
Skidmore Owings & Merrill. (2013a). Timber Tower Research Project.
Skidmore Owings & Merrill. (2013b). Timber Tower Research Project Sketches.
Stiemer, S. F. (2012). Position Paper on Hybrid (Timber / Steel) Structures. Vancouver, Canada.
Techniker. (2010). Tall Timber Buildings - The Stadthaus, Hoxton, London: Applications of Solid
Timber Construction in Multistorey Buildings.
UNESCO. (n.d.). Urnes Stave
http://whc.unesco.org/en/list/58
Church.
©
UNESCO.
Retrieved
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17,
2013,
from
Willmott Dixon. (2011). Case Study: Bridport House.
Wood Solutions Auckland. (n.d.). New 9 Storey X-Lam Social Housing. Wood Solutions Auckland.
Retrieved June 11, 2013, from http://www.woodsolutions.com.au/Blog/New-9-storey-X-Lam-socialhousing
WoodWorks. (2010a). Bell Tower Earns Distinction of First Non-residential Cross Laminated Timber
Structure in the U .S. Woodworks for Non-Residential Construction.
WoodWorks. (2010b). Case Study: Wood Buildings Aim High, Benefits and Engineering Challenges of
Podium Design.
WoodWorks. (2011). Case Study: Maximizing View & Value with Wood - Marselle’s 5 1/2 Storey
Podium Design Takes Wood to New Levels. Seattle, Washington.
WoodWorks. (2013). Case Study: Bullitt Center.
WoodWorks. (n.d.). Innovative Applications of Engineered Wood.
Zangerl, M., & Tahan, N. (2012). LCT One. Wood Design & Building, pp. 25-28.
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Design Considerations and Input Parameters for Connections and Assemblies
4.2
Lead Author: Kevin Below
Co-Authors: Robert Malczyk, Mohammad Mohammad, Chun Ni, Marjan Popovski
Peer Reviewers: Steven Boyd, Ilana Danzig, Bernhard Gafner, Andrew Harmsworth, Grant Newfield,
Thomas Tannert, Jasmine Wang
Abstract
Detailing that ensures good structural and serviceability performance of the building starts with a good
understanding of the materials, in this case of the wood and wood-based products. With the widespread
use of computer aided design and manufacturing, unique structural systems and connections can be
proposed. Where a history of past performance is lacking, the designer may benefit from undertaking
testing to either develop the needed input data, or carry out some limited testing to validate design
assumptions.
This section deals with the main design considerations and input parameters that need to be determined,
usually from testing, to support analysis and design of tall wood buildings. Certain design considerations,
while important for all wood buildings, will have more impact in tall buildings.
Most of the commercially available design tools to be used in the design of tall wood buildings will likely
be developed using methodologies applicable to concrete and steel structures, where connections have
little influence. Analysis methods which take account of the particular behaviour of connections and
assemblies, as described in Section 4.3, are more appropriate. However, no guidance or standard
procedure has been established for obtaining or developing the input data for the parameters required for
these methods. This section provides guidance as to how to develop the test data needed for the analysis,
how to evaluate the performance of connections and assemblies through laboratory testing, how to
interpret the data available in other jurisdictions and how to use proprietary non-standard connectors.
Certain parameters of new connections and assemblies, and even of some existing elements in the context
of tall buildings, are required for dynamic analysis and will need to be determined. Evaluation and testing
will be necessary to some extent, depending on the amount of information already available. Individual
connections and elements will require more extensive testing than assemblies or systems of wellunderstood elements and connections. There are a multitude of possibilities for connections in wood
structures involving wood, steel and even concrete, and many proprietary systems (not covered in
Canadian standards) have been developed, mostly in other jurisdictions. These connections can be very
useful in producing efficient and economical solutions for design of tall wood buildings.
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4.2.1
Introduction
The National Building Code of Canada (NBCC) requires (with exceptions such as buildings in low
seismic zones), that tall buildings above 20 m high (about 6 storeys) be designed using dynamic analysis.
This is the common practice for tall steel and concrete buildings, and the design of tall wood buildings
will have to satisfy the same requirements.
It is well established that connections in timber structures play an essential role in providing stability and
stiffness to the structure. Consequently, the connections’ performance in terms of strength, stiffness,
ductility and damping may govern the performance of the lateral load resisting system and the overall
performance of a timber building, and this is also critical for tall wood buildings.
Connections in timber structures are also the main source for the ductility and damping required for the
seismic design, and they play a role in controlling the vibrations. In many cases, timber connection details
govern the sizing of the various structural members. The efficient design and fabrication of such
connection systems determine the level of success of tall wooden buildings when compared to other
buildings made of steel or concrete and to a certain extent, the success of wood systems in hybrid
construction.
Generally, connections for mass timber construction need to be:
•
•
•
•
•
•
•
Structurally efficient (with sufficient ductility when needed)
Fire resistant
Aesthetically attractive (when visible)
Of good performance for serviceability (e.g., decouple where needed)
Cost-effective & readily available
Easy to apply or adapt to other locations (e.g. repeatable)
Easy to assemble (with achievable tolerances)
The emergence of structural composite wood products (as summarized in Section 4.1.) and the recent
interest in developing tall and hybrid systems require the development of innovative connection systems
and designs. Although some of these systems been developed over the last 20 years, the lack of broadly
accepted technical information and simple design guidelines are making them difficult to be adopted by
engineers.
Because most of these systems and assemblies are not covered in the Canadian codes and standards, it is
necessary to establish some guidelines for evaluating their performance through laboratory testing so that
they can be incorporated in the development of alternate solutions.
Although NBCC provides objectives and functional statements, it does not specify exact performance
levels for the performance-based design of buildings under various loading conditions. If performancebased design solutions are pursued, the designers should use appropriate performance criteria to satisfy
those objectives based on the available literature such as (ASCE7-10) or other building codes. For seismic
design, NBCC does specify broad objectives and expected performance and those are given in Section
4.3. NBCC Commentary A also has guidelines that should be used for deriving strength and stiffness
properties of new materials. NBCC Commentary A also has guidelines that should be used for deriving
strength and stiffness properties of new materials. Commentary A indicates that the resistances of new
materials should be defined on the basis of a 5% exclusion limit (with the exception of compression
perpendicular to grain which is based on deformation at 1.0 mm) and their material stiffness should be
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defined on the basis of a 50% exclusion limit; where statistical sampling is used, a 75% confidence level
is recommended for the estimate of the exclusion limit. It is recommended that these criteria be used for
determining the design resistance of new products and connections used in tall wood buildings.
This section provides guidance on the various design parameters that need to be considered and general
design considerations. It also provides procedure for deriving and interpreting the various design
properties from experimental data for use as input into the appropriate analysis procedures presented in
Section 0.
4.2.2
Wood-Related Analysis and Design Considerations
Wood as a structural material is in many ways different from other structural materials such as steel,
concrete or masonry. Some of the most important aspects related to unique properties of wood and wood
products are discussed in this section to help designers better understand the behaviour of wood members
and connections in tall wood building applications. However, the focus of this section is to provide
fundamental design considerations that are relevant to tall wood buildings. While certain design aspects
are equally applicable to low- and mid-rise wood buildings, focus is mainly on design issues associated
with tall wood buildings.
4.2.2.1
Mechanical Properties of Wood
Wood density is a general indicator of the strength properties of connections, with denser woods or wood
products usually having higher connector capacities. Wood has different strength properties in different
directions relative to the grain and it is therefore categorized as an anisotropic material. For that reason
the Canadian Standard for Engineering Design in Wood, CSA Standard O86 (CSA, 2009), provides
values of specified strengths for wood species and wood products for parallel- and perpendicular-to-grain
resistance, with different properties for tension, compression and bending. Consequently, when modeling
the local stresses in wood members, strength and stiffness properties have to be entered parallel and
perpendicular-to-grain, one set for tension and another for compression. Proprietary engineered wood
products such as Laminated Veneer Lumber (LVL), Parallel Strand Lumber (PSL), Laminated Strand
Lumber (LSL) and Cross Laminated Timber (CLT) have properties that differ from those of lumber or
heavy timber members. Designers should contact manufacturers to obtain the appropriate properties and
resistances of various wood products. When using wood products that are included in CSA O86, such as
Glued Laminated Timber (Glulam), values for their strength should be taken from the standard.
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Compression
Stress
Parallel to
Grain
Perpendicular
to Grain
Strain
Strain
Brittle Failure
Tension
Stress
Figure 79 Typical Load-deformation relationships for wood parallel and perpendicular to grain
When designing members, connections, assemblies and
The density of wood varies by
structural systems of wood or wood-based products, details
species, and is a general indicator of
that stress the wood in tension perpendicular-to-grain should
the strength of connections.
be avoided as much as possible. Tension perpendicular-tograin strength values are not only low, but because it is a brittle mode of failure (Figure 79)., its value is
quite variable. Consequently, tension perpendicular-to-grain strength design values are generally not
provided except for very specific conditions. Compression parallel-to-grain behaviour of wood is ductile
and tends to have the linear elastic and non-linear part of the response more clearly defined. The failure
mode of a beam in bending can be a combination of compression parallel to grain in the compression
zone, and a brittle failure mode in the tension zone of the member. Some of the other properties of wood
and wood products that affect their strength and stiffness properties are mentioned in continuation of the
section.
4.2.2.2
Size Effect
Some strength properties of wood and wood products depend
In some loading conditions, strength
on the size of the member. This was explained by the weakest
decreases as size increases, because of
link concept (Weibull theory) which states that the strength
the
increased
probability
of
of a member depends solely upon the strength of its weakest
encountering a major defect.
link. Since the probability of encountering major defects
(knots and other imperfections) is greater in a larger volume of wood than in a smaller volume, the
strength decreases as the size of the member increases in some loading applications. The effect of size on
the strength property is taken into account in CSA O86 by the KZ size factors. For example, for glulam,
effects on tension perpendicular to grain are taken into account by the factor KZtp, for bending by factor
KZbg, for compression by KZcg, for bearing by factor KZcp and for shear by incorporating the beam
volume in the calculation of the shear resistance.
Due to larger gravity loads, members with larger cross-sections are to be expected in design of tall wood
buildings. This will be especially the case for columns of structural systems of a spatial frame type, and
less so for the buildings that utilize walls panels such as LVL, LSL or CLT for the gravity or lateral loadresisting system. Size factors for elements made of proprietary engineered wood products should be
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determined based on the information from the manufactures, from CCMC evaluation reports or based on
the available research information. If such information is unavailable, a size factor based on testing and
analysis should be determined, or a conservative estimate should be made based on the available data for
a similar product.
4.2.2.3
Compression Perpendicular to Grain
As wood buildings are built taller, the traditional platform
type of construction adopted in low-rise buildings, where
the post is resting directly on beams and girders, becomes
challenging. Gravity loads accumulate in the lower storeys
and may cause excessive compression perpendicular to
grain at the interface between the posts and the beams or
massive slabs (Figure 80).
Due to the inherent anisotropy,
compression resistance perpendicular
to grain (bearing) in wood and wood
products is much lower than the
compression parallel to the grain.
Also, the overall shortening of the building is much greater, because shrinkage and compression creep are
significantly higher perpendicular to grain than parallel to grain.
The failure mode of compression perpendicular to grain is a generally ductile failure mode and rarely
causes catastrophic failures, except when a deep and relatively narrow member is loaded in the manner
covered by Clause 5.5.7.3 of CSA O86. However, it can be the limiting and governing design aspect in
many structural details related to tall wood buildings. Therefore, designers should develop connection and
interface details to minimize potential compression perpendicular to grain, which could be achieved using
the following approaches:
Good detail
Bad detail
Figure 80 Post to beam connection detail avoiding excessive compression perpendicular to grain due
to gravity loads
1. In the post and beam type of construction, use continuous posts. Beams and girders can be
connected to the posts through metal hangers, concealed plates or other connection systems
attached to the posts. Alternatively, the beam or girder can be designed to have a larger section
profiled to rest partially on the post. Both posts and beams can be profiled to optimize sections
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(e.g., CSN Fondaction Building in Quebec, Figure The failure mode of compression
81). It is critical that the bearing support be sufficient perpendicular to grain is generally a
to transfer the floor load. Solutions for reducing ductile failure mode and rarely can
compression perpendicular-to-grain stresses include cause catastrophic failures. However,
increasing the bearing areas (using larger it can be the limiting and governing
columns/walls), or using steel or other materials in design aspect in many structural
the critical sections (Figure 80).
details related to tall wood buildings.
The connections in some historic tall post and beam wood Different design details are available
buildings in Canada that are 8 and 9 storeys tall have been to mitigate that effect.
seismically upgraded to transfer the load from upper storey
posts to those below without subjecting the timber beams or girders to compression perpendicular-tograin stresses through their depths. Similar concepts have been adopted recently in certain modern midrise buildings in Europe (e.g., 6-storey post and beam building in Berlin).
2. In tall wood buildings with post-to-post connections, it is recommended to have the post-to-post
connection located at some distance from the beam-to-post connection location for simplified
design and ease of assembly. This arrangement is typically used in steel design to avoid complex
connections.
3. For massive construction, both the floor slab and the wall could be profiled along their full width
to fit together like a tongue and grove. This would provide a partial bearing of the floor slab on
the wall while allowing the direct transfer of gravity loads from storeys above to walls below
through end grain and not through the floor slab. An example of that is shown in Figure 81 below.
This solution, while technically feasible, may be expensive to fabricate. There are other
possibilities, such as the use of hardwood plates to spread the load or the use of self-tapping
screws in combination with steel plates. More information is provided in Chapter 5 of the CLT
Handbook (Gagnon & Pirvu, 2011).
Figure 81 Example of continuous posts in mid-rise wood building
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Figure 82 Minimizing compression perpendicular to grain and shrinkage using different strategies in
massive construction (Source: Eurban)
Alternatively,
new
systems
such
as
CREE
(www.creebuildings.com)
and
FFTT
(http://wecbc.smallboxcms.com/database/rte/files/Tall%20Wood.pdf) could be used to ensure an efficient
transfer of gravity loads from upper to lower storeys without subjecting the wood-members of the floor
system to compression perpendicular-to-grain stresses through their depths (Figure 83). More details
about the CREE system can be found in Section 0.
The Timber Tower Research Project presents yet another interesting system called “Concrete Jointed
Timber Frame” which utilizes mass timber, concrete and steel. The system relies primarily on mass
timber for the main structural elements, with supplementary reinforced concrete at the highly stressed
locations of the structure: the connecting joints.
More details can be found at
https://www.som.com/publication/timber-tower-research-project .
(a)
(b)
Figure 83 (a) CREE hybrid concrete-wood system and (b) FFTT system for tall wood buildings
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4.2.2.4
Shrinkage and Swelling
Wood is a hygroscopic material and gains or loses moisture depending on the environmental conditions.
The loss and gain of moisture affects dimensional stability of the wood and causes shrinking or swelling,
respectively. Shrinkage is different in the three principal directions in wood, with the shrinkage amount in
the parallel-to-grain direction being approximately 1/40 of the shrinkage (dimensional change) in the
perpendicular-to-grain orientation. For this reason, shrinkage parallel to grain is usually ignored in most
low-rise wood buildings. For tall buildings, however, it is recommended that both parallel-to-grain and
perpendicular-to-grain dimensional changes be included in the shrinkage calculations. Figure 84 shows
typical shrinkage values of wood in the three orientations, with the shrinkage values expressed as
percentages of the green dimensions. In service, wood never experiences drying from “green” to “ovendry”, therefore the related shrinkage amounts are usually much smaller.
Figure 84 Typical shrinkage values of wood in the three different orientations
Moisture Content (MC) is a measure of how much water is Changes in the moisture content of
present in wood or wood product and is expressed as a wood-based members cause wood to
percentage of the weight of the water in the wood with respect swell and shrink. The cumulative
to the oven-dry weight. For example lumber is generally dried effect of shrinkage becomes very
before being shipped to users and the MC at the time of important in a tall wood building.
surfacing is shown on the grade stamp. The “S-Dry”
(Surfaced Dry) on a North American grade-stamp indicates that the lumber was surfaced at a MC of 19%
or less, and “KD” (Kiln Dried) indicates that the lumber has been kiln dried to a MC of 19% or less
(usually between 13% and 19%). Panel products and other engineered wood products (EWP) are
manufactured at a lower MC. For example, Plywood and Oriented Strand Board (OSB) are usually
produced at a MC of 4% to 8%. The MC for PSL, LSL, and LVL at the time of manufacturing is between
4% and 12%, while it is between 7% and 14% for Glulam and Cross Laminated Timber (CLT). More
information can be found in the CWC publication “Introduction to Wood Design” (CWC 2011).
For wood materials, building movements are primarily related to shrinkage or swelling caused by
moisture loss or gain when the MC is below approximately 30% (species dependent wood fiber saturation
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point, Wood Handbook, 2010). Differential movements due to environmental condition changes and
loads can be accommodated relatively easily in low-rise wood structures. In mid-rise and tall wood
buildings, however, proper detailing to accommodate building movement (whether it is due to
shrinkage/swelling, member deformation, or construction tolerances) is important due to their cumulative
effect over the height of the structure. It should also be noted that in addition to the effect on vertical
movement, shrinkage may contribute to overall lateral drift calculations, but may be mitigated using
shrinkage compensators and materials subject to less shrinkage. For example, in the case of mid-rise
wood-frame buildings, the APEGBC Technical and Practice Bulletin (APEGBC, 2011) provides general
design guidance and recommends the use of engineered wood products and dimension lumber with 12%
moisture content for floor joists to reduce and accommodate differential movement in 5 and 6-storey
wood frame buildings.
Designers should pay special attention to the MC of the wood and wood-based products at the time of
purchase, MC of the product during installation, MC when the building is closed in, and the Equilibrium
Moisture Content (EMC) that the wood and wood-based products will reach in service. The EMC that
solid wood will achieve in service mainly depends on the relative humidity and temperature of the
environment and varies by region and fluctuates throughout the year within a certain range. Due to the
manufacturing processes and adhesives used in EWP, the EMC of EWP will have slightly lower EMC,
even though it may be of the same species as a solid sawn member. Typical EMC values of solid wood
are given in Table 3.
Table 3
Typical EMC for different regions of Canada
Location
West Coast
Prairies
Central
Canada
East Coast
Indoors
Outdoors under cover
Indoors
Indoors
Indoors
Average (%)
10 – 11
15 – 16
6–7
11 – 12
7–8
13 – 14
8–9
14 – 15
Winter (%)
8
18
5
12
5
17
7
19
Summer (%)
12
13
8
10
10
10
10
12
NBCC specifies that the moisture content of wood-based structural members must not exceed 19%.
Design values for wood-based products in CSA O86 are based on an average moisture content of 15% or
less while not exceeding 19%. The shrinkage of a wood member can be estimated using published
shrinkage coefficients of wood (CWC, 2011).
Particular attention should be paid to proper detailing of Connections need to account for
metal connections to avoid splitting of wood members due to shrinkage of connected elements.
dimensional movement. If a connection restrains wood from Differential shrinkage between wood
movement, tension stresses perpendicular to grain may and other materials may affect
develop and cause timber to split. An example of poor a poor elements and overall behaviour.
detail and suggestions for improvement is shown in Figure 85
below. This type of splitting often occurs with treated timber, which generally has high moisture content
compared to non-treated timber. The splitting of the timber at the support decreases the shear strength of
the member.
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Figure 85 Example of poor detailing practice and suggestions for improvement to avoid splitting
(Source CWC Wood Design Manual, CWC 2010)
To mitigate potential shrinkage and swelling and their adverse effects in tall wood building design, the
following practice must be considered.
1. Reducing the initial moisture content of wood and wood products will greatly reduce the
amount of vertical movement that may occur.
2. It is very important to protect wood and wood products from water sources during
construction and in service. Outdoor storage of wood products on the construction site should
be minimized or eliminated. Materials should be delivered just in time for installation in order
to prevent potential wetting. All products should arrive on site wrapped. Plans should be
made in advance to minimize on-site wetting. Wood-based composites and engineered wood
products usually require more attention during storage and handling, as most of them are
manufactured at a low MC with more end grains and other surfaces exposed and more gaps
introduced during manufacturing. Therefore, these products may be more susceptible to
moisture uptake during wetting incidents than lumber or timber. Use of a weather-protected
construction site is highly recommended.
3. Good construction sequencing also plays an important role in reducing wetting and the
resulting shrinkage and other moisture-related issues. Swelling due to moisture from on-site
wet construction such as pouring of a concrete topping should be taken into consideration,
and should be completed at early stages. The use of dry prefabricated wood-based or concrete
elements is recommended. 0 gives additional information regarding prefabrication.
4. Wood products under protected conditions can dry out naturally when they are well ventilated
and the humidity level of the air is not too high, and sufficient time should be provided for
this drying to occur. Walls and roofs should not be enclosed until the framing materials have
dried to an acceptable level of moisture. In cold and damp conditions, the use of space
heating can efficiently dry wood and improve construction efficiency. Rigid components
(services, pipes, elevator shafts, rigid cladding), should be installed as late as construction
allows, to minimize subsequent settling of the wood structure.
5. Besides shrinkage, other causes for vertical movement in wood structures that may need to be
considered in the design include effects of vertical loads (deformation due to compression
loads, including instantaneous elastic deformation and creep), and effects of closing of gaps
between members (settlement).
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6. Differential shrinkage between the various types of wood-based products and between wood
and other materials such as steel and concrete must also be taken into account. Examples of
that occur where the wood elements in the structure are connected to rigid components of
other materials such as masonry (e.g. cladding), concrete (e.g. elevator shafts), mechanical
services and plumbing, and where mixed wood products such as lumber, timbers, and
engineered wood products are used. The use of slots in steel components of the connections
that allow movement of dowels and prevention of direct contact between concrete, masonry
and wood-based members are some of the common preferable design details. Other
examples of good connections detailing can be found in the CWC Wood Design Manual
(CWC, 2010). Recommendations and solutions as to how to deal with differential movement
can be found in the design fact sheet entitled ‘Vertical Movement in Wood Platform Frame
Structures: Design and Detailing Solutions’ (FPInnovations, 2013).
7. Wood and wood products have a high modulus of elasticity (MOE) parallel to the
grain/strand, therefore the deformation of members loaded parallel to grain is very small and
is usually neglected in low rise and mid-rise buildings. This may not be the case in tall wood
buildings where, due to cumulative effects of multiple storeys, the accumulation of this
movement may need to be considered (See Section 4.2.2.3 above). In addition to the vertical
movement, the deformation on the compression side in combination with the vertical
elongation of the wall anchorage system may contribute to the lateral drift of mid-rise or tall
wood buildings.
8. Wood members loaded perpendicular to grain may undergo significant instantaneous and
time-dependent deformations under load and these needs to be taken into account in the
design. Instantaneous deformation will occur with the application of load, while the timedependent deformation (also called creep) is additional deformation which develops over
time. The time-dependent deformation is most pronounced where wood members are
subjected to high levels of sustained loads in environments with frequent large changes in
MC or under continuously wet service conditions. More details about creep can be found in
Section 4.2.2.6.
9. Due to imperfections in product manufacturing and building construction, small gaps
between wood members in walls and floors may be created during building construction,
which will eventually close (settle) after the completion of the building. The amount of
settlement can vary greatly with different products, construction practice and techniques. As
the construction proceeds, these gaps are usually gradually reduced as the gravity load
increases. The impact of such settlement depends on the building components and should be
considered in the design.
4.2.2.5
Tension Perpendicular to Grain
Wood and wood products are weakest in tension perpendicular to grain/strand. The failure mode of wood
subjected to tension perpendicular to grain is brittle and such stresses should be avoided in any
connections or wood members especially in buildings located in high seismic zones. Members with
tensile perpendicular-to-grain stresses include double-tapered curved and pitch-cambered beams, notched
members, members with holes, tension or compression brace members with eccentricity in connections,
moment-resistant connections with dowels, or members with cross connections. Moment-resistant
connections should be detailed to minimize tensile perpendicular-to-grain stresses as they rotation and
yield.
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Tensile perpendicular-to-grain stresses can also be introduced in members due to changes in the moisture
content. If in some areas of the structural system tensile perpendicular-to-grain stresses are expected and
cannot be avoided, strengthening of that part using self-tapping screws, fully threaded rods, drilled and
glued-in rods or other means of reinforcement is necessary. More information about reinforcement
techniques is provided in Section 4.2.2.8 below.
4.2.2.6
Duration of Load and Creep
One of the key characteristics of wood is that its strength and stiffness is affected by the intensity and
duration of load.
Generally, wood is able to carry short-term loads of a higher magnitude than those that it will support for
a long time. Creep, the time-dependent increase of deformation or deflection under constant load, affects
the serviceability design of all wood structures and its effects should be considered for tall wood
buildings.
In CSA O86, the duration of load factor, KD, is used to account for the effect of the duration of load on
strength.
Values for the KD factor for wood products and connections are given in Table 4.3.2.2 of CSA O86. KD
is 1.0 for standard-term duration of loading, while KD =1.15 for short-term such as wind and seismic, and
KD = 0.65 long-term loading such as dead loads. For standard-term loads where long-term load exceeds
live load or snow load, a load duration factor of KD =0.65 for long-term loads may be used or the factor
can be calculated as follows:
Where:
𝑃𝐿 = specified long-term load;
𝑃𝑠 = specified standard-term load.
𝑃𝐿
𝐾𝐷 = 1.0 − 0.50𝑙𝑜𝑔 � � ≥ 0.65
𝑃𝑆
[1]
Like most other construction materials, wood experiences creep deformations. Wood is generally
considered to be a viscoelastic material. If a load is applied to a wood member, reaching its full value
after an instantaneous time, tinst, an instantaneous deflection (deformation), uinst, will be developed
(Figure 85).
The polymeric nature of wood components makes it sensitive to temperature; the higher the temperature
the greater the creep amplitude. In normal use, however, when the temperature does not exceed 50°C, the
temperature effects on creep are very small and can be neglected.
For wood-based products with long-term load behavior similar to that of solid wood, ASTM Standard
D6815 provides a procedure for testing and evaluating duration-of-load and creep effects of wood-based
materials (ASTM, 2010). The intent is not to develop specific duration-of-load or creep factors for woodbased materials, but to demonstrate the engineering equivalence of wood-based products used in dry
service conditions in terms of duration-of-load and creep effects compared to those of solid lumber. If all
criteria are satisfied, the product is considered acceptable for using the duration-of-load and creep factors
applicable to lumber (Karacabeyli, 2001).
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4.2.2.7
Punching Shear
Punching shear is a type of failure associated mainly with massive slabs subjected to high localized forces
at column support points. Massive wood-based panels such as CLT, LVL or LSL supported by wooden or
steel columns (Figure 86) may be subjected to high shear forces, as such forces could cause excessive
compression perpendicular to grain and potentially a rolling-shear failure. This phenomenon is not
particular to tall wood buildings, but will be important in situations where panels are used as part of a
two-way floor system. However, there are considerations unique to each wood product because of
wood’s anisotropic nature and in the case of SCL the manner in which the SCL is manufactured.
Based on European research, several options are available to mitigate punching shear and reduce
excessive compression perpendicular to grain associated with such applications:
•
Reinforcement techniques using self-tapping screws. Recent research studies have indicated that
installing inclined self-tapping wood screws can substantially improve the load-carrying capacity
of CLT panels against rolling-shear failures (Mestek et al., 2011). This is particularly critical
when CLT is manufactured with unglued cross lamina or when relief grooves are used. The
amount of improvement depends very much on the number of screws, screw spacing, screw
diameter and type, and the screw installation angle. Under two-way load-carrying action, an even
more significant increase in load-carrying capacity of the plates could be achieved.
•
Other options involve distributing the load at the interface between the column and the CLT
though a wide metal bearing plate. Alternatively, a wooden cap (i.e., a small massive-wood
panel) could be used to further reinforce the slab, help distribute the load over a larger area and
mitigate stress concentration.
Figure 86
4.2.2.8
Massive-wood floor plate on posts with potential punching-shear issue (Source: KLH)
Transverse Reinforcement of Connections
The tensile and the compressive strengths of timber perpendicular to the grain are much lower than the
respective strength values parallel to the grain. For example, the characteristic tensile strength
perpendicular to the grain for solid timber is about 1/25 to 1/60 of the tensile strength parallel to the grain.
Examples of structural details where tensile stresses perpendicular to the grain occur are notched beam
supports, connections loaded perpendicular to grain, beams with holes, and connections with multiple
fasteners in a row.
Various reinforcing techniques have been adopted over the years for timber members and connections
(dowelled connections in particular), such as glued-on plywood plates, truss plates and fibre-reinforced
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composite (FRC) or internal reinforcement using glued-in threaded rods to avoid splitting of wood
members at the connection (Mohammad et al, 2006). While not particular to high-rise construction, an
understanding of this method of reinforcement will be an important tool for the designer.
Transverse reinforcement using self-tapping screws is slowly
Self-tapping screws or glued-in rods
becoming a commonly used reinforcement technique due to
can be used for transverse
their high withdrawal resistance and ease of installation
reinforcement. Fully threaded selfcompared to other means of reinforcement which require
tapping screws are preferred due to
surface preparation and drilling. Self-tapping screws come in
their ease of installation and their
various shank and head shapes and threads. Some are partially
effectiveness in reinforcing structural
threaded while other types are fully threaded. Screws with
members and connections.
continuous threads in particular present an alternative to the
traditional reinforcement methods and show new and economic possibilities as reinforcement in
connections and beam supports (Blass and Bejtka, 2004). With diameters up to 12 mm and lengths up to
600 mm, fully threaded screws may be used in many structural members as tensile reinforcement
perpendicular to the grain. Self-tapping screws are regularly installed at an angle to the interface which is
contrary to the traditional practice of using lag and wood screws which are driven perpendicular to the
interface between members. In driving screws at an angle, shear transfer across the interface results in a
tensile force component within the screws resulting in substantially higher capacity and stiffer
connections. This is particularly important for timber connections that are designed for high loads because
their details govern the overall design of other structural members.
Research conducted in Europe and Canada has shown that transverse reinforcement with self-tapping
screws not only enhances lateral resistance of connections, but also improves stiffness and ductility (Blass
and Schmid, 2001). Continuously threaded self-tapping wood screws are preferred over partially threaded
as they are capable of developing higher withdrawal resistance. Reinforcement may be necessary in cases
where end and edge distances of dowel-type fasteners are small, increasing the risk of brittle failure
modes (Figure 87). Such screws are currently commercially available in Canada.
Figure 87 Transverse reinforcement of bolted connections using self-tapping screws
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4.2.2.9
Fire Performance (also refer to Section 5)
Currently, there is no specific guidance given in the NBCC
regarding the fire resistance of timber connections in wood
buildings. In principle, connections should have the same
degree of fire resistance as that of the structural elements or
assemblies in which they are used.
Connections with exposed metal parts may perform poorly
under fire conditions because the intensity of a well-developed
fire is sufficient to weaken or plasticize metal components and
hardware. For connections carrying gravity loads in tall wood
buildings, a strategy must be developed to prevent collapse of
the building in the event the connections are exposed to fire.
Connections should have at least the
same degree of fire resistance as that
of the structural elements or
assemblies in which they are used.
Division B of NBCC is generally
interpreted to permit steel connections
in
45-minute
heavy
timber
construction to be unrated, however
this is not applicable or appropriate
for a tall wood building
It is not necessary to combine fire loading with lateral loading, unless a specific component is responsible
for the majority of a building’s lateral resistance. Therefore, exposed steel connections carrying lateral
loads are generally acceptable as long as it can be shown that one fire incident is unlikely to significantly
impair the overall building’s lateral resisting system.
In a fire, serviceability failures, such as crushing of wood or Under fire condition, mass timber will
deflection may be acceptable as long as they do not lead to char, and the potential depth of char
collapse of the building floors or walls or failure of fire safety can be reliably calculated (i.e.,
systems. This may facilitate development of alternate load roughly 0.6mm/min or 1.5in per
paths that do not rely on exposed steel connections. As hour). Steel connections located
discussed in detail in Chapter 5, in a fire, mass timber will entirely outside the potential char
char and the potential depth of char can be reliably calculated layer are considered appropriately
as roughly 0.6 mm per minute or 1.5 in per hour. It is protected.
generally accepted that if steel connections are located
entirely outside the potential char layer, they are considered appropriately protected. Thus, in an exposed
mass timber element in a 2-hour fire-rated building, if connections are covered with approximately 3
inches of wood, the connection can be considered protected. Calculation of the actual depth of char is
further discussed in Chapter 5. When designing concealed connections, it is necessary to recess the
dowels and bolts in the side wood members and use wooden caps for further protection. If this is not
done, the dowels and bolts can carry heat into the connection and char the connection from the interior
creating a potential failure mechanism.
In many cases, the mass-timber element may be protected with gypsum wallboard or other material to
protect the wood and reduce the level of charring, and making this protection continuous over the steel
connections may provide the required protection for the connections. Again this is further discussed in
Chapter 5.
Connections can also be protected with various fire protection products used for protection of steel, such
as spray-on fire-resistance material (commonly called ‘fireproofing’). Care must be taken at the interface
between the wood and the protective material to ensure the continuity of the protection when the wood
chars in a fire, and that the temperature of the steel is not sufficient to cause charring of the wood.
One such material that has been used for protection of steel connections is intumescent paint. However,
recent testing has indicated that intumescent coating may not perform as well as expected when used for
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protection of steel components in heavy timber connections. Cracking and peeling may occur, especially
at the interface between wood and steel were observed, and while still structurally sound, the steel may
get hot enough to char the wood, causing premature failure of the connection. These concerns may be
alleviated if specific fire testing or analysis is available to demonstrate effective protection of the
connection.
To enhance connection performance under fire conditions, the following should be considered:
1. Use of concealed connections whenever possible, with all metal components embedded inside the
structural wood members or beneath the potential char layer (sacrificial wood). Current CNC
technology and availability of innovative connection systems facilitate such a design approach
(Figure 88). When using a single steel plate however, it is important to follow the specified
maximum distance between the outer rows of connectors as per CSA O86 to avoid potential
splitting. The depth of the char layer and thickness of protection should be calculated as discussed
in Chapter 5.
2. In certain connection systems (e.g., post to beam using two-way moment-resisting metal plates
just like in steel construction), wooden caps could be used to cover the steel plates and dowels at
the interface between posts and beams for fire protection. The thickness of the wooden caps
should be calculated based on char, as discussed in Chapter 5.
3. Where the wood is encapsulated or protected with gypsum wallboard or other material, this
protection may be made continuous over the connections. Further analysis will be required based
on the material properties of the steel, the wood and the protective material.
4. It is often possible to design connections to be redundant, allowing for an alternate load path to
develop in the case of yielding of exposed metal components under fire conditions as has been
adopted in historical tall wood buildings in North America (Figure 89). A modern version of this
detail, not requiring fire protection, can be found in Figure 80, where the crushing failure shown
is acceptable.
5. Care should be taken with spray-on fire-protection material at the interface between wood and
steel, and with the charring effect that hot steel may have on the mass timber.
6. Intumescent paint should be avoided unless specific fire testing or analysis is available to
demonstrate effective protection of the connection.
Figure 88 Concealed post to beam connection systems
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Figure 89 A post-to-beam connection detail in a seismically upgraded historical tall wood building in
North America
4.2.2.10 Cost Considerations (also refer to Chapter 8)
The lack of costing information for connection systems used in tall wood buildings is a challenge, but as
these buildings become more common, more costing information will become available. One possible
strategy to minimize the risk of cost overruns is to use a systems approach. New systems like CREE are
becoming available in North America, but their number is currently very limited.
Cost analysis should include all construction costs including transportation, installation and finishing. The
choice or selection of the type of assembly should also take into account the costs of maintenance during
the expected life of the structure.
Typically, the cost of the different types of connection
systems depends on several factors. As a rule of thumb,
generic traditional fasteners generally cost less than
proprietary fasteners, especially if the latter are imported.
However, some proprietary connection systems and fasteners
provide more efficient and reliable performance that makes
the connection more economical for certain applications.
In estimating the overall cost of connection systems, one
must account for the following key elements:
•
•
•
•
•
•
•
•
In estimating the cost of connection
systems, one must account for the
following key elements:
•
•
•
•
Availability
Ease of design
Reliability in performance
Buildability
Availability
Ease of design (complex systems take more time to design)
Reliability in performance
Buildability (how quick and easy it is to assemble and potentially disassemble without
sophisticated or specialized tools)
Serviceability
Compatibility with other mechanical and electrical installations
Need to conceal for aesthetic reasons
Fire protection
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4.2.3
Input Data for Connections and Assemblies
4.2.3.1
Strength
The strength of the connection or assembly corresponds to the
maximum load resisted by the test specimen as shown on the
load-displacement curve during tests. The capacity of the
connection or assemblies should be determined through
appropriate engineering mechanics based on the specimen
configuration and test setup. Assumptions used for deriving
strength need to be stated and verified.
Moisture content and load duration
are important factors in connection
design. Brittle failures should be
avoided, especially in high seismic
zones.
Connections or assemblies must be designed to resist the effects of the transfer of one or more types of
force flow between components or between substructures. In order to avoid attracting unintended forces, a
connection or assembly should be able to resist the force flows assumed in the design of the system or
substructure to which it belongs. It is also important to note that the stiffness of a component (a bolt or a
screw for example) might be significantly different from the stiffness of the whole connection or
assembly. Additionally the flow of forces into a connection is hard to anticipate partially because some
structural elements may not work as assumed (some walls work as deep beams for example) and some
non-structural members may attract some load because of their rigidity.
Capacity demands on connections or assemblies in wood construction need to be defined in terms of:
•
•
•
Effects of peak external force levels
Fatigue
Reversals in force flows
For connections or assemblies, if wood failure governs the
The behaviour of timber structures
strength then the duration for which external forces are
under seismic and wind loads are
sustained (static fatigue) becomes an issue. Typically, types
largely controlled by the connections,
of connections or assemblies for which strength is controlled
which must be ductile and maintain
by wood failure (brittle failure in particular) should be
integrity under overload.
avoided (i.e., design for ductile failure wherever possible),
especially in high seismic areas. Consideration must be given to how moisture and other service factors
influencing the strength of wood (or other materials) are integral to any connection or assembly design.
Guidance is given in CSA O86 and other material design standards such as EC5 in Europe and NDS in
the US on how to adjust the capacity to take account of the wood moisture content and other service
conditions. CSA O86 is the standard that is referenced by the building codes in Canada, and it should be
used in design. The use of NDS or EC5 modification factors is at the discretion of the designer, provided
they are not directly applied to resistances calculated with CSA O86.
Currently, moment-resisting connections are not explicitly covered under CSA O86. However, current
design information in the standard could be used to design a moment connection. Several proprietary
moment-resisting systems are available that allow engineers to design portal and braced frames that have
moment-resisting connections. Designers should consult with individual manufacturers of those
connection systems for technical information on their performance.
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4.2.3.2
Stiffness
Stiffness is a measure of the resistance offered by a connection or an assembly to deformation,
demonstrated by the slope of the load-deformation curve (Figure 90(a)). Since the stiffness of most
connections and assemblies varies with the load in a non-linear fashion, it is useful to make certain
simplifications of the load-deformation curve, to the degree necessary for the appropriate analysis method
(See Section 0).
Usually, an initial linear stiffness is defined and it can be determined in several ways, such as the secant
stiffness between two load points of the load-displacement curve. The first load point can be either the
load at zero or the load at 10% of the maximum load, for example, while the second load point may be the
load at 40% of the maximum load, as shown in Figure 90(b). The initial stiffness can then be determined
as follows:
ke =
where
𝑃1 , 𝑃2 –
∆1 , ∆2 –
P2 − P1
∆ 2 − ∆1
[2]
Loads at the first and second points, N
Displacement corresponding to the first and second load points, mm
A consistent method should be used for deriving stiffness values, so that they are compatible in numerical
modelling and analysis.
Depending on the type, connections may be designed to have the ability to limit deformations of the
structural system. It is critical that any connection only provide stiffness against the flow of forces as
assumed in design of the system or substructure to which it belongs, in order to avoid unintentionally
attracting forces which may exceed the connection capacity. When stiffness is required (e.g., for wind and
seismic), it has to be controlled against the effects of axial, shearing and moment force flows assumed in
the design.
As most joints in timber construction are assumed as “pinned” joints, the stiffness of connections or
assemblies are not required by the designer. The CSA O86 provides the load-slip curves of nailed joints.
They can be used to derive the stiffness of nailed joints or assemblies using nailed joints. For other
connections and assemblies using other types of fasteners such as bolts and dowels, the stiffness can be
determined by tests or based on available information in other jurisdictions.
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Ppea
Load (P)
Pu
Pyie
Δyie
Δpe
Δu
Displacement
(a) Typical load-displacement curve
(b) EEEP elastic-plastic curve
Figure 90 Definitions of various ductility parameters
4.2.3.3
Ductility
Connections should be designed not only to resist the design loads, but also to absorb energy and
maintain the integrity of the structural system in the event of overloading. The proper design of
connections plays a critical role in determining the overall performance of timber structures. This is
especially important for seismic design stemming from the fact that the energy dissipation of timber
structures under earthquake is mainly achieved through the
ductility of connections. It has been demonstrated by several Connections should be designed not
researchers that a structure with ductile and dissipative only to resist the design loads, but
connections, if appropriately designed, can resist much also to absorb energy and maintain the
higher seismic motions than the same structure with rigid integrity of the structural system in
the event of overloading.
and non-dissipative connections.
In seismic design, the term “ductility” is usually defined as the ability of an assembly or a structure to
undergo large deformations in the inelastic range without substantial reduction in strength. Apart from
system irregularities such as a soft storey or an inadequate lateral load-resisting system, inadequate or
inappropriate connection design or detailing is responsible for most of the collapses and damage that
occur during extreme wind and seismic events.
To estimate ductility, it is necessary to determine when the connection or assembly begins to yield. As
discussed in Section 4.2.3.2 above, the yield point of an assembly is defined as the load (or stress) at
which a material or an assembly begins to plastically deform (i.e. irreversible deformation).
Ductility can be determined based on analysis of test data on timber connections and assemblies. The
ductility is usually expressed as the “ductility ratio (μ)”, which is defined as the ratio of the displacement
at the maximum (peak) or failure (ultimate) load to that at the yield load (Figure 90) as follows.
𝜇=
∆𝑢
∆𝑦𝑖𝑒𝑙𝑑
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where
𝛥𝜇 = displacement at failure load, 𝑃𝑢
𝛥𝑦𝑖𝑒𝑙𝑑 = displacement at yield load, 𝑃𝑦𝑖𝑒𝑙𝑑
For wood connections or assemblies, the point where yielding starts is usually not obvious. Different
methods exist for the determination of the yield point for timber structures, but none has yet been adopted
in the Canadian standards. According to ASTM E2126 (ASTM, 2012), the yield point can be determined
by using the equivalent energy elastic-plastic (EEEP) curve, as shown in Figure 90(b), and it is
recommended that this method be used for determining the yield point of wall, floor and roof assemblies.
For connections, the CEN bilinear elastic-plastic approach is proposed (Ceccotti, 1995) in Europe,
whereas in the USA, the ASTM standard uses the 5% diameter offset for connections with dowels. An
agreed universal approach for the yield point and ductility would help the harmonization of standard
testing and analysis procedures needed for seismic design of timber systems. The definition of ultimate
displacement in ISO Standards 16670 and 21581 are accepted universally.
Ductility Categories
In Eurocode 8, three ductility classes for timber connections have been proposed, depending on the type
of fastener (e.g., nails, screws and dowels), loading conditions and failure mode (CEN, 2001). Ductility
categories were proposed (see Table 2), where connections or components could be classified based on
the failure mode as “brittle”, “low ductility”, “moderate ductility” or “high ductility” (Smith et al., 2006).
Table 4
Proposed ductility classes for connections by Smith et al. (2006)
Classification
Brittle
Low ductility
Moderate ductility
High ductility
Average ductility ratio (µ)
µ≤2
2<µ<4
4<µ≤6
µ>6
The underlying assumption is that failure mechanisms individually or in combination control the global
system failure mode. One of the main objectives of this proposed classification system is to link
connection behaviour to that of the overall system. For example, once a target building system ductility
value (𝑅𝑑) is specified, the corresponding connection ductility that satisfies that system ductility class is
designed. It is important that designers distinguish between the connection ductility and that of the
system.
4.2.3.4
Damping
Damping is a measure of how oscillations of a system decay after a disturbance. Since the decay is due to
the absorption of the energy imparted to the system by the disturbance, it is a measure of the ability of a
building to dissipate the energy from wind or seismic load.
In timber structures, seismic and wind energy is dissipated through several mechanisms such as internal
friction, friction between structural elements, and plastic deformation. Under extreme seismic events, a
large portion of this energy dissipation is achieved through nonlinear deformation of the mechanical
connections due to the yield of metal connectors and the bearing of wood members. Where timber is used
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in long, tall and light structures, the dynamic response to wind load and human-induced vibrations can be
critical and joints contribute significantly, in terms of damping and stiffness, to the way the structure as a
whole behaves.
Unlike the stiffness of a structure, the damping coefficient cannot be calculated from the structure. For an
actual structure, the damping coefficient can be estimated by free- or forced-vibration experiments. The
measured rate at which motion decays in free vibration will provide a basis for evaluating the equivalent
damping coefficient at deformation amplitudes within the linear elastic limit of the overall structure.
Additional energy is also dissipated due to inelastic behavior of the structure at larger deformations (e.g.,
for seismic design). Under cyclic forces or deformations, this behaviour implies formation of a forcedeformation hysteresis loop as shown in Figure 91 (Labonnote, 2012). The damping energy dissipated
during one deformation cycle between deformation limits is given by the area within the hysteresis loop.
This energy dissipation due to inelastic deformations of structures expected during strong earthquakes
cannot be modeled by equivalent viscous damping (Chopra, 1995). Instead, the most common and direct
approach to account for the energy dissipation through inelastic behavior is to recognize the inelastic
relationship between resisting force and deformation, such as shown in Figure 91, in the dynamic
analysis. Such force-deformation relationships can be obtained from the experiments in Sections 4.2.3.5
and 4.2.3.6.
Figure 91 Structural damping hysteresis loops a) idealized and b) simplified (Labonnote, N. 2012)
In design, the damping coefficient can be estimated from
Where timber is used in long, tall and
existing buildings similar in size and structural type.
light structures, the dynamic response
FPInnovations has conducted ambient vibration testing of
to wind load and human-induced
several completed wood low- and mid-rise buildings ranging
vibrations can be critical and joints
from light wood frame to post and beam and CLT and
contribute significantly, in terms of
combinations of various systems. Findings indicate that for
damping and stiffness, to the way the
completed buildings with finishes and partitions, most
structure as a whole behaves
buildings will have an equivalent damping coefficient in the
range of 2% to 4% (Hu, 2010). Based on the limited damping data measured so far, it seems to be
reasonable to use 3% for critical viscous damping for wind design. FPInnovations continues to collect
field measurements on new built mid- to high-rise wood buildings and is collaborating with other
researchers to enhance the database.
For seismic design, it is recommended that an equivalent viscous damping coefficient of 5% be used with
the equivalent static force method and linear dynamic analysis with consideration of additional energy
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dissipation from inelastic deformations. If the inelastic deformations are considered in a nonlinear time
history dynamic analysis, then 1% to 2% should be used for the equivalent viscous damping coefficient.
4.2.3.5
Evaluating, Testing and Detailing of Connections and Assemblies
4.2.3.5.1 Evaluation of Connections
Non-conventional types of connection systems and fasteners
are evaluated based on test data produced by the Testing of connections should be
manufacturer with interpretations of evidence performed by performed in accordance with
product assessment organizations such as CCMC in Canada established testing protocols such as
or ICC-ES in the USA. Such assessment organizations ASTM D1761, D5764 and D7147.
produce reports with their recommended design values for
that particular type of fastening system. The proposed design properties become valid only if local
authorities having jurisdiction accept them.
The following provides a common approach in testing and deriving the design values for proprietary
connection systems that fall beyond current CSA O86 generic types of fastening systems based on
accepted standards such as ASTM.
1. Test assemblies should be realistic and
representative of typical connection systems in
the field. Sampling strategy of wood-based
components of the connection assembly should
comply with standardized ASTM or international
practice. Sampling based on density is a common
practice.
General considerations must be taken
into account prior to testing
connections and assemblies including:
•
•
•
Specimen configuration
Material moisture content of
components
Load application, rate of
loading and type of loading
Workmanship in terms of
assembly fabrication.
2. The number of test assemblies should reflect
•
variability in the wood-based elements and other
components integral to the connection assembly
and samples should be representative of actual
assemblies in service in terms of fabrication and moisture conditions. Guidance on the
minimum required sample size is given in ASTM D5457 (ASTM 2012). Depending on the
type of connection assembly, a minimum of ten (10) replicates for each configuration is
considered acceptable, although a smaller sample size could be adopted if justified. Product
assessment organizations such as CCMC or an engineer must be allowed to determine the
number of tests required based on the reliability of the results.
3. Testing should be done in accordance with established testing protocols. In North America,
ASTM D1761, D5764 and D7147 testing procedures are adopted depending on the type of
fastening system and objectives (ASTM, 2012). However, other testing protocols such as the
universally agreed to ISO Standard 16670 could also be used. The following considerations
must be taken into account prior to testing:
4. Material moisture content (assemblies must be conditioned to the target service conditions
prior to testing)
5. Load application, rate of loading and type of loading need to be determined (i.e., static vs.
cyclic or reverse cyclic, short term vs. sustained loading, etc.)
6. Workmanship in terms of assembly fabrication.
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7. Care must be taken to collect all necessary data to allow the determination not only of
ultimate capacity, but also deformation characteristics of the loaded assembly.
8. Observations of failure modes must be recorded. Measurements of density and moisture
content within the vicinity of the connection must be taken.
4.2.3.5.2 Evaluation of wall, floor or roof assemblies
The following criteria should be considered when developing a test program for evaluating mechanical
properties of wall, floor and roof assemblies:
1. Specimen configuration: The assembly configurations selected for testing should be
sufficiently broad to capture the full range of assembly applications. Selected configurations
should cover all failure modes that can reasonably occur in reality.
2. Specimen size: The specimens should be full-size assemblies unless it can be shown by
theory or experimentation that testing of reduced-scale specimens will not significantly affect
behavior.
3. Specimen fabrication: Specimens should be constructed in a setting that simulates commonly
encountered field conditions. The assembly should be of a construction quality that is
equivalent to what will be commonly implemented in the field.
4. Boundary conditions: The boundary conditions of assembly tests should be representative of
constraints that an assembly would experience in a typical structural system. Boundary
conditions should not impose beneficial effects on seismic behavior that would not exist in
common system configurations.
5. Load application: Loads should be applied to test specimens in a manner that replicates the
transfer of load to the assembly as it would occur in common system configurations. For an
assembly that resists gravity and overturning loads, test loading should include these loads,
unless it can be shown that they do not significantly influence component performance.
A minimum of two test specimens should be included for each assembly configuration. A minimum of
three tests should be included in any of the following situations: (1) if rapid and unpredicted deterioration
occurs (such as that caused by brittle fracture); (2) if the strength varies by more than 15% between the
two tests; or (3) if the ultimate deformation capacity varies by more than 20% between the two tests.
Both monotonic and cyclic displacement-controlled tests should be performed:
1. Monotonic loading: Specimens should be tested to deformations large enough to achieve a
20% reduction in applied load. Test specimens should be tested in both directions for
assemblies that have significant asymmetric behavior.
2. Cyclic loading: Specimens should be tested with the cyclic displacement schedules in ASTM
2126 (Standard test methods for cyclic (reversed) load test for shear resistance of vertical
elements of the lateral force resisting systems for buildings). The specimens should be tested
to deformations large enough to achieve a 20% reduction in applied load.
For each test specimen, the load-displacement curve and failure modes should be recorded. The following
parameters should be obtained from the test data:
1. ultimate load,
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2. ultimate deformation, taken as the deformation at
80% of the ultimate load on the descending
portion of the curve;
3. initial stiffness, based on force and deformation
at 40% of the ultimate load;
Parameters that need to be determined
from test data include:
Ultimate load and deformation
Initial stiffness
Yield load and deformation
Assembly ductility
4. yield deformation, determined in accordance
with the Equivalent Energy Elastic-Plastic (EEEP) approach in ASTM E2126;
5. ductility, which is the ultimate deformation divided by yield deformation
For the cyclic test data, values of each parameter should be measured from both positive and negative
portions of the envelope curve, as illustrated in Figure 92. For components with reasonably symmetric
behavior, values of ultimate load, ultimate deformation, initial stiffness, effective yield deformation and
the effective ductility should be calculated as the average of their respective values determined from the
positive and negative portions of the envelope curve. For components with significant asymmetric
behavior, positive and negative values of ultimate load, ultimate deformation, initial stiffness, effective
yield deformation and effective ductility capacity should be calculated and evaluated separately for each
loading direction.
Figure 92
Envelope curves of cyclic test data for assemblies
4.2.3.6 Deriving Design Values for Connections and Assemblies based on Test Data or Design Data from
Other Jurisdictions
4.2.3.6.1 Connections
To derive the specified design values which correspond with design values given in CSA O86 standard
for generic fastening systems, the following procedure may be adopted:
•
Establish the characteristic strength property from test data
The characteristic value is determined from test data (determined in 4.2.3.5) on connections or
assemblies following ASTM D5457 standard procedure (ASTM, 2012). The characteristic design
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value is taken as the lower 5% percentile estimate determined as per established parametric or
non-parametric procedure based on 75% confidence level. Weibull or Normal distributions
(whichever provides a better fit for the test data) should be adopted for parametric analysis.
Typically test data are fitted to Weibull 2-parameter distribution models (CSA O86 2001).
•
Determine the coefficient of variation (CoV)
CoV is determined using the full data set, or lower tail data, that was used to establish the
characteristic strength. A 2-parameter Weibull model is recommended for establishing CoV from
the lower tail distribution.
•
Determine the Data Confidence Factor (Ω)
Because of potential limitations on the sample sizes for some connections assemblies for practical
reasons, it is necessary to adjust the characteristic value by a factor called “Data Confidence
Factor” (Ω) as given in ASTM D5457 to compensate for uncertainty in the estimated
characteristic property.
•
Compute the reliability normalization factor
Guidance is given in CSA O86 document “Standard Practice Relating Specified Strengths of
Structural Members to Characteristic Structural Properties” published in 2001 regarding the
appropriate target level of the reliability index, β, which is used to compute the reliability
normalization factor for bending, shear, tension and compression members (CSA, 2001).
•
Determine the specified design value
The specified design value is established by adjusting the characteristic 5th % value by the data
confidence and reliability normalization factors then by applying a 0.8 factor to bring it from
short-term laboratory test to standard duration to make it consistent with current design values in
CSA O86 standard.
•
Factored resistance
The specified design value is further adjusted by a
resistance factor in accordance with current design
provisions for fasteners in CSA O86 standard. The
appropriate material factor to apply depends on the
ductile or brittle capacities of the connection
assemblies.
4.2.3.6.2 Wall, Floor and Roof Assemblies
Procedure to derive the design values
for connections and assemblies should
be based on standard procedure as
specified in ASTM D5457 and other
established methods recognised in
other jurisdictions and shall be
consistent with CSA O86 standard
procedure.
Because of the costs for conducting assembly tests, the number of assembly specimens is limited. As a
result, the test data can provide a reasonable estimation of mean properties of the assemblies, but it is
difficult to derive the characteristic values of the assemblies.
For wall, floor and roof assemblies, the test data can be used to verify the design values which are derived
through modelling or engineering mechanics using material properties information provided in CSA
Standard O86 (2009). The design values can also be developed based on established methods recognised
in other jurisdictions.
In CSA O86, the specified shear strengths of shear walls and diaphragms are derived from the US
allowable stresses using a factor of 1.863. The allowable shear force for a shear wall was approximately
equal to the average ultimate load-carrying capacity of a tested shear wall divided by a load factor of 2.5.
Therefore the specified shear strength of a shear wall can be established as follows:
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where
𝑃𝑚𝑎𝑥 –
𝐿
–
α
–
𝑉𝑑 ≤
1 𝑃𝑚𝑎𝑥
× 1.863
𝛼 𝐿
[4]
Average maximum load resisted by the specimen, N
Length of specimen, m
design scale factor (2.5 ≤ α ≤ 5.0)
The above equation would provide the same target safety index as the shear walls recognised in CSA
O86. The α is given a range such that if an assembly does not meet the ductility compatibility criteria
using α equal to 2.5, a larger value for α up to a maximum limit of 5.0 can be selected so that the
corresponding yield deflection results in a smaller value to meet the ductility compatibility criteria.
4.2.3.7
Requirements for Proprietary Connections
Proprietary innovative connections are becoming more common due to increasing numbers of
contemporary wood buildings and tall and large wood buildings, where they are expected to add design
interest and innovation as well as perform their fundamental role.
Proprietary types of metal fasteners are generally divided into
two groups according to product standards, namely 'dowel- Proprietary innovative connections are
type fasteners' and 'connectors' such as punched metal plate becoming more common due to
fasteners. Metal dowel-type fasteners are efficient, so the increasing numbers of contemporary
relative weakness of timber in the direction perpendicular to wood buildings and tall and large
grain becomes the governing factor in design. Where metal wood buildings where they are
fasteners are concealed, their appearance is unimportant, but expected to add design interest and
where they are exposed, detailing has to be carefully innovation, as well as perform their
considered. Common dowel-type fasteners are self-tapping fundamental role. Examples include
self-drilling
screws and self-drilling dowels. These are quite popular in self-tapping screws,
Europe and are becoming popular in North America and dowels and glued-in rods.
elsewhere as a method of attaching steel plates to large wood
components. Self-drilling dowels incorporate a drilling bit that can drill directly through steel plate link
elements (side-plates or embedded plates of about 6 mm thickness) making for rapid installation. They
are also applicable for plain wood-to-wood connections and are commonly used in CLT assemblies.
Proprietary metal fasteners are also used as shear connectors in composite concrete and steel beam and
plate applications, where higher loads and longer spans can be achieved through composite action.
Glued-in or bonded rods are also a common type of fasteners that are used mainly in Europe for carrying
heavy loads and for moment-resisting connections. The rods are inserted into predrilled holes and
subsequently grouted. Compressive, tensile and shear forces can be accommodated and rigid connections
can be achieved. End-grain connections can be established which allow for the transfer of heavy loads.
Several innovative concealed systems have been developed by various manufacturers mainly in Europe.
Designers should be cautioned though that some large glued-doweled moment connections are
particularly susceptible to shrinkage checking. Research has shown that generally, using mild steel as well
as more rods of smaller diameter properly spaced from each other and from the edges will achieve better
ductility (Tlustochowicz, 2012).
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Modern concealed metal hardware for wood construction is invariably proprietary and design information
is based on recommendations of third-party product-assessment organizations such as the Canadian
Construction Materials Centre CCMC, ICC-ES in the USA and technical approvals in Europe. Products
vary in complexity. There is typically a balance intended between structural capability, ease of
construction, and no doubt also aesthetic requirements.
4.2.4
References
ANSI/AF&PA. 2005. National Design Specification (NDS) for Wood Construction. Washington DC.
ANSI. 2012. Standard for Performance-Rated Cross-Laminated Timber (ANSI/APA PRG 320-2012).
New York (NY): American National Standards Institute.
American Society of Civil Engineers. 2010. Minimum Design Loads for Buildings and Other Structures,
ASCE 7-10, Reston, VA, 2010.
American Society for Testing and Materials. 2012. Annual Book of ASTM Standards. West
Conshohocken, PA. ASTM Standard.
APEGBC. 2011. Practice Bulletin. Structural, fire protection and building envelope professional
engineering services for 5 and 6 storey wood frame residential building projects (Mid-rise buildings).
Association of Professional Engineers and Geoscientist of BC.
Blass, H.J. and Bejtka, I. 2004. Reinforcement perpendicular to the grain using self-tapping screws.
Proceedings of the 8th World Conference on Timber Engineering, Volume I, Lahti, Finland, 2004.
------------- and Schmid, M. 2001. Self-tapping Screws as Reinforcement Perpendicular to the Grain in
Timber Connections. RILEM Symposium: Joints in Timber Structures. Stuttgart, 2001, S. 163–72
CEN. (2004). Eurocode 5 : Design of timber structures - Part 1-2 : General - Structural fire design.
Brussels, Belgium: European Committee for Standardization.
Chopra, A.K. 1995. Dynamics of Structures – Theory and Applications to Earthquake Engineering.
Prentice-Hall Inc.
Canadian Standard Association. 2009. Engineering Design in Wood. CSA O86. Canadian Standard
Association, Mississauga, Ontario.
--------------------------------------. 2001. Practice Relating Specified Strengths of Structural Members to
Characteristic Structural Properties. CSA O86. Special publication. Revised April 2001.
Canadian Wood Council. 2011. Introduction to Wood Design. Canadian Wood Council. Ottawa, Ontario.
------------------------------. 2010. Wood Design Manual. Canadian Wood Council. Ottawa, Ontario
Hockey, B. and F. Lam, and H.G.L. Prion. 2000. “Truss plate reinforced bolted connections in parallel
strand lumber,” Canadian Journal of Civil Engineering. 27:1150-1161.
Ceccotti, A. 1995. Timber connections under seismic actions. In: Timber engineering–STEP 1. 1st
Edition. STEP/EUROFORTECH. The Netherlands, ISBN 90-5645-001-08. Pp. C17/1-C17/10.
CEN 2004. Eurocode 5: Design of timber structures – Part 1-1: General – Common rules and rules for
buildings (EN 1995-1-1:2004: E).
------ 2001. Eurocode 8 – Design of Structures for Earthquake Resistance, Part -1. European Standard
prEN 1998-1. Draft no. 4. Brussels: European Committee for Standardization.
FPInnovations. 2010. CLT Handbook. Special Publication SP-528E. FPInnovations, Quebec, QC
Karacabeyli, E. 2001. Wood : creep and creep rupture. Reprint: 5p. Encyclopedia of Materials: Science
and Technology : 9616-9620.
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Labonnote, N. 2012. Damping in Timber Structures. PhD Thesis. Faculty of Engineering Science and
Technology, Norwegian University of Science and Technology, Norway.
Hu, L. 2012. Guide for wind-vibration design of wood-frame buildings. Report for project No.
201004697 and for Canadian Forest Service No. FRII-3.19, FPInnovations, Quebec.
Mastschuch, R. 2000. Reinforced Multiple Bolt Timber Connections. M.Sc. Thesis, University of British
Columbia, Vancouver, BC, Canada.
Tlustochowicz, G., Fragiacomo, M., and Johnsson, H. 2012. Provisions for Ductile Behavior of TimberSteel Connections with Multiple Glued-In Rods. J. Struct. Eng., 10.1061/(ASCE)ST.1943541X.0000735.
Mestek P., H. Kreuzinger, and S. Winter (2011) Design concept for CLT - Reinforced with selftapping
screws. In proceedings of the 44th Meeting International Council for Research and Innovation in
Building and Construction Working Commission W18 - Timber Structures CIB W18 Alghero, Italy.
Paper 44 - 7 – 6. 14 pp.
Mohammad, M., W. Muñoz, P. Quenneville, A. Salenikovich. 2010. Stiffness and Ductility of Bolted
Connections. Proceedings of the 11th World Conference on Timber Engineering (WCTE), Italy.
-------------------, P. Quenneville and A. Salenikovich. 2006. Reinforcement of Bolted Timber
Connections using Self Tapping Screws. Proceedings of the 9th World Conference on Timber
Engineering (WCTE), Portland, OR.
Smith, I., A. Aziz, M. Snow and Y.H. Chui. 2006. Possible Canadian /ISO Approach to Deriving design
Values from test data. Proceedings of the CIB-W18 meeting, Florence, August 28-31.
Soltis, L. A., R.J. Ross, and D.F. Windorski. 1998. “Fiberglass-reinforced bolted wood connections,”
Forest Products Journal. 48(9):63-67.
Wood Handbook, 2010. Forest Products Laboratory. General Technical Report FPL-GTR-190. Madison,
WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory: 508 p.
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Advanced Analysis and Testing of Systems for Design
4.3
Lead Authors: Mahmoud Rezai and Marjan Popovski
Co-Authors: Lin Hu, John Sherstobitoff
Peer Reviewers: Ron DeVall, Erol Karacabeyli, John van de Lindt, Jasmine Wang
Abstract
Section 0 provides some of the fundamental background information on advanced analysis of tall wood
buildings. The information guides the reader to available procedures, methodologies, and steps taken for
analysis of tall wood buildings under gravity, wind and seismic loads. For the gravity structural systems,
aspects such as analysis and design approaches, structural integrity, progressive and partial collapse, blast
protection, and compatibility of the gravity system for lateral load demand are discussed. Under analysis
and design of structural systems for earthquake loads, force modification factors are discussed, along with
procedures for determining and suggesting R-factors for dual and hybrid systems. Methods for seismic
analysis appropriate for design of tall wood buildings are also described, along with the input parameters
unique to wood structures needed for the analyses such as effective damping. In addition, the main
aspects of force-based design, displacement-based design, and performance-based design are presented.
The main aspects of capacity-based design procedures when applied to timber structures are also
discussed, along with the impact of wood diaphragm flexibility on the seismic response. The subsection
on analysis and design for wind loads deals with various aspects related to wind design and performance
including static and dynamic analyses, vortex shedding, experimental analyses and testing methods,
deflections and wind-induced vibration-controlled design (deflection controlled and acceleration
controlled), as well as testing of wood systems needed to support wind load analyses and design.
The analysis and design of tall wood buildings require complex and innovative thinking and as such, the
designers should review the available literature and the state-of-the-art on the analysis, design and
detailing for seismic and wind loading. The analysis of tall wood buildings for seismic loading and
detailing for capacity design should utilize nonlinear analysis in conjunction with dynamic and modal
analysis to identify the load-displacement backbone curve of the system in a global sense and the loaddisplacement/rotation/curvature demand on the ductile elements, which in the case of timber structures is
generally the connections. The appropriate force reduction factor used in the design should be established
based on rigorous analysis and experimental testing (if the available data and literature do not adequately
address the system being considered). In a case of hybrid system where ductile elements are defined as
steel or reinforced concrete components while wood elements carry gravity loads only, the analysis and
design for lateral loads should be in line with conventional practice for structures made of respective
materials. The wood-based gravity system, however, shall be compatible with the lateral load resisting
elements in undergoing the lateral drift while carrying gravity loads.
The analysis and design of a tall wood building should be carried out by an experienced and
knowledgeable structural engineering team well versed in dynamic and nonlinear analysis. The analysis
and design performed should be peer reviewed by experts in the field of structural analysis and timber
design.
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4.3.1
National Building Code of Canada
The National Building Code of Canada (NBCC, 2010) is an objective-based National Model Code that
when adopted by provincial and territorial governments, becomes a regulation. The NBCC is a model
code in the sense that it provides consistency among the provincial and territorial building codes. Since
NBCC sets out technical provisions for design and construction of new buildings, the structural and
serviceability performance of all newly built tall wood buildings should comply with the NBCC and
provincial or territorial code requirements.
4.3.1.1
The NBCC code requirements are published in objective-based format by establishing requirements to
address the main code objectives. The objectives describe, in very broad terms, the overall goals what the
NBC’s requirements are intended to achieve and describe undesirable situations and their consequences
that the Code aims to avoid occurring in the buildings. The objectives are classified in four main
categories: Safety (OS), Health (OH), Accessibility (OA), and Fire and Structural Protection (OP). The
structural and serviceability performance of the tall wood buildings should satisfy the NBCC objectives
specified under:
•
•
•
OS2 – Structural safety;
OP2 – Structural sufficiency of the building;
OH4 – Vibration and deflection limitation
The NBCC objective-based code requirements are supported by a set of functional statements that are
more detailed than the objectives. The functional statements describe conditions in the building that help
satisfy the objectives. The functional statements and the objectives are interconnected; there may be
several functional statements related to any one objective and a given functional statement may describe a
function of the building that serves to achieve more than one objective. Like objectives, functional
statements are entirely qualitative and are not intended to be used on their own in the design and approval
process. The structural and serviceability performance of the tall wood buildings should satisfy the NBCC
functional statements under:
•
•
•
•
4.3.1.2
F20 – To support and withstand expected loads and forces;
F22 – To limit movement under expected loads and forces;
F80 – To resist deterioration resulting from expected service conditions;
F82 – To minimize the risk of inadequate performance due to improper maintenance or lack of
maintenance.
Building Code Compliance
The structural design of tall wood buildings should provide compliance to the NBCC, CSA material
standards and provincial and territorial codes by using alternative solutions that will achieve the level of
performance required in the Codes in the areas defined by the objectives in Section 4.3.1.1. These
requirements should be considered as the minimum acceptable measures required to adequately achieving
the listed objectives.
4.3.1.3
Performance Levels
NBCC does not specify exact performance levels for the performance-based design of the buildings under
various loading conditions. If performance-based design solutions are pursued, the designers should use
appropriate performance criteria based on the available literature such as (ASCE7-10) or other building
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codes. However, NBCC does specify objectives and expected performance for seismic design and those
are given below. NBCC Commentary A also has guidelines that should be used for deriving strength and
stiffness properties of new materials. Commentary A indicates that the material resistance of new
materials should be defined on the basis of a 5% exclusion limit and their material stiffness should be
defined on the basis of a 50% exclusion limit; where statistical sampling is used, a 75% confidence level
is recommended for the estimate of the exclusion limit”. It is recommended that these criteria are used for
determining design resistance of new wood products and connections used in tall wood buildings.
The seismic design according to NBCC has the following intents, which are consistent with the overall
objectives of the NBCC:
1.
2.
3.
to protect the life and safety of building occupants and the general public as the building responds
to strong ground shaking;
to limit building damage during low to moderate levels of ground shaking, and
to ensure that post-disaster buildings can continue to be occupied and functional following strong
ground shaking, though minimal damage can be expected in such buildings.
The NBCC Commentary states:
“The damage caused to buildings by earthquake ground motions is a direct consequence of the
lateral deflection of the structural system. The ability of a building to withstand such ground
motions arises largely from the capability of the structural system to deform without significant
loss of load-carrying capacity. Article 4.1.8.13 is concerned with both the determination of
lateral deflections and limits on those deflections to ensure satisfactory performance.”
NBCC then gives explicit guidance in determination of realistic values of anticipated maximum
deflections including the effects of torsion.
The NBCC Commentary also states:
“The deflection parameter that best represents the potential for structural and non-structural
damage is interstorey deflection, also known as interstorey drift. Sentence 4.1.8.13(3) in NBCC
specifies limits on the largest interstorey deflection at any level of the structure. Ordinarily the
limit is 0.025hs (where hs=interstorey height) except for post-disaster buildings and schools, for
which the limits are 0.01hs and 0.02hs respectively. As noted by DeVall (2003), the limit of
0.025hs represents the state of ‘near collapse’ (equivalent to ‘extensive damage’), but not
collapse.”
It should be noted that, cyclic and shake table tests on some wood-based systems have shown that the
near collapse state can occur at much higher storey drifts (Ceccotti, 2008; Karacabeyli and Ceccotti,
1998). The more stringent drift limit of 0.01hs in NBCC for post-disaster buildings reflects the need for
facilities such as hospitals, power generation stations, and fire stations to remain operational following an
earthquake. A detailed discussion on “Performance-based design” is included later in Section 0.
The input ground shaking is defined as having a probability of exceedance of 2% in 50 years at a median
confidence level. This corresponds to a 0.04% annual probability of exceedance. Although stronger
shaking than this could occur, in most situations it is typically economically impractical to design for such
rare ground motions; hence the 2% in 50 year level may be termed as the maximum earthquake ground
motion to be considered, or more simply the Design Ground Motion (DGM). This ground motion should
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be taken as an input for the seismic analysis and design of tall wood structures. Even though design forces
for wind may be greater than seismic design forces in some situations of tall wood buildings (i.e., wind
"governs" the design), the design should clearly define the Seismic Force Resisting System (SFRS) and
detailing that corresponds to the seismic forces calculated for the building according to capacity based
design should be provided.
4.3.2
Analysis and Design for Gravity Loads
4.3.2.1
General Analysis and Design Approach
The general approach for design of tall wood buildings entails column/wall load take-down for gravity
loads such as dead (including superimposed), live and snow loads. The load take-down includes live load
reduction factors, as indicated in NBCC.
There are a number of linear elastic structural analysis programs that can be utilized in modelling of the
system including wall panels and floors to arrive at the design loads for the gravity system (hand
calculations could be done for sanity checks). Construction staging analysis should also be utilized to
ensure the design follows the construction sequencing for loading various support members including
transfer elements. The design of the elements of the gravity system should be conducted according to the
requirements in CSA Standard O86 (2009).
4.3.2.2
Structural Integrity and Progressive/Partial Collapse
NBCC does not currently have any criteria directly related to the structural integrity and collapse of
buildings. However, some material standards such as the CSA A23.3-04 standard that deals with design
of concrete structures provide some requirements for structural integrity of precast panels. The goal of the
design requirements in NBCC and material standards is to ensure that the structures have minimum
interconnectivity of their elements, and that a complete lateral force-resisting system is present with
sufficient lateral strength to provide stability under both gravity and lateral forces. Conformance with
these criteria will provide tall wood buildings with structural integrity for normal service and minor
unanticipated events that may reasonably be expected to occur throughout the lifetime of the structure.
For tall wood buildings that house large numbers of persons, or which house functions necessary to
protect the public safety or occupancies that may be the subject of intentional sabotage or attack, more
rigorous protection may need to be incorporated into designs than provided by these sections of NBCC.
For such structures, for example, additional precautions may be taken in the design of the structures to
limit the effects of local collapse and to prevent or minimize progressive collapse. Some aspects of these
additional precautions are discussed in this section. Requirements from some international building codes
and standards that offer guidance in this area, such as ASCE7-10 and the United Kingdom Building
Regulations (UKBR, 2004), are discussed. Also, basic aspects related to blast protection of buildings are
mentioned.
4.3.2.2.1 Progressive and Disproportional Collapse
Progressive collapse is defined as the spread of an initial local failure from element to element, resulting
eventually in the collapse of an entire structure or a disproportionately large part of it. Some authors have
defined resistance to progressive collapse to be the ability of a structure to accommodate, with only local
failure, the notional removal of any single structural member (e.g., a column/wall). Aside from the
possibility of further damage that uncontrolled debris from the failed member may cause, it appears
prudent to consider whether the abnormal event will fail only a single member. Accidents, misuse, and
disruptions are normally unforeseeable events. Likewise, general structural integrity is a quality that
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cannot be stated in simple terms. Consequently, it is the purpose of this section to direct attention to the
problem of local collapse, present guidelines for handling it that will aid the design engineer, and promote
consistency of treatment in all types of structures and in all construction materials. This Guide does not
intend to establish specific events to be considered during design or for this standard to provide specific
design criteria to minimize the risk of progressive collapse.
In addition to unintentional or willful misuse, some of the incidents that may cause local collapse
(Leyendecker et al., 1976) are explosions caused by ignition of gas or other liquids; boiler failures;
vehicle impact; impact of falling objects; effects of adjacent excavations; gross construction errors; very
high winds such as tornadoes; and sabotage. Generally, such abnormal events would not be a part of
normal design considerations.
There are a number of factors that contribute to the risk of damage propagation in structures (Breen
1976). The most important and most often ones are listed below:
•
•
•
•
•
There is an apparent lack of general awareness among engineers that structural integrity against
collapse is important enough to be regularly considered in design.
To have more flexibility in floor plans and to keep costs down, interior walls and partitions are
often non-load-bearing and hence may be unable to assist in containing damage.
In attempting to achieve economy in structure through greater speed of erection and less site
labor, systems may be built with minimum continuity, ties between elements, and joint rigidity.
In roof trusses and arches there may not be sufficient strength to carry the extra loads or sufficient
diaphragm action to maintain lateral stability of the adjacent members if one collapses.
In eliminating excessively large safety factors, code changes over the past several decades have
reduced the large margin of safety inherent in many older structures. The use of higher-strength
materials permitting more slender sections compounds the problem in that modern structures may
be more flexible and sensitive to load variations and, in addition, may be more sensitive to
construction or design errors.
4.3.2.2.2 Design Alternatives
Experience has demonstrated that the principle of taking precautions in design to limit the effects of local
collapse is realistic and can be satisfied economically. There are a number of ways to obtain resistance to
progressive collapse. In Ellingwood and Leyendecker (1978), a distinction is made between Direct and
Indirect Design. Direct design is related to an explicit consideration of resistance to progressive collapse
during the design process through either Alternate path method or Specific local resistance method.
Alternate path method allows local failure to occur but seeks to provide alternate load paths so that the
damage is absorbed and major collapse is averted. Specific local resistance method, on the other hand,
seeks to provide sufficient strength to resist failure from accidents or misuse. In case of Indirect Design,
this is an implicit consideration of resistance to progressive collapse during the design process through the
provision of minimum levels of strength, continuity, and ductility.
The general structural integrity of a tall wood structure should be tested by analysis to ascertain whether
alternate paths around hypothetically collapsed regions exist. Alternatively, alternate path studies may be
used as guides for developing rules for the minimum levels of continuity and ductility needed to apply the
indirect design approach to enhance general structural integrity. Specific local resistance may be provided
in regions of high risk because it may be necessary for some element to have sufficient strength to resist
abnormal loads for the structure as a whole to develop alternate paths. Specific suggestions for the
implementation of each of the defined methods are contained in Ellingwood and Leyendecker (1978).
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For these analysis, an understanding of the full strength distribution of the wood components involved in
preventing progressive collapse may be useful. Some guidance is provided in Section 4.2 for developing
this information.
4.3.2.2.3 General Building Design Guidelines for Better Structural Integrity
To have good structural integrity, connections between structural components should be ductile and have
a capacity for relatively large deformations and energy absorption under the effect of abnormal loading
conditions. This criterion can be met in many different ways, depending on the structural system used.
Details that are appropriate for resisting low to moderate wind loads and especially seismic loads often
provide sufficient ductility. There are number of ways of designing for the required integrity to carry
loads around severely damaged walls, trusses, beams, columns, and floors. A few examples of the design
concepts and details are listed below:
•
•
•
•
•
•
•
•
Proper plan layout of walls and columns is an important factor in achieving good building
integrity. In wall-bearing structures, there should be an arrangement of interior longitudinal walls
to support and reduce the span of long sections of cross-walls, thus enhancing the stability of
individual walls and of the structures as a whole. In the case of local failure, this will also
decrease the length of walls likely to be affected;
Provide an integrated system of ties among the principal elements of the structural system. These
ties may be designed specifically as components of secondary load-carrying systems, which often
must sustain very large deformations during catastrophic events.
Use of perpendicular walls (returns on walls) and their proper connection to the main walls to
make the main load carrying walls more stable;
When using one-way floor slabs, one should reinforce them to carry some load in the
perpendicular direction as well. In such a case if a support is removed in the principal direction of
the slab, the collapse of the slab will be minimized by providing an alternate load path;
The interior walls must be capable of carrying enough load to achieve the change of span
direction in the floor slabs;
Where single direction slabs cannot be designed to change loading direction (to act as two-way
slabs), the span will increase if an intermediate supporting wall is removed. In this case, enough
resistance in the slab should be present to sustain the increased load due to longer span, although
very large deflections will result and they should be acceptable;
Use redundant structural systems that can provide a secondary load path. Example may be the use
of an upper-level truss or transfer girder system that allows the lower floors of a multistory
building to hang from the upper floors in case of an emergency. Such detailing allows frames at
the bottom floor to survive removal of key support elements;
Avoid low-ductility detailing in all elements that might be subject to dynamic loads or very large
distortions during localized failures. Ductile connections will be able to better cope with
implications of increased loading under the influence of building weights falling from above;
4.3.2.2.4 United Kingdom Regulations on Disproportionate Collapse
CSA Standard O86 has a general design requirement for Structural Integrity:
The general arrangement of the structural system and the interconnection of its members shall
provide positive resistance to widespread collapse of the system due to local failure.
Other than this statement, there are no other requirements or guidance in the NBCC and CSA material
standards. When considering the structural system for the first tall wood buildings, it is useful to keep in
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mind what measures can be utilized to minimize the changes of disproportionate collapse. This section
provides a brief summary of existing code regulations in this field that are part of the UK Building
Regulations, so that the designers can refer to them if disproportional collapse assessment is needed.
The UK Building Regulations (UKBR, 2004) has led with requirements for the avoidance of
disproportionate collapse, which were formulated in the aftermath of the 1968 Ronan Point collapse.
These provisions may be used by designers for designing tall wood buildings. The requirements include:
a) prescriptive “tying force” provisions which are deemed sufficient for the avoidance of disproportionate
collapse, b) “notional member removal” provisions which need only be considered if the tying force
requirements could not be satisfied, and c) “key element” provisions applied to members whose notional
removal causes damage exceeding the prescribed limits.
It should be noted that some UK designers already have commented on the shortcomings of the
provisions (Izzuddin et al., 2007). First, the tying force provisions are unrelated to real structural
performance. Second, the ductility considerations are neglected at all levels of the provisions. Indeed, the
tying force requirements are intended to provide resistance to gravity loading by means of catenary action
upon removal of a vertical member, yet the associated ductility demands for specific structural systems
can be unrealistically large, thus rendering the provisions unsafe. On the other hand, the alternative
notional member removal provisions are more performance-based, but these are applied with
conventional design checks, and hence they ignore the beneficial effects of such nonlinear phenomena as
catenary and arching actions. Third, the notional member removal provisions assume a static structural
response, when the failure of vertical members under extreme events, such as blast and impact, is a highly
dynamic phenomenon. In this respect, sudden column loss represents a more appropriate design scenario,
which includes the dynamic influences yet is event-independent. While such a scenario is not identical in
dynamic effect to column damage resulting from impact or blast, it captures the influence of column
failure occurring over a relatively short duration, and it can also be considered as a standard dynamic test
of structural robustness.
When a multi-storey building is subjected to sudden column loss (Figure 93), the ensuing structural
response is dynamic, leading for the real buildings to have a considerable concentration of deformations
in the connections within the floors above, provided the remaining columns can take the redistributed
gravity load. The failure of these floors on the lower parts of the structure is largely determined by the
maximum deformation demands on the connections in relation to their ductility capacity. This mode of
failure defines a limit state which forms the basis for quantifying the robustness of multi-storey buildings
under sudden column loss scenarios. The proposed approach can be applied in three main stages for
evaluating the above limit state:
i.
ii.
iii.
Use of nonlinear static modeling and response, which considers the damaged structure under
gravity loading;
Simplified dynamic analysis to estimate the maximum dynamic response; and
Ductility assessment, which establishes the ductility demand in connections at the maximum
dynamic response and compares it to the ductility capacity.
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Figure 93
An example of sudden column loss in a multi-storey building.
4.3.2.2.5 Blast Protection of Buildings
NBCC doesn’t have any criteria directly related to the design of buildings for blast protection. If such
design is needed for a tall wood building, engineers are encouraged to obtain information either from the
CSA standard S850 on Design and Assessment of Buildings Subjected to Blast Loads, or from the
ASCE/SEI Standard 59-11: Blast Protection of Buildings. Some aspects of blast protection that can be
related to the general robustness and structural integrity of the building include:
•
•
•
4.3.2.3
Provide additional element (wall) resistance and robustness to resist blast and load reversal when
blast loads are considered in the design;
Consider the use of compartmentalized construction in the design of new buildings when
considering blast protection;
Although not directly adding structural integrity for the prevention of progressive collapse, the
use of special, non-frangible glass for fenestration can greatly reduce risk to occupants during
exterior blasts. To the extent that non-frangible glass isolates a building’s interior from blast
shock waves, it can also reduce damage to interior framing elements (e.g., supported floor slabs
could be made to be less likely to fail due to uplift forces) for exterior blasts.
Wall/column to Foundation Interface
The connection of walls/columns carrying gravity loads to the footings is typically accommodated using
steel plates with direct bearing on concrete. The interface bearing stresses on wood (e.g., bearing stresses
including eccentricities) and concrete (e.g., punching shear) must be examined for code check. A
minimum axial load eccentricity of 5% of the wall/column width should be considered for the gravity
loads.
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The interface of soil and concrete footing may also be modelled, as area springs, in consultation with the
geotechnical consultant providing upper and lower bounds of the soil modulus for the subgrade.
4.3.2.4
Testing to Support Gravity Load Analyses and Design
Where the information from the literature is lacking, testing should be conducted for gravity load carrying
elements to identify the axial/buckling strength of the gravity system (e.g., columns/walls). The testing
should be conducted full scale, as assembly and multi-bay multi-storey systems.
When a new gravity system or assembly is used in a tall wood building to support gravity loads, testing
(or combination of testing and analysis) should be conducted to determine the main system properties and
performance such as:
1. Axial/buckling resistance (strength) of the main gravity load carrying elements (e.g., columns or
walls);
2. Resistance and performance of the connections connecting the vertical-to-horizontal elements
(e.g., beam-to-column connections);
3. Ability of the gravity system to “ride along” with the Lateral Load Resisting System (LLRS)
when the LLRS is subjected to lateral loads, and be able to sustain the gravity loads at the largest
expected lateral drifts;
4. Susceptibility of the gravity system to compression perpendicular to grain stresses (if applicable);
5. Susceptibility of the gravity system to duration of load and creep deformation;
6. Deformation and vertical movement compatibility of the LLRS and the gravity one, especially if
the LLRS is a non-wood-based system,
7. Any effects on the system due to shrinkage and swelling,
The testing should be conducted in full scale on a connection, element and assembly level. A full-scale
testing on a system level is also desirable, if possible, consisting of at least two-bay, two storey systems.
When designing the testing program, the inherent variability of the wood should be taken into
consideration. It is recommended that testing is conducted at an accredited laboratory and that the test
plan be developed in consultation with experts familiar with the wood products. More information on
sampling, testing of connections and assemblies, and the parameters that need to be obtained from testing
is included in Section 0 of the Guide.
4.3.2.5
Compatibility of Gravity System for Lateral Load Demand
It is of paramount importance to ensure that the gravity load carrying system in tall wood buildings can
accommodate the lateral drift associated with the seismic response of the buildings. The building drift
would produce secondary forces and moments in the gravity system that must be taken into account in the
design. It is noted that the larger and stiffer the gravity load carrying system, the more it will interact with
the SFRS in a tall wood building. The entire structural system should be designed to sustain the
anticipated P-δ effects.
Sprinkler systems should also be designed to accommodate the deflections/drift arising from the seismic
loads and be functional after the design earthquake to limit the damage of potential post-earthquake fires.
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4.3.3
Analysis and Design for Earthquake Loads
4.3.3.1
Seismic Force Resisting Systems SFRS and Force modification factors
The earthquake forces acting on the buildings are resisted by Seismic Force Resisting Systems (SFRSs).
One of the key steps in the seismic design of tall wood buildings is the determination of force
modification factors for the structural system that will be used. In the 2010 edition of the National
Building Code of Canada (NBCC, 2010) there are three wood-based SFRS included in the table of
systems, namely: wood-frame shearwalls, braced frames and moment resisting frames. Each of these
systems has its own force modification factors (R-factors) based on their seismic performance and design
detailing. The force modification factors for wood-frame shearwalls and braced timber frames in
Canadian codes were validated based on full scale testing and dynamic analysis in the last two decades
(Karacabeyli and Ceccotti, 1997; Ceccotti and Karacabeyli, 2002; Popovski et al. 2003; Popovski 2008).
In an effort to develop and validate procedures for Performance Based Seismic Design (PBSD) of woodframe buildings, a shake table tests on a six storey wood-frame building on one storey steel podium were
conducted as a part of the NEESWood project in the US (Figure 94). The six-storey building performed
well at maximum considered earthquake level sustaining only gypsum wall board damage and no
structural damage. This level of performance satisfied the performance expectations outlined during the
design process thereby validating the PBSD philosophy used to design the building (van de Lindt et al.
2010). In an effort to raise the 4-storey limit for the platform wood-frame construction in British
Columbia and Canada to six storeys, an analytical study was undertaken to quantify the seismic
performance of 4-staorey and 6-storey wood-frame buildings (Ni et al. 2010). The results have shown that
the seismic performance of the 6-storey buildings is similar if not superior of that of the 4-storey
structures.
Figure 94 Six storey wood-frame plus steel podium building tested as a part of the NEESWood
project
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The R-factors in general account for the over-strength and the capability of the structure to undergo
ductile nonlinear response, which dissipates energy and increases the building period. This allows the
structure to be designed for seismic forces smaller than the forces that would be generated if the structure
remained elastic, without increasing the displacements from the seismic loads. In NBCC the elastic
seismic load is reduced by two types of R-factors, the 𝑅𝑜 -factor that is related to the over-strength of the
system and 𝑅𝑑 -factor that is related to the ductility of the structure. The R-factors for each SFRS reflect
their seismic performance during past earthquakes, and the ability to undergo nonlinear response with
limited loss of strength as the structure goes through several cycles of motion. Historically, there is little
theoretical or experimental background currently given in the codes for determining the numerical values
of the R-factors. Consequently, the process of assignment of R-factors requires considerable committee
judgment of individual systems. Most of the current values for the R-factors in the building codes are
based on past seismic performances of the structural system and some results from non-linear time history
dynamic analyses, if available.
If the tall wood building uses an established concrete, steel or masonry SFRS while the wood system
carries only the gravity loads, the SFRS should be designed using the R-factors in NBCC and the design
guidelines in the applicable CSA material standards. In case when a wood-based SFRS not included in
NBCC is used, the designer has to decide what R-factors for that system should be used (this is based on
the available information in the literature; some methods are provided in the subsequent sections). The
𝑅𝑜 -factor that is related to the over-strength can be calculated using the equation [5] given in Mitchell
et.al. (2003).
where:
𝑅𝑜 =𝑅𝑠𝑖𝑧𝑒 𝑅𝜑 𝑅𝑦 𝑅𝑠ℎ 𝑅𝑚𝑒𝑐ℎ
[5]
𝑅𝑠𝑖𝑧𝑒 is the over-strength arising from restricted choices for sizes of members and elements and rounding
of sizes and dimensions; 𝑅𝜑 is a factor accounting for the difference between nominal and factored
resistances, equal to 1/𝜑, where φ is the material resistance factor as defined in the CSAO86 standard; 𝑅𝑦
is the ratio of “actual” yield strength to minimum specified yield strength; 𝑅𝑠ℎ is the overstrength due to
the development of strain hardening; and 𝑅𝑚𝑒𝑐ℎ is the overstrength arising from mobilizing the full
capacity of the structure such that a collapse mechanism is formed.
In an effort to quantify the seismic performance of CLT structures, full scale shaking table tests were
conducted on a three storey and seven storey CLT structure (Figure 95). The tests were conducted as a
part of the SOFIE project of the Trees and Timber Institute of the National Research Council of Italy in
collaboration with National Institute for Earth Science and Disaster Prevention in Japan (NIED),
Shizuoka University, and the Building Research Institute (BRI) in Japan. Shake table tests on a 3-storey
house conducted in the laboratories of the NIED in Tsukuba, Japan showed that the CLT construction
survived 15 destructive earthquakes without any severe damage (Ceccotti and Follesa, 2006). Based on
the tests carried out to-date in Europe, Japan and Canada (Popovski et al., 2011) and the corresponding
analytical studies (Pei et al., 2012 and 2013), the recommended R-factors for Cross-Laminated Timber
(CLT) tall buildings of a platform type with ductile connectors such as nails and screws, are 𝑅𝑜 = 1.5 and
is 𝑅𝑑 = 2.0. The proposed force reduction factors are to be used for CLT walls that respond in
predominantly rocking motion, have aspect ratios (panel height vs. length) of 1:1 or higher, and have
hold-downs or other devices to take the overturning moments.
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Figure 95 Six storey plus attic CLT building tested as a part of the SOFIE project
Currently, there are no specific procedures in Canada that deal with development of 𝑅𝑑 -factors for new
systems. There are some procedures that have been developed in the US that are suggested for the
development of 𝑅𝑑 -factors. A brief summary of these suggested procedures is given below.
Designers are cautioned that there is a difference between the R-factors in Canadian and US codes with
the US ones having generally higher values for the R-factors. R-factors should always be used in the
context of the code as they represent more than just the ductility of the system. Also, these factors must be
used only in conjunction with the corresponding ground motion design level. Basically for tall wood
buildings there needs to be a systematic analytical and testing investigation to arrive at a suitable force
modification factors based on the ductility/detailing levels and the robustness of hysteresis loops for the
ductile elements/connections.
4.3.3.1.1 FEMA P-695 Procedure
Applied Technology Council (ATC) Project 63 in the USA has developed the FEMA P-695 document
(FEMA, 2009) that contains a procedural methodology where the inelastic response characteristics and
performance of typical structural systems could be quantified, and the adequacy of the structural system
provisions to meet the desired safety margin against collapse. The methodology directly accounts for the
potential variations in structural configuration of buildings, the variations in ground motion to which
these structures may be subjected, and the available laboratory data on the behavioural characteristics of
structural elements. The developed procedure establishes a consistent and rational method for evaluating
of building system performance and the response parameters (R, Cd, Ω0) used in current building codes in
the USA. The primary application of the procedure is for the seismic evaluation of new structural
systems, so that they have equivalent margin against collapse for the maximum considered earthquake
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(Figure 96). The drawbacks of the FEMA P695 procedure are that it is quite complex, very time
consuming and therefore very expensive. A large number of non-linear dynamic analyses are required on
a number of different building models with different configurations. In addition, the types of analyses
required are sophisticated and that FEMA P-695 requires peer panel oversight throughout the process.
Figure 96 Schematic flowchart of FEMA P-695 methodology for system performance assessment
4.3.3.1.2 FEMA P-795 Procedure
ATC also led the development of the FEMA P-795 methodology (FEMA, 2011) for evaluating the
seismic performance equivalency of structural elements, connections, or subassemblies whose inelastic
response controls the collapse performance of a seismic-force-resisting system. The recommended P-795
Component Equivalency Methodology (referred to as the Component Methodology) is a statistically
based procedure for developing, evaluating and comparing test data on new components (proposed
components) that are proposed as substitutes for selected components (reference components) in a current
code-approved seismic force resisting system. The Component Methodology is derived from the general
methodology contained in FEMA P-695. Like the general methodology in FEMA P-695, the intent of the
Component Methodology is to ensure that code-designed buildings have adequate resistance against
earthquake-induced collapse. In the case of component equivalency, this intent implies equivalent safety
against collapse when proposed components are substituted for reference components in the reference
SFRS. The equivalency is established in terms of the component’s resistance, ductility capacity and
energy dissipation. Proposed components found to be equivalent by the Component Methodology can be
substituted for components of the reference SFRS, subject to design requirements and seismic design
category restrictions on the use of the reference SFRS. Reference SFRSs include the seismic-forceresisting systems contained in ASCE/SEI 7-10 Minimum Design Loads for Buildings and Other
Structures (ASCE, 2010). In general this methodology is much easier to use and can be applied to
introduce any new substituting element in a structural system that is already implemented in the code
(Figure 97).
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Figure 97 Conceptual boundaries defined by applicability criteria of the Component Methodology
4.3.3.1.3 AC-130 Equivalency Approach
According to the ICC-ES acceptance criteria document AC130 (ICC-ES, 2009), assignment of an Rfactor for new prefabricated wood shear-resisting wall assemblies can be made by showing equivalency in
their seismic performance criteria (maximum load, ductility, storey drift, etc.) obtained from quasi-static
cycle tests, compared to the same properties already observed from tests on lumber-based nailed shear
walls. Although the main lateral load resisting elements or assemblies of the new structural system used
in a tall wood building will most likely differ from wood frame shear walls, the equivalency criteria given
in AC-130 can be used in assessing the seismic behaviour of the new assemblies, since the criteria are
equivalency-based ones. To use the AC 130 equivalency approach to assess the performance of the new
system assemblies, an assumption should be made that the design values (lateral resistances) for the new
system will have the same “safety margin” as that of regular wood-frame shear walls according to CSA
O86. In other words, it should be assumed that the design values for lateral load resistance of the new
system are derived in the same way as if they were determined for wood-frame shear walls. The specified
strengths for shear walls in Canada were soft converted from the Allowable Stress Design (ASD) values
of the Uniform Building Code (UBC) in the USA. The ASD values in UBC were derived using the
average maximum load obtained from monotonic pushover tests divided by a safety factor of 2.8, or the
average maximum load from cyclic tests divided by a safety factor of 2.5. The same factors are
recommended for use for developing design values for the new system that is evaluated. If the new
system satisfies the AC-130 equivalency criteria, then the new system can be assigned the same R-factors
as those for wood-frame shearwalls for preliminary analyses. It should be noted that performance
equivalency on a component level does not necessarily mean equivalency on a system level.
4.3.3.1.4 Other Procedures
To calibrate the ductility related force modification factor (𝑅𝑑 -factor) for the new wood-based system,
designers can also use the preliminary procedure that was presented in Pei et al. (2013). The procedure
assumes that the drift is the main parameter that directly relates to structural damage (and failure) of the
structural system and the appropriate 𝑅𝑑 -factor is chosen based on the accepted probability of exceeding a
defined maximum drift limit that is related to collapse prevention state. The main steps of the procedure
are the following:
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1.
2.
3.
4.
5.
6.
7.
8.
Develop an 𝑅𝑜 -factor for the system based on the equation [5];
Develop design (resistance) values for the main LLRS based on cyclic tests of its main
components. A design level in the range of 35 to 40% of the ultimate load is deemed appropriate;
Determine the seismic demand for the structure according to the Equivalent Static Force
Procedure in 2010 NBCC for the given seismicity of the location and a series of 𝑅𝑑 -factors (say
1.5, 2.0, 2.5 and 3.0);
Develop a building design and an analytical model in a computer software for each selected
𝑅𝑑 -factor;
Choose a suite of earthquake records for the site to be used in conducting the non-linear dynamic
analyses. The number of records should be larger than seven;
Conduct the non-linear dynamic analyses;
Based on the results from the analyses, develop the fragility curves (cumulative distribution
curves) for each of the buildings (models) with different 𝑅𝑑 -factors;
Choose the appropriate 𝑅𝑑 -factor based on the probability of non-exceedance of a certain value of
the storey drift. An 𝑅𝑑 -factor that will allow the building to have 10% probability of exceeding
the maximum specified drift limit under a 2% in 50 years earthquake can be chosen as an
appropriate one.
The main drawbacks of this procedure (similar to that of FEMA P-695) are that it is complex and
relatively time consuming. The procedure may be applied to the building of interest, or for the general
structural system used in the building. For development of R-factors for a structural system in general, the
procedure might not be considered robust enough unless a large portfolio of representative building
archetypes is used.
It should be noted that the Commentary of NBCC Section 4.1.8.13(3) specifies a interstorey drift limit at
near collapse state, but not the corresponding probability of non exceedance of this limit for a building
designed according to it. The Commentary of NBCC suggests a drift of 2.5% as near collapse state,
however, based on the experience with wood-based systems, a drift limit of 3 to 4% is deemed
appropriate as a near collapse drift limit. Regarding the probability of failure, ASCE 7-10 in the US, for
example, requires that buildings designed according to it have a probability of failure of 10%. This is also
approximately in line with the accepted probability of failure in FEMA P-695 procedure.
4.3.3.1.5 R-factors for Dual and Hybrid Systems
If the LLRS consists of two different systems that have different R-factors (dual or hybrid system),
NBCC requires that the dual system is designed with the lower R-factors of the two. It is recommended
that this approach is used in case of tall wood buildings with dual or hybrid systems. As this solution is a
conservative one, some research has been conducted on assuming intermediate values for the hybrid
system consisting of mid-rise wood-frame shearwalls and portal frames (Chen et al., 2013); however, this
method still needs to be proved for other systems and higher buildings.
In cases of tall wood hybrid structural systems where the entire SFRS is made of material other than
wood, such as steel or concrete, the SFRS should be designed according to the applicable material
standard. If such system is included in the table of SFRSs in NBCC, the appropriate R-factors should be
used. If the non-wood SFRS is not listed in NBCC, derivation of the R-factors should follow the same
procedures as presented in previous sections. Compatibility of the non-wood SFRS and the wood-based
gravity system should be taken in the account in the design, such as the ability for the gravity system to
undergo the required lateral deformation of the SFRS without collapse.
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4.3.3.2
Methods for Seismic Analysis
In order to design a structure to withstand an earthquake the forces on the structure must be specified. The
exact forces that will occur during the life of the structure cannot be known. A realistic estimate is
important, however, since the cost of construction, and therefore the economic viability of the project
depends on a safe and cost efficient final product. The seismic forces in a structure depend on a number of
factors including the size and other characteristics of the earthquake, distance from the fault, site geology,
and the type and characteristics of lateral load resisting system. The need for immediate post-earthquake
use and the consequences of failure of the structure may also be of a concern in the design. These factors
should be included in the specification of the seismic design forces. The most common procedures used
for analysis of the buildings under seismic loads are given in this section.
4.3.3.2.1 Equivalent Static Procedure
The response of a structure to earthquake-induced forces is a dynamic phenomenon. Consequently, a
realistic assessment of the design forces can be obtained only through a dynamic analysis of the building
models. Although this has long been recognized, and the dynamic analysis is a default analysis in 2010
NBCC, it is used infrequently in routine designs of timber buildings, and the equivalent static procedure
as defined in Section 4.1.8.11 of NBCC are normally used. In the equivalent static force procedure, the
inertial forces are specified as equivalent static forces using empirical formulas. The empirical formulas
do not explicitly account for the "dynamic characteristics" of the particular structure being analyzed. The
formulas were, however, developed to adequately represent the dynamic behaviour of what are called
"regular" structures, which have a reasonably uniform distribution of mass and stiffness and vibrate
predominantly in their fundamental mode in each direction. The main aspects of the equivalent static
procedure are the following:
1.
2.
3.
4.
5.
6.
The fundamental period of the building is calculated based on the given empirical formulae, or
other established methods of mechanics with upper limit as specified in NBCC;
The elastic design base shear for the structure is determined based on the seismic weight of the
building; spectral acceleration value for the location determined from a uniform hazard linear
elastic response spectrum;
The minimum lateral design force for the building is then calculated as the elastic base shear
divided by two force modification factors, 𝑅𝑑 -factor related to the system ductility and 𝑅𝑜 -factor,
related to the system over-strength;
The storey shear forces are distributed along the height of the building using the inverse
triangular distribution;
Higher mode effects are simplified by adding an additional force at the top of the building and the
higher mode factor 𝑀𝑣 ;
Overturning moments and torsional effects of the building are calculated using simplified
formulae.
Equivalent static analysis can, therefore, work well for some tall wood building that satisfy the criteria in
NBCC for use of such analysis stated in NBCC 4.1.8.7.1. Such structures also should not have significant
coupling of the lateral and torsional modes. For buildings taller than 60 m and with periods higher than
2s, or in all buildings where second and higher modes and torsional effects are important, a dynamic
analysis should be used to specify and distribute the seismic design forces.
In wood structures the non-structural elements and components can have a profound effect on the
building period, and the influence of non-structural components most-probably will have to be taken into
account when determining the period of the tall wood structure. According to Section 4.1.8.3 of NBCC, if
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stiffness is imparted on the Seismic Force Resisting System (SFRS) from elements that are not part of the
SFRS should be used in calculating the period of the structure if the added stiffness decreases the
fundamental period of the building by 15%. Such elements that are not part of the SFRS should also be
taken into account when calculating the irregularity of the structure.
4.3.3.2.2 Linear Dynamic Analyses
There are two types of linear dynamic analyses: Response spectrum or modal analysis, and Time history
linear dynamic analysis. Both analyses can be performed using most of the commercially available
software used for structural analysis and design. Linear elastic models of the tall wood structures that
include structural elements and connections need to be developed in the chosen software package to
perform these types of analyses.
4.3.3.2.2.1
Modal analysis
With presence of powerful desktop computers, this type of analysis has become the norm in analysing the
seismic behaviour of tall buildings, and as such it is a default analysis in NBCC. Modal analysis is
applicable for calculating the linear response of multi-degree-of-freedom structures and is based on the
fact that the building response is superposition of the responses of individual natural modes of vibration,
with each mode responding with its own particular pattern of deformation (the mode shape), with its own
frequency (the modal frequency), and with its own modal damping. The response of the structure,
therefore, can be modeled as the response of a number of single-degree-of-freedom oscillators with
properties chosen to be representative of the mode and the degree to which the mode is excited by the
earthquake motion. For certain types of damping, this representation is mathematically exact. Numerous
full-scale tests and analyses of earthquake response of structures have shown that the use of modal
analysis, with viscously damped single-degree-of-freedom oscillators describing the response of the
structural modes, is an accurate approximation of the building’s linear response.
The purpose of modal analysis is to obtain the maximum response of the structure in each of its important
modes, which are then summed in an appropriate manner. This maximum modal response can be
expressed in several ways. In NBCC the modal forces and their distribution along the height of the
structure are emphasised to highlight the similarity to the equivalent static method traditionally used in
the code. This correlation helps clarify the fact that the simplified modal analysis is simply an attempt to
specify the equivalent lateral forces on a structure in a way that directly reflects the individual dynamic
characteristics of the structure. Once the story shears and other response variables for each of the
important modes are determined and combined to produce design values, the design values are used in
basically the same manner as the equivalent lateral forces.
For many wood buildings of a moderate height, three modes of vibration in each direction may be
sufficient to determine the earthquake response of the structure and the design forces. For high-rise
structures, however, more than three modes may be required to adequately determine the forces for
design. A simple rule may be used that the combined participating mass of all modes considered in the
analysis should be equal to or greater than 90 percent of the effective total mass in each of two orthogonal
horizontal directions of the building.
Natural periods of vibration are required for each of the modes used in the subsequent calculations. The
periods of the modes contemplated are those associated with moderately large, but still essentially linear,
structural response. The period calculations (models) should include only those elements that are effective
at these amplitudes. Such periods are usually longer than those obtained from ambient vibrations on
completed structures because the later include the stiffening effects of non-structural and architectural
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components of the structure at small amplitudes. During response to strong ground-shaking, however,
measured responses of structures have shown that the periods lengthen, indicating the loss of the stiffness
contributed by the non-structural components.
A wide variety of methods for calculation of natural periods of linear models and associated mode shapes
exist, so no particular method or software is suggested here. It is essential, however, that the software
employs one of the generally accepted principles of linear dynamics such as those given in well-known
textbooks on structural dynamics and vibrations (Clough and Penzien, 1975; Newmark and Rosenblueth,
1971; Chopra, 2011), and that the software’s accuracy and reliability are well documented and widely
recognized.
A central feature of modal analysis is that the earthquake response is considered as a combination of the
independent responses of the structure vibrating in each of its important modes. As the structure vibrates
back and forth in a particular mode at the associated period, it experiences maximum values of base shear,
story drifts, floor displacements, base (overturning) moments, etc. Modal forces at each level are
expressed in terms of the seismic weight assigned to the floor, the mode shape, and the modal base shear.
Design values are usually obtained using the square root of the sum of the squares (SRSS) of the modal
quantities, or similar methods. Although such analysis gives satisfactory results, it should be noted that
this may not always be a conservative predictor of the earthquake response as more adverse combinations
of modal quantities than are given by this method of combination can occur. The most common instance
where combination by use of the SRSS method is unconservative occurs when two modes have nearly the
same natural period. In this case, the responses are highly correlated and the designer should consider
combining the modal quantities more conservatively (Newmark and Rosenblueth, 1971). The complete
quadratic combination (CQC) technique provides somewhat better results than the SRSS method for the
case of closely spaced modes.
4.3.3.2.2.2
Linear response history analysis
Linear response history analysis, also commonly known as linear time history analysis, is a numerically
involved technique in which the response of a linear structural model to a specific earthquake ground
motion record is determined through a process of numerical integration of the equations of motion. The
ground shaking record is digitized into a series of small time steps, typically on the order of 1/100th of a
second or smaller. Starting at the initial time step, a numerical integration algorithm is followed to allow
the calculation of the displacements of each node and the forces in each element of the model for each
time step of the record.
The principal advantages of response history analysis, as opposed to response spectrum analysis, is that
response history analysis provides a time dependent history of the response of the structure to a specific
ground motion, allowing calculation of the path dependent effects such as damping. It also provides
information on the stress and deformation state of the structure throughout the entire period of the
response. A response spectrum analysis, however, indicates only the maximum response quantities and
does not indicate when during the response period these occur, or how response of different portions of
the structure is phased relative to that of other portions. Response history analyses are highly dependent
on the characteristics of the individual ground shaking records and subtle changes in these records can
lead to significant differences with regard to the predicted response of the structure. This is why, when
response history analyses are used in the design process, it is necessary to run a suite of ground motion
records. The use of multiple records in the analyses allows observation of the difference in response,
resulting from differences in record characteristics. Although NBCC doesn’t specify the number of
records to be used in this type of analysis, a suite of minimum 7 different records (ASCE7-10) should be
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used. Suites containing even larger numbers of records are preferable, since when more records are run, it
is more likely that the differing response possibilities for different ground motion characteristics are
observed. In order to encourage the use of larger suites, it is required that when a suite contains fewer
than seven records, the maximum values of the predicted response parameters be used as the design
values. When seven or more records are used, then mean values of the response parameters may be used.
This can lead to a substantial reduction in design forces and displacements and typically will justify the
use of larger suites of records. Where possible, ground motion records should be scaled from actual
recorded earthquake ground motions with characteristics (earthquake magnitude, distance from causative
fault, and site soil conditions) similar to those which control the design earthquake for the site. There are
extra complexity and cost associated with the use of response history analysis rather than modal response
spectrum analysis. As a result, this procedure is rarely used in the design process. One exception is for the
design of structures with energy dissipation systems comprising linear viscous dampers. Linear response
history analysis can be used to predict the response of structures with such systems, while modal response
spectrum analysis cannot.
The major advantages and disadvantages of the response spectrum analysis (RSA), compared with the
more complex time-history analysis described later, can be summarised as follows:
1. In case of RSA, the size of the problem is reduced to finding only the maximum response of a
limited number of modes of the structure, rather than calculating the entire time history of
responses during the earthquake. This makes the problem much more tractable in terms both of
processing time and (equally significant) size of computer output;
2. Examination of the mode shapes and periods of a structure gives the designer a good feel for its
dynamic response
3. The use of smoothed envelope spectra makes the analysis independent of the characteristics of a
particular earthquake record;
4. RSA can very often be useful as a preliminary analysis, to check the reasonableness of results
produced by linear and non-linear time-history analyses.
Offsetting these advantages are the following limitations:
1. RSA is essentially linear and can make only approximate allowance for nonlinear behaviour;
2. The results are in terms of peak response only, with a loss of information on frequency content,
phase and number of damaging cycles, which have important consequences for low-cycle fatigue
effects. Moreover, the peak responses do not generally occur simultaneously; for example, the
maximum axial force in a column at mid-height of a moment-resisting frame is likely to be
dominated by the first mode, while its bending moment and shear may be more influenced by
higher modes and hence may peak at different times;
3. Recall also that the global bending moments calculated by RSA are envelopes of maxima not
occurring simultaneously and are not in equilibrium with the global shear force envelope;
4. Variations of damping levels in the system (for example, between the structure and the supporting
soils) can only be included approximately.
5. Modal analysis as a method begins to break down for damping ratios exceeding about 0.2,
because the individual modes no longer act independently (Gupta 1990).
4.3.3.2.3 Nonlinear Static Analyses
In performance assessment and design verification of building structures, approximate nonlinear static
procedures (NSPs) (also known as pushover analysis) are becoming common engineering practice to
estimate seismic demands. In fact, some seismic codes have begun to include them to aid in performance
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assessment of structural systems (e.g., Eurocode 8; Japanese Design Code). Although seismic demands
are best estimated using nonlinear time-history analyses, NSPs are frequently used in ordinary
engineering applications to avoid the intrinsic complexity and additional computational effort required by
the former. As a result, simplified NSPs recommended in ATC-40 and FEMA-356 have become popular.
These procedures are based on monotonically increasing predefined load patterns on the defined nonlinear building model until some target displacement is achieved.
In Nonlinear Static Procedures, the basic demand and capacity parameter for the analysis is the lateral
displacement of the building. The generation of a capacity curve (base shear vs. roof displacement defines
the capacity of the building uniquely for an assumed force distribution and displacement pattern. It is
independent of any specific seismic shaking demand and replaces the base shear capacity of conventional
design procedures. If the building displaces laterally, its response must lie on this capacity curve. A point
on the curve defines a specific damage state for the structure, since the deformation for all components
can be related to the global displacement of the structure. By correlating this capacity curve to the seismic
demand generated by a specific earthquake or ground shaking intensity, a point can be found on the
capacity curve that estimates the maximum displacement of the building the earthquake will cause. This
defines the performance point or target displacement. The location of this performance point relative to
the performance levels defined by the capacity curve indicates whether or not the performance objective
is met. Thus, for the Nonlinear Static Procedure, a static pushover analysis is performed using a nonlinear
analysis program for an increasing monotonic lateral load pattern. An alternative is to perform a step by
step analysis using a linear program. The base shear at each step is plotted against the roof displacement.
The performance point is found using the Capacity Spectrum Procedure (Chopra et al.). The individual
structural components are checked against acceptability limits that depend on the global performance
goals. The nature of the acceptability limits depends on specific components. Inelastic rotation is typically
one of acceptability parameters for beam and column hinges. The limits on inelastic rotation are based on
observation from tests and the collective judgement of the development team.
However, these simplified procedures based on invariant load patterns are sometimes inadequate to
predict inelastic seismic demands in buildings when modes higher than the first mode contribute to the
response and when the inelastic effects alter the height-wise distribution of inertia forces (Gupta and
Kunnath, Goel and Chopra). In order to overcome some of these drawbacks, a number of enhanced
procedures considering different loading vectors (derived from mode shapes) were proposed (Kalkan and
Kunath 2006). These procedures attempt to account for higher mode effects and use elastic modal
combination rules while still utilizing invariant load vectors.
4.3.3.2.4 Nonlinear Dynamic Analysis
This method of analysis is very similar to linear response history analysis, described earlier, except that
the mathematical (analytical) model of the structural component is a non-linear one. In other words the
model is formulated in such a way that the stiffness, strength, and even connectivity of the elements is
directly modified based on the deformation state of the structure. This permits the effects of element or
connection yielding and other nonlinear behaviour of structural response to be directly accounted for in
the analysis. It also permits the evaluation of other nonlinear behaviour such as foundation rocking,
opening and closing of gaps, and nonlinear viscous and hysteric damping. This ability to directly account
for these various nonlinearities allows nonlinear response history analysis to provide relatively accurate
evaluation of the response of the structure subjected to strong ground motion provided the model is set up
correctly.
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In wood structures, the connections are usually the main providers of the non-linearity in the system.
Adequate non-linear models should be chosen that properly represent the non-linear performance of the
connections or assemblies. The models chosen should adequately account for the strength, stiffness and
ductility of the modeled connection/component in both the initial (virgin) cycle, as well as in all
subsequent cycles (Figure 98). The model should also properly account for the strength and stiffness
degradation as well as the hysteretic properties of the modeled connection/component. The models used
in the analyses should be verified against available test data.
Load
Virgin curve
Connector moving
through gap
(reduced stiffness)
Deformation
Strength
deterioration
Connector
regaining contact
(wood crushing)
Figure 98 Typical story of a CLT structure with various connections between the panels (drawing
courtesy of A. Ceccotti)
Similar to its linear counterpart, when nonlinear response history analysis is used in the design process, a
suite of at least seven ground motion records must be considered. It would be also appropriate to perform
sensitivity studies, in which the assumed hysteretic properties of elements are allowed to vary, within
expected bounds, to allow evaluation of the effects of such uncertainties on predicted response. Although
use of nonlinear time history analysis in the design process is usually related to special and complex
structures only, it may be used for design and quantifying the seismic response of tall wood buildings.
The NBCC provisions for design using linear methods of analysis including the equivalent lateral force
procedure or the modal response spectrum analysis are highly prescriptive. They limit the modeling
assumptions that can be employed as well as the minimum strength and stiffness the structure must
possess. Further, the methods used in linear analysis have become standardized in practice such that it is
unlikely that different designers using the same technique to analyze the same structure will produce
substantially different results. However, when nonlinear analytical methods are employed to predict the
structure’s strength and its deformation under load, many of these prescriptive provisions are no longer
applicable. Further, as these methods are currently not widely employed by the profession, the
standardization that has occurred for linear methods of analysis has not yet been developed for these
techniques. As a result standardization has not yet been developed for use of non-linear dynamic analyses
in assessment and design of structures, and the designer using such methods must employ a significant
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amount of independent judgment in developing appropriate analytical models, performing the analysis,
and interpreting the results to confirm the adequacy of a design. Since relatively minor changes in the
assumptions used in performing a nonlinear structural analysis can significantly affect the results obtained
from such an analysis, it is imperative that the assumptions used be appropriate. It is generally required
that designs employing nonlinear analysis methods be subjected to independent design peer-review in
order to provide a level of assurance that the independent judgment applied by the designer when using
these methods is appropriate and compatible with that which would be made by other competent
practitioners.
4.3.3.2.5 Input needed for the Analyses
The input properties that are needed to develop numerical models of the structure to perform static and
dynamic analyses are discussed in detail in Section 0 of the Guide. Some of the more important
parameters are listed below.
1. Element properties
Elements in wood-based tall buildings will most probably consist of engineered wood products either
used as beam/column elements, structural wall panels, or their combination. In hybrid buildings some
steel, concrete or even masonry elements may be present. For analysis and modeling purposes (inclusion
into linear static and dynamic analysis models), the effective stiffness of the engineered wood
products/panels with and without the effect of the connections must be determined. Lower bound, upper
bound, and best estimates of the strength and stiffness properties (backbone curves, initial elastic
stiffness) should be determined. This can be done by reviewing the available test/analytical data provided
in the literature (engineered wood products producers, proprietary connection developers), or can be
obtained by carrying out additional experimental tests for specific applications.
2. Effective damping
Effective viscous damping for steel and concrete structures is usually assumed in the range of 2% to 5%.
Wood buildings in general have slightly higher values of viscous damping. Consequently, the damping
values of the main load resisting elements and assemblies for inclusion into linear dynamic response
history models of wood structures may be assumed in the range of 3-5% unless there is a justification
from an available experimental data to use higher values. It should be noted that the effect of hysteresis
damping will be explicitly included in nonlinear dynamic models. No viscous damping should be
assigned for members that contain friction devices.
3. Input earthquake motions for analyses
Input earthquake motions should be provided by geotechnical consultant for a site using the area
seismicity and soil profile. It is noted that University of British Columbia (UBC) has developed an
ensemble of input earthquake records for various earthquake scenarios for the Lower Mainland area for
soil site class C. Also NBCC will be publishing recommended procedure for establishing site specific
time-histories along with some recommended time-history records for various sites in the 2015 edition of
the model code.
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4. Hysteresis and backbone models for connections and assemblies
The non-linear load-deformation relationships (backbone and hysteresis curves, Figure 98) for main parts
of the LLRS (connections and assembles) should be obtained if non-linear dynamic analyses are
conducted. If information from the literature is lacking for a specific geometry and application, tests
should be carried on representative samples (e.g., representative of the materials to be used, and the
tolerances of the manufacturing and assembly) by accredited laboratories to establish the
hysteresis/backbone curves suitable for the analyses. Please refer to Section 0 for more information.
5. Soil properties and soil structure interaction
For tall buildings the interaction between the soil and the foundation could affect the overall performance
of the building. Consequently, the soil-structure interaction should be modeled accordingly when
developing the linear or non-linear dynamic analyses. Usually the soil properties are modeled using a
series of horizontal and vertical springs. There are well established procedures for determining the
properties of vertical and horizontal soil springs. The geotechnical consultant should be contacted for
providing suitable properties for the soil springs for the prescribed geometry of the foundation and soil
conditions at the base of the footings. The spring properties for both upper and lower bounds should be
used, bounding the response of the building for both force and displacement demands.
From the preceding procedure for analysis of tall wood buildings it is recommended to carry out linear
static (code prescribed procedure) and dynamic response spectrum analysis for determining the overall
seismic demand at various floors followed by nonlinear static analysis to gauge the sequence of yielding
and formation of hinges at various levels. The information provided including sensitivity analyses will
help to establish the overall backbone curve of the lateral load resisting system and thus establishes the
global displacement ductility demand and local axial/rotation/curvature ductilities.
4.3.3.3
Analytical Models, Software AND Model Verification
4.3.3.3.1 Software and Analytical Models
In general, a three-dimensional numerical model of a building is necessary to study the seismic behaviour
of the building. There are numerous commercially available software packages that can help engineers to
develop linear and nonlinear models of buildings. Examples of general purpose finite elements software
for practicing engineers include ETABS, SAP2000, Perform 3D, Drain 3DX, STAAD, RISA, RAM,
ABAQUS, ADINA, ANSYS, Structural System, S-Frame, P-Frame, ST STRUDEL, Visual Tools and
many others. Other specialized software for nonlinear time-history analysis includes OpenSees,
SeismoStruct, Nonlin & Nonlin-Pro.
When using a general purpose software package for modeling of the wood building, the wood beam,
column, or wall (panel) elements in most cases should be modeled as linear elastic beam or shell
elements, with their strength and stiffness properties included for all different directions. The geometry
and effective stiffness of wall or frame assemblies under lateral loads should be determined and modeled
adequately. Since most of the non-linearity in wood structures occurs in the connections, they should be
modeled using hysteretic models that will allow for adequate capture of the strength and stiffness
degradation, as well as the pinching of the loops that is characteristic for most wood connections. The
input parameters needed for the analytical models are discussed in detail in Section 0 of the Guide. It
should be noted that for general purpose commercial software, one may need to arrive at the effective
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stiffness of the engineered wood products/panels of the structural system with and without the effect of
the connections for carrying out linear static and response spectrum (dynamic modal) analysis.
Unfortunately, most of the general purpose software packages may not have hysteretic models that can
adequately model the behaviour of the wood connections. Efforts should be made to choose models that
can best capture the behaviour of the modeled connection or assembly. The building mass should be
computed using the floor masses and modeled as either lumped in the nodes of the structural model or
uniformly distributed along the diaphragm.
Example of a software that was developed specifically for non-linear analysis of wood-frame structures is
SAPWood (van de Lindt and Pei, 2010). This analysis program was developed based on SAWS and
CASHEW concepts (Folz and Filiatrault, 2002) that are able to model and generate hysteresis loops for
wood-frame shearwalls that including the pinching effects. It is aimed at providing both researchers and
practitioners with a user-friendly software package that is capable of performing nonlinear seismic
structural analysis and loss analysis for wood-frame structures. SAPWood has the ability to build and
analyze light frame wood shearwalls using nonlinear connector (nails, hold-down devices, or screw)
elements. This enables the analysis of woodframe structures beginning at the fastener level when
assembly (shearwall) test data is not available. The diaphragms in the building as assumed to be rigid, and
all non-linear action is occurring in the wood-frame shearwalls. In addition to time domain analysis,
several modules that support the NEESWood PBSD efforts are also included.
4.3.3.3.2 Model Verification and Comparison of Results
The analytical models or analogues used to model the components, connections, and assemblies of the tall
wood building have to be verified against the available test result data, to ensure that equivalent properties
used in the static/dynamic model produce reasonable results. These simplified and verified models may
then be extrapolated to develop more complex and robust models of tall wood buildings.
Also, it is suggested that the results from the developed numerical model of the building (static and
dynamic analyses) be checked against simplified methods of calculation. For example, the results of the
analytical model such as column or wall gravity load takedown should be compared with hand
calculations for calculating column and wall loads under dead and live loads.
The fundamental periods of the tall wood building computed by modal analysis should be compared with
available empirical formulas and any available similar test data to ensure the results are not biased.
4.3.3.4
Methods of Seismic Design
4.3.3.4.1 Force-Based Design
Earthquakes motions induce displacements and forces in structures. Traditionally, seismic structural
design has been based primarily on forces. The reasons for this are largely historical, and related to how
we design for other actions, such as dead and live load (Priestley et al., 2007). For such cases we know
that force considerations are critical: if the strength of the designed structure does not exceed the applied
loads, then failure will occur. Consequently, seismic design provisions included in NBCC (and other
building codes in the world) currently use a forced-based seismic design approach. In force based design
approach the seismic design forces are determined either using dynamic analysis or equivalent static
procedure while displacements are checked later in the design process. Force-based design procedures can
be used for design of tall wood buildings if they fulfill the code requirements for such procedures. For
example the equivalent static procedure for wood structures includes the following steps:
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1. Estimate the structural dimensions of members, walls, and connections;
2. Estimate the member/connection stiffness;
3. Estimate the natural period of the structure based on the code formula, or other established
methods of mechanics with upper limit as specified in NBCC;
4. Calculate elastic forces on the structure based on the spectral acceleration of the site of the
building;
5. Select the ductility and overstrength related force modification factors 𝑅𝑑 and 𝑅𝑜 ;
6. Obtain the total seismic design shear force by dividing the elastic one with the R-factors. If the
period of the building is determined by mechanics based approach or other means and the
selected structural system is susceptible to development of soft storey failure mechanism, the
designer may increase the base design shear by 20% ;
7. Distribute the forces along the height of the building using the inverse triangular rule, with a
portion of the total seismic force (Ft) being applied to the top of the building, if the building
period exceeds 0.7 s.;
8. Define the location of connections that will act as inelastic components;
9. Analyse the structure under the seismic design forces;
10. Design the plastic hinge connections;
11. Calculate lateral deflections of the building and compare them to the code limits. If they are
within the specified limits continue to point 12. If the deflections are larger than the limits, revise
the stiffness characteristics and go back to point 3. If an alternative method for deriving the
period was used, use of code formula for the period may result in lower deflection, so that option
may be explored as an alternative. If the deflection based on code period formula still could not
meet the drift limit requirement, the preliminary design should be revised and a stiffer system be
chosen (back to Step 3);
12. Apply capacity design for the connections and members that are not yielding.
If dynamic analysis procedure is used, the period under point 3 above will be calculated based on the
developed analytical model of the structure, as will be the distribution of forces along the height of the
building. The force-based seismic design approach is relatively simple to use and economically viable.
For these reasons the method has been widely used during the last 50 years, and still remains the
cornerstone of seismic design requirements included in current editions of design codes (Filiatrault and
Folz, 2002). The force based design in general and when used in wood buildings has several
shortcomings, and some of them are itemized below:
1. One of the main problems with the force-based design is related to the selection of appropriate
member/connection stiffness. Assumptions must be made about the member/wall/connection
sizes before the seismic design forces are determined. In case of rigid diaphragms, these forces
are then distributed among members/walls in proportion to their assumed stiffness. Clearly, if
member sizes are modified from the initial assumption, the calculated design forces are no longer
valid, and recalculation and use of iterative procedure is required, which is very seldom
performed by the designers;
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2. Inherently in the procedure, the member stiffness is treated as independent of its strength for a
given member cross section, which is not always true;
3. The force-based design process is initiated with an estimate of the elastic fundamental period of
the structure. The empirical period equations provided by design codes are not tailored for wood
buildings. In fact, the whole notion of elastic period is fallacious since most wood buildings
exhibit inelastic response over the entire range of lateral deformations;
4. The force modification factors (R-factors) are assigned in the codes for only a handful of
structural systems. Using a system that is not listed in the code would require a significant
research on the global system response and ductility. Since the 𝑅𝑑 factor is closely related to the
ductility of the structure, a proper definition of the displacement ductility for wood buildings is
needed. Currently, there is no consensus within the research and engineering community on the
appropriate definition of yield and ultimate displacements for wood based lateral load-resisting
systems. It is suggested that the Equivalent Energy Elastic Plastic (EEEP) approach provided in
ASTM Standard E2126 is used for determining these values;
5. Deformation limit states are not directly addressed by the force-based design procedure. Limiting
deformations is paramount for any system. For example, large portion of the structural and nonstructural damage to wood framed buildings resulting from recent earthquakes has been
associated with excessive lateral displacements (Filiatrault and Folz, 2002);
6. The reduction of the elastic base shear by the R factors implies that the maximum displacement
that the structure would undergo if it would remain elastic is equal to the maximum displacement
of the actual inelastic structure. This equal displacement approximation is inappropriate for short
period structures. However, it is expected that this assumption will be valid for tall wood
buildings.
4.3.3.4.2 Displacement-Based Design
The design procedure known as Direct Displacement-Based Design (DDBD) has been developed over the
past decade with the aim of mitigating the deficiencies in current force-based design, discussed
previously. Since deflection and inter-story drift in particular is a key parameter for the control of damage
in structures, it was rational to examine a procedure where displacements are considered at the beginning
of the seismic design process. The DDBD characterizes the multi-degree of freedom (MDOF) structure as
a single-degree-of-freedom (SDOF) system (representation) with equivalent elastic lateral stiffness and
viscous damping properties representative of the global behaviour of the structure at the target peak
displacement response (Figure 99, Priestley et al., 2007). The fundamental philosophy behind the design
approach is to design a structure which would achieve, rather than be bounded by, a given performance
limit state under a given seismic intensity. This would result in essentially designing uniform-risk
structures, which is philosophically compatible with the objectives of the design codes. The design
procedure determines the strength required at designated plastic hinge locations to achieve the design
goals in terms of defined displacements. It must then be combined with capacity design procedures to
ensure that inelastic dissipative regions (plastic hinges) occur only where intended, and that non-ductile
modes of deformation do not develop. These capacity design procedures must be calibrated to the
displacement- based design approach and can result in generally less onerous requirement than those for
force- based designs, resulting in more economical structures.
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Figure 99 Single degree of freedom simulation of a multi-storey structure; b) Determining the
effective stiffness of the structure.
A comprehensive reading on the Displacement-based Seismic Design of Structures is given in
Priestley et al., 2007. An effort to introduce the procedure in seismic design in wood-frame structures is
presented in Filiatrault and Folz, 2002 and Newcombe, 2010. The basic steps of the procedure are given
below.
a)
b)
c)
d)
e)
Definition of Target Displacement and Seismic Hazard ∆T. The first step in the design procedure
is the definition of the target displacement that the building should not exceed under a given
seismic hazard level. The seismic hazard associated with the target displacement must then be
defined in terms of a design relative displacement response spectrum that is obtained from the
acceleration spectrum.
Selection of Structural System. Once the design performance level and associated seismic hazard
have been defined, the main wood-based lateral load-resisting system must be specified.
Performance of the chosen system under lateral loads should be investigated by testing, if there is
not ample information in the literature to define the performance of the system under lateral
loads.
Determination of Equivalent Viscous Damping ζeq. In order to capture the energy dissipation
characteristics of the structure at the target displacement, an equivalent viscous damping ratio
must be determined. For this purpose, a database of damping values must be established for the
selected structural system based on its hysteretic behaviour obtained from testing. The equivalent
damping should account for the energy dissipation characteristic of structural and non-structural
elements in the building. For tall wood structures damping contribution from non-structural
components taken as a 2% of the critical is expected to yield reasonable results.
Determining the Equivalent Elastic Period 𝑇𝑒𝑞 . Knowing the target displacement and the
equivalent viscous damping of the building at that target displacement, the equivalent elastic
period of the building 𝑇𝑒𝑞 can be obtained directly from the design displacement response
spectrum.
Determining the Required Equivalent Lateral Stiffness Kreq. Representing the building as an
equivalent linear SDOF system the required equivalent lateral stiffness can be calculated based on
the period, the effective seismic weight acting on the building and the acceleration of gravity.
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Determination of Actual Equivalent Lateral Stiffness Kaeq. The actual equivalent lateral stiffness
of the building at the target displacement can be determined from the results of a static pushover
analysis.
g) Lateral Stiffness Verification. The actual equivalent lateral stiffness of the building must be
compared to the required equivalent lateral stiffness. If these two stiffness values differ
substantially, the lateral-load resisting system of the building must be modified by returning to
point b.
h) Computation of Design Base Shear Vb. If the actual lateral stiffness of the building is nearly
equal to the required lateral stiffness, the design process is completed by computing the required
base shear capacity of the building, 𝑉𝑏 , as a product of the actual stiffness and the target
displacement. This base shear can then be used to design the other elements of the structure.
f)
It is noted that Pang and Rosowsky, 2007, 2008, have presented improved procedure for displacement
based design for wood buildings. This procedure may be used instead of the one presented above.
As can be seen, the direct-displacement procedures have some advantages over the traditional forcedbased design. They are mostly related to the facts that:
1.
2.
3.
4.
5.
No estimation of the elastic period of the building is required;
Force modification factors 𝑅𝑑 and 𝑅𝑜 do not enter the design process;
The displacements drive the entire design process;
Relationships between the elastic and inelastic displacements are not required;
The yield displacement does not enter the design process.
The direct-displacement design strategy, on the other hand, requires detailed knowledge of the global
nonlinear monotonic load-displacement behaviour (pushover) of the main lateral load resisting system, as
well as the variation of the global equivalent viscous damping with displacement amplitude. If no
information in the literature is available, connection, assembly and LLRS level testing is required in
parallel with the development of non-linear structural analysis models for the wood structure.
4.3.3.4.3 Performance-Based Design
As mentioned previously, design and construction of wood buildings in Canada is regulated at the
provincial level and enforced on local level, using provincial codes that are based on the NBCC (NRC,
2010) that is the national model building code, and the CSA O86 as the material standard for engineering
design in wood. Building codes and material standards are intended to establish minimum requirements
for providing safety to life and property from seismic hazards. The objectives of the seismic design
according to NBCC and other codes in North American Codes are the following:
1.
2.
3.
4.
Resist minor earthquakes without damage;
Resist moderate earthquakes without structural damage but with some non-structural damage;
Resist major earthquakes with significant structural and non-structural damage; and,
Resist the most severe earthquakes ever likely to affect the building, without collapse.
This goal is accomplished through the specification of prescriptive criteria that regulate acceptable
materials of construction, identifying approved structural and non-structural systems, specifying required
minimum levels of strength and stiffness for elements and connection, and controlling of deflections and
detailing of the building. Although the prescriptive criteria of model building codes are intended to result
in buildings capable of providing certain levels of performance, the actual performance capability of
individual building designs is not assessed as part of the traditional code design process. As a result, the
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performance capability of buildings designed to prescriptive criteria can be variable and, for a given
building, may not be specifically known. The performance of some buildings designed to these
prescriptive criteria can be better than the minimum standards anticipated by the code, while the
performance of others could be worse.
Building owners and occupants generally believe that if the building satisfies the code provisions for
safety, minor damage of the building is anticipated during a seismic event. Experiences form the
earthquakes at the end of the 20th century (e.g., the 1994 Northridge and 1995 Kobe Earthquakes), as
well as those in the 21st century (2011 Christchurch earthquake), has forced recognition that damage,
sometimes severe, can occur in buildings designed in accordance with code. Property and insured losses
during this earthquakes, led to awareness that the level of structural and non-structural damage that could
occur in code-compliant buildings may not be consistent with public notions of acceptable performance.
Furthermore, recognition that code-based strength and ductility requirements applicable for seismic
design of new buildings are not always suitable for evaluation and upgrade of existing buildings has led to
the development of performance-based engineering methods for seismic design (FEMA, 2012).
The performance-based seismic design process explicitly evaluates how a building is likely to perform
given the potential hazard it is likely to experience, considering the uncertainties inherent in the
quantification of potential hazard and uncertainties in the assessment of the actual building response. In
performance-based design, identifying and assessing the performance capability of a building is an
integral part of the design process, and guides the many design decisions that must be made. Figure 100
(FEMA 2006) shows a flowchart that presents the key steps in the performance-based design process. It is
an iterative process that begins with the selection of performance objectives, followed by the development
of a preliminary design, an assessment as to whether or not the design meets the performance objectives,
and finally redesign and reassessment, if required, until the desired performance level is achieved.
Performance-based design begins with the selection of seismic design criteria stated in the form of one or
more performance objectives. Each performance objective is a statement of the acceptable risk of
incurring specific levels of damage, and the consequential losses that occur as a result of this damage, at a
specified level of seismic hazard. Losses can be associated with structural damage, non-structural
damage, and can be expressed in the form of casualties, direct economic costs, or downtime (time out of
service), resulting from damage.
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Figure 100 Simplified diagram of the performance-based seismic design procedure
Documents related to performance-based seismic design procedures have evolved during the past 20
years in the US. That initial provisions were included in the FEMA 273 Report, the NEHRP Guidelines
for the Seismic Rehabilitation of Buildings (FEMA, 1997a) and its companion document, FEMA 274, the
NEHRP Commentary on the Guidelines for the Seismic Rehabilitation of Buildings (FEMA, 1997b).
Although both documents addressed seismic upgrade of existing buildings, the procedures were widely
used for new buildings as well. Concurrently, the Structural Engineers Association of California
developed the Vision 2000 Report: Performance-Based Seismic Engineering of Buildings (SEAOC,
1995), which described a performance-based seismic design framework for design of new buildings.
These first-generation documents outlined the initial concepts of performance levels related to
damageability and varying levels of hazard. In FEMA-273, performance of structural elements and nonstructural elements are segregated into structural performance levels and non-structural performance
levels. A building performance level is then obtained by combining a structural performance level with a
non-structural performance level. Three structural performance levels are defined, consistent with the
performance states most frequently sought by building owners. These are termed as Immediate
Occupancy, Life Safety and Collapse Prevention levels.
The Collapse Prevention (Structural Stability) level is intended to represent a state of incipient collapse in
which the lateral force resisting system has experienced substantial stiffness and strength degradation.
The gravity load resisting system, while also potentially compromised, is anticipated to retain sufficient
integrity to continue to support basic dead and live loads. Structures performing to the Collapse
Prevention level are anticipated to be potential complete economic losses, however, because collapse has
not occurred, they present only a moderate level of risk to occupants during the earthquake. The Life
Safety level is also a state in which significant damage has occurred to the lateral force resisting system;
however this damage is reduced relative to the Collapse Prevention level. While structures responding to
the Collapse Prevention level are expected to have little, or no, remaining margin against collapse (Figure
101), structures responding to the Life Safety level are expected to retain significant margin. Such
structures are expected to be repairable, though perhaps not economically so, and it is expected that they
represent a very low level of risk to occupant safety during the design earthquake. The Immediate
Occupancy level is a state of minor damage. Structures performing to this level are expected to
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experience limited stiffness degradation and no significant strength degradation. Such structures are
expected to present a negligible risk to life safety both during and after the earthquake and are therefore
immediately available for post-earthquake re-occupancy, presuming that damage to non-structural
elements does not preclude this. FEMA-273 is a displacement-based document and structural
performance levels are using the concept of a static pushover curve (Figure 101). Such curves are
obtained for a structure by subjecting it to a monotonically increased pattern of lateral forces,
representative of the inertial forces that would be experienced by the building when subjected to actual
ground shaking demands. Each point on the curve, represented by a ♦ in the figure, corresponds to an
event at which some structural element has experienced degradation in stiffness.
Figure 101 Typical push-over curve with the structural performance levels
A major limitation in the approach taken by FEMA-273 relates to its reliance on component based
acceptance criteria without simultaneous consideration of global structural behaviour, even though the
performance levels are described in terms of the global behaviour. Specifically, the procedures presented
in FEMA-273 will substantially underestimate the probable performance of the structure because system
effects are neglected. The FEMA 273 philosophy is retained in the next document (the 1997 NEHRP
Provisions), though it has been somewhat modified (Figure 102). The diagonal lines indicate the
performance objectives expected for various types of buildings. Specifically, it is anticipated that regular
buildings would meet the Life Safety level for “design level earthquake demands” (10% in 50 years
earthquake) while the Collapse Prevention level for maximum considered earthquake (MCE) demands,
having a 2% probability of exceedance in 50 years, and the Immediate Occupancy level for frequent
earthquake demands having a 50% probability of exceedance in 50 years. These performance levels have
remained the same to this date and are implemented in the current Canadian and US codes. The
qualitative performance levels of FEMA 273 with simplified sketches of damaged states of a masonry
wall element, where the damaged states can be more clearly shown that on a wood one, are shown in
Figure 102.
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Figure 102 NEHRP 1997 Performance objectives
The second generation documents, that include FEMA 356, Prestandard and Commentary for the Seismic
Rehabilitation of Buildings (FEMA, 2000b) and the American Society of Civil Engineers (ASCE)
Standard ASCE/SEI 41-06, Seismic Rehabilitation of Existing Buildings (ASCE, 2007) define the current
practice for performance-based seismic design in the US. In ASCE 41-06 the building performance is
expressed in four discrete Structural Performance Levels (SPL) and two intermediate Structural
Performance Ranges. The discrete SPL are Immediate Occupancy (S1), Life Safety (S3), Collapse
Prevention (S5) and Not Considered (S6), See Figure 103. The intermediate Structural Performance
Ranges are the Damage Control Range (S2) and Limited Safety Range (S4). For each of the performance
levels, damage states and drift limits are prescribed for some of the most frequently used LLRS. For
example, for wood-frame shearwalls under the Collapse Prevention SPL, the following damage is
expected at 3% transient or permanent drift: Connections loose; Nails partially withdrawn; some splitting
of members and panels; Veneers dislodged. Performance levels are also applied to the non-structural
components, and the total structural performance is assessed based on both structural and non-structural
components at a specified seismic hazard level.
Figure 103 Qualitative performance levels of FEMA 273/356
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Four levels of analysis are provided in ASCE 41, which give designers progressively detailed information
about the structural performance. The first two levels match the model code style of force-based design,
and cannot be used on buildings with long periods or significant irregularities. Some tall wood buildings
may fit this category. The second two are based on displacement and serve to directly determine the post
yield capability of the buildings. In addition to a set of general analysis requirements, each analysis
method is defined in terms of specific modeling requirements and procedures. A common acceptance
criterion is provided for linear methods (force based) and also for the non-linear methods (displacement
based). The criteria are extensive, organized by material type, and based on the amount of available
information, including applicable test results.
The first level of analysis is the Linear Static Procedure (LSP) that provides an equivalent lateral force,
vertical distribution of forces and rules for modeling, and acceptance criteria. This procedure is similar to
the equivalent lateral force procedure from the codes, except that the base shear is much higher and the
ductility factors are much smaller. It is intended to be simple and very conservative to allow one and two
story buildings of regular configuration to pass because of their excessive strength. The second level of
analysis is the Linear Dynamic Procedure (LDP) that uses modal analysis and site-specific response
spectra to determine the force demands. The LDP also includes modeling rules that encourage
consideration of soil structure interaction and appropriate acceptance criteria. It is much more beneficial
than the LSP analysis since it utilizes a site-specific response spectra, calculated building periods, and the
beneficial effects of multiple modes. It does have a serious limitation in that it cannot properly evaluate a
building with significant redundancy, that is, significant strength even after damage begins to occur. The
rules for judging the building to be adequate still triggers unacceptable performance when the first
significant element within the stiffest lateral system exceeds its limits. For some buildings, this technique
is satisfactory since once significant yielding occurs, there is nothing else to step in and provide
resistance.
The first of the displacement-based procedures is the Non-linear Static Procedure (NSP), commonly
referred to as the pushover method. Using analytical techniques, accessible in commercially available
advanced computer programs, a model of the building is subjected to increasing deflection while the
impact on the lateral force resisting elements is monitored. As the yield limits are exceeded, the elements
are allowed to yield and the computer program tracks their post yield displacement to determine when the
building loses its lateral force resisting ability. In the process, first significant yield does not signal a
problem; instead, it signifies that other elements need to step up and take over. Using a series of
approximations, a target displacement is calculated based on site-specific response spectra. If there is a
lateral system within the building that can arrest the movement to within the target displacement, the
building is judged adequate. If not, then there is one more level of analysis, if the building is worth the
cost of running it. This process of analysis matches the way buildings behave in earthquakes since it
estimates the building’s actual movement and resulting damage.
The second displacement based method is a Non-linear Dynamic Time History Procedure (NDTHP) that
uses time history records to represent the possible shaking that the site could experience. The frequency
content of the record is used directly to determine the displacement demand and gives a more accurate
representation. Also, the number of cycles of non-linear behaviour can be monitored and used to more
accurately predict the extent of damage that will result from the non-linear behaviour. Buildings that need
to rely on a high level of non-linear behaviour to achieve their target displacement benefit most from the
NDTHP. Buildings that are heavily damaged in earthquakes, but remain standing straight up, illustrate the
beneficial effects of time history record. Only NDTHP is capable of predicting such behaviour.
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The performance-based procedures are constantly improving. FEMA initiated a series of projects that are
referred to as the ATC-58/ATC-58-1 Projects, to further improve the latest procedures. The first step was
publishing of the FEMA 445, Next-Generation Performance-Based Seismic Design Guidelines for New
and Existing Buildings (FEMA, 2006), while the last one was the final document: FEMA P-58-1
Performance Assessment of Buildings (FEMA, 2012a, FEMA 2012b). Also, an updated version of the
ASCE 41 standard (ASCE 41-13) is available.
Although it has been a topic of research work and development for the last 15 years, the advancements in
Performance-Based Seismic Design (PBSD) of wood structures have been slower than for other
structures. In addition, almost all of the work is related to wood-frame structures, the predominant system
used in North America. A short summary of the work that may be useful to the designers is given below.
The shortcomings of using force-based design for wood structures have been identified (Filiatrault and
Folz, 2002). The Direct Displacement-based Design (DDD) procedure that was first suggested by
Priestley (1998) was adopted and presented one possible displacement-based design procedure for wood
structures. New steps towards defining a PBSD for wood-frame structures were discussed at a workshop
in Colorado (van de Lindt, 2005). This design procedure was applied to a two-story wood-frame building
(Filiatrault et. al., 2006). An experimental study has shown a strong correlation between
displacement/drift and the level of damage observed during shake table tests of a wood structure (van de
Lindt and Liu, 2006). A design procedure that focuses on limiting inter-story drift as a rational approach
for PBSD of engineered wood-frame buildings when damage limitation is one of the design objectives
was developed by Pang and Rosowski (2007) that includes the fragility analysis for wood-frame shear
walls (Kim and Rosowski, 2005). Also, efforts to develop damage based seismic reliability concepts for
wood-frame structures were made (van de Lindt and Gupta, 2006). The DDD procedure was used in
performance-based design and determining the preliminary force modification factors for CLT structures
in Canada and the US (Pei, Popovski and van de Lindt 2010, 2013).
Designers of tall wood buildings who are interested in performing PBSD are advised to follow the latest
FEMA and ASCE documents. Designers may find that following these procedures may be difficult, time
consuming, and not cost competitive, unless the owners have a request to follow a strict PBSD. As an
alternative, the Direct Displacement Based Design procedure for wood structures developed by Filiatrault
and Folz (2002) presented in the previous section can be used. The method requires nonlinear pushover
analysis of the complete structure, as well as an estimate of equivalent viscous damping at a target drift
limit. As pointed out by the authors, these can be considered as disadvantages to a generalized procedure
since nonlinear pushover analysis often requires the use of complex finite element programs.
Furthermore, equivalent viscous damping ratios of tall wood structures have to quantified, along with
their damping values.
Second alternative is the PBSD procedure that was developed by Pang and Rosowski (2007). Building on
the foundation of the SDOF displacement-based design methodology, this procedure for design of
multiple-degree-of-freedom (MDOF) systems requires neither nonlinear pushover analysis nor the
equivalent viscous damping ratio. Although the DDD PBSD procedure was tailored specifically for
multistory wood-frame structures when developed, it can be used for any wood-based structural system.
The methodology is based on the assumption that damage in wood structures can be directly related to the
displacement demand. The general steps for the procedure are as follows:
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1.
2.
3.
4.
5.
6.
7.
8.
Define multiple performance levels in terms of limiting inter-story drifts for given seismic hazard
levels,
Calculate or estimate the mass and stiffness ratios (relative to first floor) for each floor,
Perform normalized modal analysis on the equivalent linear MDOF system to obtain inter-story
drift factors and natural frequency parameters,
Construct inter-story drift spectra for the most severe hazard level (2% in 50 years) and determine
the required equivalent stiffness for each floor,
Select the number of resisting elements (walls, frames) needed in the lateral force resisting
system of choice from a table of developed design (resistance) values. The table should also
include backbone responses of the elements and equivalent stiffnesses at various drift levels,
Check the design using the actual stiffness ratios (based on the resisting elements selected in step
5). Revise the element selection if necessary,
Repeat steps 2 - 6 for each performance level using the actual stiffness ratios of the selected
resisting elements. Revise the design if drift limits are exceeded at any performance levels,
Compute design base shear, story shear and uplift force using the actual nonlinear backbone
curves of the selected resisting elements,
If needed, the performance of the designed building can be further verified using a series of time history
dynamic analyses of a non-linear model of the building as conducted in Pei et al. (2010). A suite of
records for the site should be chosen for the analyses scaled to the 2% in 50 years hazard. Based on the
maximum drifts obtained from the analyses, the fragility curves for the building performance can be
obtained, and the performance of the building assessed in terms of probability of exceeding a certain drift.
A probability of failure of 10% for a 2% in 50 years seismic hazard is acceptable by the model codes
(ASCE7-10) and other documents such as FEMA P-695 (FEMA, 2009).
4.3.3.5
Capacity-Based Design Procedures
The concept of capacity design is of major importance in seismic design. Capacity design is widely used
for seismic design of concrete, steel and masonry structures, and must be used in the seismic design of tall
wood buildings. This design approach is based on the simple understanding of the way a structure
sustains large deformations under severe earthquakes. By choosing certain modes of deformation of the
lateral load resisting system (LLRS), certain parts of it are chosen and suitably designed and detailed for
yielding and energy dissipation under the imposed severe deformations. These critical regions of the
LLRS, often termed as plastic hinges" or "dissipative zones", act as energy dissipaters to control the force
level in the structure. All other structural elements can be designed as non-ductile and are protected
against actions that could cause failure, by providing them with strength greater than that corresponding
to the development of maximum feasible strength in the potential plastic hinge regions. In other words,
non-ductile elements, resisting actions originating from plastic hinges, must be designed for strength
based on the over-strength rather than the code-specified factored strength (resistance), which is used for
determining required strengths of hinge regions. This “capacity” design procedure ensures that the chosen
means of energy dissipation can be maintained. An example of a desirable and non-desirable hinge
mechanisms is shown in Figure 104.
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Figure 104
Potential choices for plastic hinges: a) hinges in columns can lead to a soft storey
mechanism; b) hinges in beams can lead to desirable weak beam - strong column design
Using capacity based design, in steel structures the members are typically designed to yield before the
connections. Also, beam failure mechanisms are preferred since they provide sufficient structural ductility
without creating undesirable mechanism of collapse (Figure 104). In timber structures, however, the
failure of wood members in tension or bending is not favourable because of its brittle characteristics.
Consequently, all non-linear deformations and energy dissipation in case of wood structures should occur
in the connections. This approach is completely the opposite of that in steel structures.
Main steps of the capacity based design procedure for wood structures are the following:
A kinematically admissible plastic mechanism is chosen for the structure and connections that
will act as plastic hinge regions within the structure are clearly defined. The mechanism chosen
should be such that the necessary overall displacement ductility can be developed with the
inelastic behaviour in the plastic regions (hinges).
2. Connections that will act as plastic hinge regions within the structure are designed to have their
strength (factored resistance) as close as practicable to the required strength (demand).
Subsequently, these connections are carefully detailed to fail in the fastener yielding mode and to
ensure that estimated ductility demands can be reliably accommodated;
3. Undesirable failure modes within the wood members containing the connections (plastic hinges),
are inhibited by ensuring that the strengths of these modes exceeds the capacity of the plastic
hinges at over-strength;
4. Those components of the structure not suited for stable energy dissipation, are protected by
ensuring that their strength exceeds the demands originating from the over-strength of the plastic
hinges. Therefore, these regions are designed to remain elastic irrespective of the intensity of the
ground shaking or the magnitudes of inelastic deformations that may occur.
1.
Using this approach, for wood braced frame systems for example; the dissipative zones should be located
in the connections connecting the braces to the rest of the frame. They should be able to produce yielding
by combination of wood crushing and fastener bending. All other connections should be designed to
remain linear elastic, with a strength that is slightly higher than the force induced on each of them when
neighbouring dissipative zones reach their over-strength. There should be a gap present between the
diagonal braces and the rest of the frame (in the corners). The presence of a gap allows for the brace
connections (dissipative zones) to deform and reach their ductility capacity. When there is no gap
between the end of the brace and the frame corners (a case of tight fit corners), the braced frame will be
stiffer, and would probably be able to carry larger lateral loads, but may also be less ductile. Eccentricities
in all connections of the braced frame, and especially in the dissipative zones, should be minimized. All
wood members should be designed to remain linear elastic at all times. The columns should be continuous
as much as possible over the height of the structure, and should be able to carry the vertical load at all
deflection levels, including the maximum allowable lateral drift. Wood diagonal braces should be
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designed not to buckle at any time. Structural integrity of the frame should be maintained at all times
while dissipative zones experience inelastic behaviour. Narrow braced frames, with aspect ratio (story
height vs. frame width) higher than 1.0, should be avoided because of their cantilever (bending) type
seismic response (Popovski, 2000). Wider frames have been shown to exhibit more of a shear type
response and make better use of the braces and their connections. However, wide frames with aspect
ratios lower than 0.67 should be avoided because the benefits of having a wider frame are usually
outweighed by the drawbacks of having a long brace, susceptible to buckling at lower force levels.
In case of platform type CLT structures, it is suggested that all non-linear deformations and energy
dissipation should occur in the connections (brackets) that connect the wall to the floor panels, in the
hold-down connections, if used, and in the vertical half-lap joints in the walls. All other connections
should be designed to remain linear elastic, with a strength that is slightly higher than the force induced
on each of them when neighboring dissipative zones reach their overstrength resistance. All connections
used for energy dissipation in CLT structures should be designed to fail in fastener yielding mode. No
wood failure modes in these connections should be allowed.
Using this strategy, the connections in horizontal step joints between floor panels (No. 2 in Figure 105)
should have sufficient over-strength and adequate stiffness to allow for the diaphragm to act as a single
unit. Similarly, connections tying up the floor panels to the walls below (No. 3 in Figure 105) should also
be over-designed, and be one of the strongest connection elements in the structure. If vertical step joints
are present in the walls (No. 4 in Figure 105), thus dividing the walls into several wall segments, the step
joint connections can be designed as yielding elements (dissipative zones) that will yield simultaneously
with the steel bracket connections (or hold-downs, if present) subjected to uplift at both ends of each wall
segment. Yielding of the bracket connections at panel ends should be followed by yielding of the rest of
the bracket connections connecting the walls to the floor. Fasteners should be randomly placed in the
available space in the steel brackets and hold-downs with the maximum fastener spacing possible. Larger
fastener spacing will help avoid load concentration in a small area of the CLT panel.
One can always use another design approach by over-designing the connections in the step joints, which
will result in the entire wall being able to act as a single panel. In this case, wall uplift will start to occur
at both ends of the wall during the seismic response, and the potential benefits of the step joints as energy
dissipating zones will be lost. The vertical joints between perpendicular walls (No.1 in Figure 105), may
or may not be included as dissipative joints. The effect of the perpendicular walls on the seismic
performance of CLT walls has not been investigated in depth so far. Until these effects are fully known
and quantified, it is suggested that vertical joints between perpendicular walls be over-designed. This
approach also slightly simplifies the seismic design procedure and gives the structure additional level of
robustness and safety.
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Figure 105 Typical storey of a multi-storey CLT structure with various connections between panels
(courtesy of A. Ceccotti); Connections 1, 2 and 3 to be elastic, while 4 to be ductile
It should be noted that capacity design is not an analysis technique but a powerful design tool. It enables
the designer to “tell the structure what to do” and to desensitize it to the characteristics of the earthquake,
which are, after all, unknown. Subsequent judicious detailing of all potential plastic regions will enable
the structure to fulfill the designer’s intentions. A capacity design approach is likely to assure predictable
and satisfactory inelastic response under conditions for which even sophisticated dynamic analyses
techniques can yield no more than crude estimates. This is because the capacity-designed structure should
not develop undesirable hinge mechanisms or modes of inelastic deformation, and as a consequence is
insensitive to the earthquake characteristics, as the magnitude of inelastic deformations are concerned.
When combined with appropriate detailing for ductility, capacity design will enable optimum energy
dissipation by rationally selected plastic mechanisms to be achieved. Moreover, as stated earlier,
structures so designed will be extremely tolerant with respect to the magnitudes of ductility demands that
future large earthquakes might impose.
4.3.3.6
Diaphragm flexibility and its Influence on Seismic Response
Floor diaphragms in buildings have two different functions. They are designed to carry the vertical dead
and live loads but also to transfer the lateral loads imposed by wind and seismic action to the components
of lateral load resisting system below. In the latter case the diaphragms rely on their in-plane strength and
stiffness to transfer the imposed loads. In multistory buildings, where the diaphragms are comprised of
reinforced concrete slabs or steel decks with structural concrete topping, the in-plane stiffness of the
diaphragm is quite large so that it acts as a rigid body. In wood-frame structures the situation is often
different, as the in-plane stiffness of wood diaphragms are much lower. The in-plane stiffness of the
diaphragms has to be taken into account when determining the response of the tall wood building, as the
diaphragm deformation alters the characteristics of the building and its response to both wind and seismic
ground motion. In case of flexible diaphragms, the components of vertical LLRS carry lateral loads from
the tributary area of the diaphragm that they support. In case of rigid diaphragms, the lateral loads must be
assigned to the components of the LLRS in proportion to their stiffness. In this case torsional response
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(including accidental torsion) has to be taken into account. It should be noted that NBCC requires that
even in case of flexible diaphragms the accidental torsion should be taken into account.
Currently, there are no criteria for classifying the diaphragms as rigid or flexible neither in 2010 NBCC
nor in CSA Standard O86. There are some guidelines on that in FEMA 356 (FEMA 2000) and its
successor ASCE 41-06 (ASCE 2006). In these documents it is recommended that a diaphragm be
classified as flexible when the maximum horizontal deformation of the diaphragm along its length is more
than twice the average interstory drift of the vertical lateral-force-resisting elements of the story
immediately below the diaphragm. On the other hand, the diaphragm may be considered as rigid if the
maximum diaphragm deformation is less than half the average interstory drift in the story below.
Diaphragms that are neither flexible nor rigid are classified as being stiff and the response of the structure
in such a case should be based on an analysis that takes into account both the in-plane stiffness of the
diaphragm and the stiffness of the vertical lateral force-resisting system. The Design Load Standard of the
American Society of Civil Engineering ASCE 7 (ASCE 2010) provides both prescriptive and calculation
based methods of classifying the diaphragms. According to these specifications, diaphragms constructed
of untopped steel decking or wood structural panels (plywood or OSB) may be idealized as being flexible
when the vertical lateral load-resisting elements are steel or composite steel and concrete braced frames,
or concrete, masonry, steel or composite shear walls, or in light wood frame construction where the nonstructural concrete toping is no greater than38mm. Although such diaphragms may not always meet the
deflection-based criteria, the provisions continue the widely followed traditional design practice and are
supported by some recent research. The research demonstrates that for regular light-framed wood
diaphragm buildings a force distribution based on flexible diaphragm assumption leads to better
performance.
In the case of tall wood buildings, it is proposed that analysis of the diaphragm flexibility should be
carried out. Whether a diaphragm can be treated as flexible, or rigid, or somewhere between the two
would depend on the in-plane stiffness of the diaphragm relative to the stiffness of the vertical structural
system and by comparing the lateral loads transferred to the components of the SFRS in both cases. In
cases where the designer is not sure if the diaphragm falls in either of the categories, an envelope
approach is recommended. In the envelope approach, the designer should analyse the structure twice, first
assuming the flexible and then the rigid diaphragm assumption, and then taking the worst scenario.
In cases where the diaphragms consist of CLT panels, in CLT or hybrid construction, the diaphragms are
recommended to be taken as rigid and analysed as such. Also, diaphragms consisting of glulam or heavy
timber elements with thick decking are expected to be in the rigid category.
4.3.3.7
Discontinuities in Plan and Elevation
In general the SFRS should carry the lateral load demand to the ground with no interruption. There are,
however, architectural challenges to overcome such as the lobby space and changes in the openings for
the core where there likely be some discontinuities to occur.
Plan discontinuities should be avoided for tall wood buildings as much as possible. However, if
unavoidable, there must be columns placed at the end of the walls to ensure the overturning associated
with the wall panels are transferred to the columns. The shear is transferred through the diaphragm to
adjacent walls. As such, the connection of wall to diaphragm and the columns supporting the SFRS
should be designed to capacity of the SFRS or R=1 (e.g., full elastic response of the system).
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NBCC 2010 identifies 8 different types of discontinuities associated with mass, stiffness and geometry of
the system.
4.3.3.8
Lateral Drifts
Lateral drifts associated with the response of SFRS and formation of plastic hinges in the system during a
seismic event must be accommodated by all non-structural elements such as cladding, sprinkler system
and the gravity load carrying system.
The lateral drift should be limited to NBCC criterion for various performance objectives.
The drift should be calculated, as per formation of plastic hinges throughout the lateral load resisting
system, as the difference between the lateral deformations of consecutive floors.
4.3.3.9
Testing needed to Support Seismic Load Analyses and Design
If test results are not available for the main strength, stiffness, and ductility properties of the connections,
structural elements, assemblies, and sections of the SFRS used, testing should be carried out to determine
such properties. More details about the testing requirements in general can be found in Section 0 of the
Guide. The tests should involve full-scale specimens as much as possible. Types of tests to be considered
include static and cyclic tests on structural elements and connections, as well as cyclic, pseudo dynamic,
or shake table tests on the main lateral load resisting assemblies or main portions of the SFRS. Important
parameters to be extracted include but not limited to: initial and post-yield stiffness, yield and ultimate
strength and defection, strength and stiffness degradation, ductility, drift capacity, and hysteresis loop
properties including energy absorption.
4.3.4
Analysis and Design for Wind Loads
4.3.4.1
Static Analysis
Commentary I of NBCC provides the procedure that can be used for carrying out static analysis of
building under wind loading.
External and internal pressures for both low- and high-rise buildings are provided for various sites and
conditions. The computed pressures should be applied on the floor diaphragms as lateral loads and based
on tributary floor areas.
Partial wind loading to accommodate the potential diagonal wind loading should also be applied to
account for tendency of structures to sway in the across-wind direction. Taller structures should be
designed to resist 75% of the maximum wind pressures for each of the principal directions applied
simultaneously. Taller structures should also be checked for the maximum additional torsion by removing
the wind pressure (or 50% of the wind pressure) from parts of the face areas.
4.3.4.2
Dynamic Analysis
NBCC clause 4.1.7.2.(1) requires use of dynamic or experimental procedures for buildings whose height
is greater than 4 times their minimum effective width, or greater than 120 m, or for other buildings whose
properties make them susceptible to wind induced vibrations. In addition, buildings whose light weight,
low frequency and low damping make them susceptible to vibration are also required to be designed by
experimental or dynamic methods.
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In addition to the calculation of wind load, the calculation of wind-induced lateral deflection, vibration
and vortex-shedding effect can also be important for some buildings that are required to be treated by the
dynamic procedure. The dynamic procedure topics are dealt with in NBCC commentary I.
4.3.4.3
Vortex Shedding
In general, wind buffeting against a bluff (angular shape) body is diverted in three mutually perpendicular
directions, giving rise to these sets of forces and moments along all six components in the space. In civil
and structural engineering the force and moment corresponding to the vertical axis (lift and yawing
moment) are of little significance, so aside from the effects of uplift forces on large roof areas, flow of
wind is considered a two-dimensional (2D) phenomenon consisting of longitudinal (along) wind and
transverse wind. The term along wind, or simply wind, is used to refer to drag forces while transverse
wind is the term used to describe crosswind. In tall wood building design, the crosswind motion that is
perpendicular to the direction of the wind can be more critical than along-wind motion and need to be
considered in the design.
When a prismatic building is subjected to a smooth wind flow, the originally parallel upwind streamlines
are displaced on either side of the building. This results in spiral vortices being shed periodically from the
sides into the downstream flow of wind. At relatively low wind speeds, the vortices are shed
symmetrically in pairs, one from each side. When the vortices are shed, that is, break away from the
surface of the building, an impulse is applied on the building in the transverse direction. At low wind
speeds, since the shedding occurs at the same instant on either side of the building there is no tendency for
the building to vibrate in the transverse direction. Therefore, the building experiences only along-wind
oscillations parallel to wind direction. However, at higher speeds, vortices are shed alternately, first from
one and then from the other side. When this occurs, the transverse impulse occurs alternately on opposite
sides of the building with a frequency that is precisely half that of the along-wind impulse. This impulse
due to transverse shedding gives rise to vibrations in the transverse direction. The phenomenon is called
vortex shedding (Simiu and Scanlan 1996). There is a simple formula to calculate the frequency of the
transverse pulsating forces caused by vortex shedding:
𝑓=
𝑉∙𝑆
𝐷
[6]
where:
𝑓 is the frequency of vortex shedding in hertz;
𝑉 is the mean wind speed at the top of the building;
𝑆 is a dimensionless parameter called the Strouhal number for the shape of the building;
𝐷 is the diameter of the building.
The Sirouhal number is not a constant but varies irregularly with wind velocity. At low air velocities S is
low and increases with the velocity up to a limit of 0.21 for a smooth cylinder shaped building. This limit
is reached for a velocity of about 22.4m/s and remains almost a constant at 0.20 for wind velocities
between 80 and 180 km/h (22.4 and 51 m/s). During vortex shedding an increase in deflection occurs at
the end of each building swing. If the damping of the building is small, the vortex shedding can cause
building displacements far beyond those predicted on the basis of static analysis. When the wind speed is
such that the shedding frequency becomes approximately the same as the natural frequency of the
building in the transversal direction, a resonance condition is created. After the structure has begun to
resonate, further small increases in wind speed will not change the shedding frequency, because the
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shedding is now controlled by the natural frequency of the structure. In this case, the vortex-shedding
frequency is “locked in” with the building natural frequency (Taranath, 2012). When the wind speed
increases significantly above the value that caused the lock-in phenomenon, the frequency of shedding is
again controlled by the speed of the wind. The structure vibrates with the resonant frequency only in the
lock-in range. For wind speeds either below or above this range, the vortex shedding will not be critical.
Vortex shedding occurs for many building shapes. The value of S coefficient for different shapes is
determined in wind-tunnel tests by measuring the frequency of shedding for a range of wind velocities.
One does not have to know the value of S very precisely because the lock-in phenomenon occurs within a
range of about 10% of the exact frequency of the structure.
Unlike steady flow of wind, which for design purposes is considered static, turbulent wind loads
associated with gustiness cannot be treated in the same manner. This is because gusty wind velocities
change rapidly and even abruptly, creating effects much larger than if the same loads were static. Wind
loads, therefore, need to be studied as if they were dynamic, somewhat similar to seismic loads. The
intensity of dynamic component of wind load depends on how fast the velocity varies and also on the
response of the structure itself. Therefore, whether pressures on a building due to a wind gust, is dynamic
or static entirely depends on the gustiness of wind and the dynamic properties of the building to which it
is applied. The action of a wind gusts depends not only on how long it takes for the gust to reach its
maximum intensity and decrease again (the gust period), but also on the fundamental period of the
structure. If the wind gust reaches its maximum value and vanishes in a time much shorter than the period
of the building, its effects are dynamic. On the other hand the gusts can be considered as static loads if the
wind load increases and vanishes in a time much longer than the period of the building. For example, a
wind gust that develops to its strongest intensity and decreases to zero in 2s is a dynamic load for a tall
building with a period of considerably larger than 2s, but the same 2s gust is a static load for a low-rise
building with a period of less than 2s.
4.3.4.4
Experimental Analysis and Testing
NBCC indicates that for unusual types of structures, specialized information such as theoretical studies,
model tests or wind tunnel experiments may be required to provide adequate design values for wind
loading.
Wind tunnel tests can be used as an alternative to the static and dynamic procedures. It is especially
recommended for buildings that may be subjected to buffeting or channeling effects caused by upwind
obstructions, vortex shedding, or to aerodynamic instability. It is also suitable for determining external
pressure coefficients for the design of cladding on buildings whose geometry deviates markedly from
common shapes.
4.3.4.5
Deflections and Wind Induced Vibrations-Controlled Design
4.3.4.5.1 Deflection Controlled Design
4.3.4.5.1.1
Design Criterion
Lateral deflection of tall buildings under wind loading is generally consideration from the standpoint of
serviceability or comfort. Wood-based structural systems (except CLT structures to some extent) tend to
be more flexible than their masonry, steel or concrete counterparts. Also, hybrid buildings can be
designed to be more flexible these days partly because adequate strength can be achieved using higher
strength materials. While the increased flexibility provides benefits such as increasing the building period
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and thus lowering the seismic input forces (seismic demand), it may not provide adequate stiffness for
reducing deflections associated with wind loading and vibrations.
NBCC indicates that unless precautions are taken to permit movement of interior partitions without
damage, a maximum lateral deflection limitation of 1/250 to 1/1 000 of the building height should be
observed. According to NBC Sentence 4.1.3.5.(3), 1/500 should be used unless other drift limits are
specified in the design standards referenced in NBC Section 4.3, or a detailed analysis is made.
Precautions must be taken to ensure that the wind deflections are within the prescribed code limits and
that interior partitions, cladding and interior finishes, can accommodate wind induced deflections.
4.3.4.5.1.2
Design Values of the Building Components
One of the fundamental design values for tall wood buildings is the determination of effective stiffness in
determining the lateral deflections under the wind and checking the vibration design criterion in the form
of peak across-wind and along-wind accelerations at the top of the building. The input for determining
building's stiffness was discussed in Section 4.3.3.2.5.
4.3.4.5.2 Wind-Induced Vibration Controlled Design
Clause No. 79 in Commentary I of the 2010 NBCC gives several peak-acceleration limits used to control
the wind inducted vibrations. For example, it states that in North America in the time period from 19752000, many of the tall buildings that underwent detailed wind tunnel studies were designed for a peak
one-in-ten-year acceleration in the range of 1.5% to 2.5% of g (acceleration due to gravity – 9.81m/s2).
The lower end of this range was generally applied to residential buildings and the upper end to office
towers; their performance based on these criteria appears to have been generally satisfactory.
Clause No. 76 of the Commentary also provides equations to calculate the peak-accelerations of buildings
as below:
Across-wind direction
Along-wind direction
2
aW = f nW
g p Wd (
aD = 4π 2 f nD2 g p
ar
)
ρ B g βW
KsF ∆
CeH β D C g
[7]
[8]
The equations above require as to input 𝑓𝑛𝑊 , 𝑓𝑛𝐷 , the fundamental natural frequencies of the building in
across-wind and along-wind directions, respectively. Any validated dynamic models of wood buildings
can be used to compute the fundamental natural frequencies of wood buildings.
The equations also require to input β𝑊 , β𝐷 , the fraction of critical damping in across-wind and alongwind directions, respectively. For information on and definitions of other parameters used in these
equations please refer to the NBCC 2010 Commentary.
4.3.4.6
Testing needed to Support Wind Load Analyses and Design
Because of the complexities in design of tall buildings for architectural aesthetics, the wind load design
values tabulated in NBCC 2010 might not cover all conditions or types of structures that occur in practice.
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As the new information may become available in the future, the designer should try to obtain the latest
and most appropriate design information available. For unusual types of tall buildings, specialized
information such as theoretical studies, model tests or wind tunnel experiments may be required to
provide adequate design values.
4.3.5
Design Methodologies for Low Seismic Damage
As previously mentioned, building owners and general public may believe that if the building satisfies the
codes provisions related to seismic design, no damage of the building is to be anticipated during a seismic
event. Experiences form the earthquakes during the last few decades (e.g., the 1994 Northridge and 1995
Kobe Earthquakes, 2011 Christchurch earthquake) have forced recognition that damage, sometimes
severe, can occur in buildings designed in accordance with the codes. For example during the
Christchurch Earthquake, apart from two buildings that actually fully collapsed during the earthquake
most of the modern buildings met the code specified goal of life-safety. In most cases, however, this was
accompanied by major structural and non-structural damage. The extent of the structural damage in many
buildings was so great that close to 1,100 buildings that “performed adequately” according to the code
objectives had to be fully or partially demolished (CERC, 2012). The number of demolitions, the cost of
repairs and insured losses related to structural and non-structural damage and the business disruption after
a large earthquake add substantial economic and social impact.
These factors have led to developing of new solutions and technologies for seismic design of buildings
that focus on lowering the sustained damage during and after major earthquakes. The solutions ensure that
the inflicted damage during a severe earthquake event is relatively small and may be easily and
economically repaired with minimal disruption and downtime for building users. For new buildings, the
low-damage technologies have been developed specifically to be incorporated into the structure at a
comparable cost to conventional systems using common construction practices. Although the low-damage
technologies are all inter-related and are not mutually exclusive, they can conveniently be described in
two main categories:
1. Methods of active or passive energy dissipation and vibration control of the seismic response.
They include base isolation with or without supplemental damping or energy-dissipation
devises, which control the response of a building by reducing accelerations and the building’s
displacements. More information on these systems is provided in Section 4.3.5;
2. Emerging new forms of low-damage technology. These solutions may incorporate rocking
mechanisms with self centering capabilities (mainly by post-tensioning) in conjunction with
energy dissipation devices that act as ductile energy absorbing areas.
4.3.5.1
Passive and Active Seismic Isolation and Vibration Control
Detailed information on the seismic design of buildings using energy dissipation devices such as viscous
dampers and base isolation, including both theory and examples of practical applications, is given in
literature (See upcoming NBCC 2015 and ASCE7-10).
The concept of base isolation is to interpose a layer with low horizontal stiffness between the ground and
the building so that the layer deforms rather than the building. The layer's low horizontal stiffness results
in a modified structure that has a fundamental period that is much longer than that of a building structure
on a fixed base. The increase in fundamental period of building (e.g., 2 to 3 seconds) significantly reduce
the demand during a seismic event.
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Supplemental energy dissipation devices, often referred to as dampers (even if damping is not the primary
dissipation mechanism), could be inserted into a structural system with the express objective of reducing
the response of the overall building by absorbing or dissipating energy within the devices.
The response of such systems needs to be evaluated using a dynamic response approach in which the
excitation is consistent with that required in the NBCC provisions. This approach would normally
comprise ground motion time-histories having spectra that are compatible with the Commentary J
specified design spectral acceleration values for a particular location, including both site effects and the
appropriate importance factor.
Other passive and active systems such as Tuned Mass Dampers (TMD) can also be explored. For example
water tanks at the top of a tall wood building that serve as a backup supply for the fire sprinkler system
can be designed to act as mass dampers (e.g., tuned liquid column dampers) to alter how the building
responds to seismic and wind loading.
4.3.5.2
Rocking Self Centering Post Tensioned Systems
The rocking mechanism as a method of resisting lateral earthquake forces has been used in structures
since ancient times. The principle is the same with modern structural rocking mechanisms, which use a
high-strength, post-tensioned rod acting as a controller to ensure that the structure is clamped back into its
original position after the shaking. Such high-performance structural system was developed the 90’s in
the US as part of the Precast Seismic Structural Systems (PRESSS) program (Priestley, 1999). The
system was first used in precast reinforced concrete moment frames or interconnected concrete
shearwalls, then in steel moment frames (Christopoulos et al., 2008), and finally, during the last decade it
has been introduced to wood structures (Palermo et al., 2006). The wood solution of this system named
Pres-Lam™ was developed at the University of Canterbury with the support of the Structural Timber
Innovation Company Ltd (STIC), a research consortium of the timber industry, universities and NZ
government. The seismic forces and movements are accommodated through a controlled rocking
mechanism between prefabricated elements that can be glulam beams, LVL, LSL and CLT panels used as
walls, or any wood-based floor systems (Figure 106). The structural elements are held together by long
unbonded high-strength steel tendons.
Figure 106 Post tensioning details of: a) beam-column frame structure, b) wall system
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Energy dissipation is provided by the yielding of short lengths of replaceable mild steel fuses or by other
energy dissipation devices such as hysteretic or friction dampers. The system is sometimes called a “flagshaped” hybrid system because of the way it self-centres itself and the shape of the cumulative (hybrid)
hysteresis curve. The combined hysteresis curve of such system is shown in Figure 107. The
posttensioning clamps the frame or the wall to its original position, whereas partially debonded mild steel
or other supplemental damping devices dissipate seismic energy through ductile yielding.
Figure 107 Self centering, energy dissipation and hybrid system hysteresis for PRES-Lam system
(CERC, 2012)
Tests of components and assemblies of this system subjected to seismic loads conducted at the University
of Canterbury have shown excellent system performance with no significant structural damage. Figure
108 shows testing on a two-thirds-scale multistorey frame and wall Press-Lam building. Consequently,
the system has been used in several multi-storey buildings in New Zealand so far (Figure 109; Buchanan
et al., 2008). A procedure suitable for both analysis and design of this system was developed by the
researchers in New Zealand (Newcombe et al., 2008; Newcombe 2010; Newcombe et al., 2008). The
procedure was verified against extensive experimental test data from quasi-static cyclic tests on wall,
column, and beam-column subassemblies. Based on detailed analytical-experimental comparison, looking
at both local and global connection behaviour, suggestions are given in terms of (a) strain penetration
contribution when dealing with internally glued mild steel reinforcement, (b) actual stiffness of the
connection when accounting for the bearing effects in the parallel or perpendicular-to-the-grain
directions. These considerations are then implemented into the design procedure in order to guarantee
control over the re-centering/dissipative system mechanism.
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Figure 108 A two-thirds-scale two-storey frame consisting of beam and wall Press-Lam elements
Use of the Pres-Lam system in tall wood building applications is possible, but further research may be
needed. Post-tensioned timber under high compressive stresses will experience some axial shortening
caused by creep and relaxation in parallel to the grain direction. The associated losses in posttensioning
have to be allowed for in the design. Since strength and stiffness perpendicular to grain of the column
elements is much lower than parallel to grain, while the shrinkage is greater, columns in post-tensioned
beam-column joints would require special reinforcement.
Figure 109 A detail of wood-based rocking wall system with energy dissipater
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The main advantage of designing tall wood buildings for low seismic damage is the potential for small
downtime and thus avoiding the long and expansive post-earthquake repairs that are needed in regularly
designed buildings.
4.3.6
Quality Assurance
The design of tall wood buildings should follow the applicable "Quality Control/Management" Procedure
in effect rigorously. This includes:
1. Documented Checks of all Engineering Work; proper and appropriate checks are fundamental
to upholding the Code of Ethics, which first and foremost requires that all professionals hold
paramount the safety, health and welfare of the public, the protection of the environment and
promote health and safety in the workplace. The consultant should maintain documented
checks which includes regular and documented checks of our engineering work including:
•
•
•
•
•
•
•
Assessing the risk and level of checking required for each job;
Scheduling, resourcing and budgeting for quality assurance and quality control;
Confirming and documenting input requirements;
Gathering and checking input data;
Self-checking or independent checking of calculations;
Checking, verifying and validating engineering work; and
Documenting and retaining appropriate records of checks, corrections and corrective
action, as needed.
2. Documented Independent Review of Structural Designs; documented independent review of
structural designs prior to construction should be carried out by a professional engineer
having appropriate experience in designing structures of a similar type and scale, and not
involved in preparing the design. The review includes representative samples of the structural
assumptions, continuity of gravity and lateral load paths, stability and detailing. Where
appropriate, numerical calculations are carried out on a sample of gravity and lateral force
resisting elements necessary to double check the design. The extent of the review is
determined by the reviewer based on the progressive findings of the review.
3. Direct Supervision; when a professional engineer in the consultant office delegates activities
involving the practice of professional engineering it should meet the intent of the following:
•
•
•
•
•
Basic and general guidance on direct supervision,
Active involvement,
Adequate supervision of field reviews
Responsibility for professional engineering decisions; and
Appropriate consideration of experience levels when delegating professional tasks,
4. Peer Reviews; the analysis and design of tall wood buildings should be accompanied by a
detailed peer review of analysis methodology and design components to ensure the design
and detailing meets the intent of NBCC for both gravity and lateral loads.
5. Use of the Seal; All professional engineers should use their seal, as needed, with signature
and date, to seal or stamp specifications, reports, documents, plans or drawings that have
been prepared and delivered by the engineer in charge, or that have been prepared and
delivered under the engineer's direct supervision.
6. Retention of Project Documentation; consultant should retain complete project
documentation including correspondence, investigations, surveys, reports, data, background
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information, assessments, designs, specifications, field reviews, testing information, quality
assurance documentation, and other engineering documents for a minimum period of 10
years.
7. Documented Field Reviews During Implementation or Construction; proper and appropriate
field reviews are fundamental to upholding the applicable Code of Ethics, and as such, all
engineering documents are prepared by, or under the direct supervision of, a professional
engineer of record. The professional engineer of record or subordinate carries out periodic
field reviews to confirm that the construction conforms to the engineering design documents
prepared for the work.
4.3.7
Recommendations for Future Work
The analysis and design of tall wood buildings require complex and innovative thinking and as such the
designers should review the available literature and the state-of-the-art on the analysis, design and
detailing for seismic loading. As such information is not as extensive as the one available for steel and
concrete structural systems, future work should include experimental testing and companion analytical
studies to better define the strength, stiffness, ductility and energy dissipation properties of various woodbased systems, or hybrid systems that will include steel and concrete elements. Experimental dynamic
vibration measurements should also be carried out on scaled models for comparative studies to identify
effective stiffness and damping ratio of tall wood buildings.
4.3.8
References
American Society for Testing and Materials (ASTM). 2009. Standard test methods for cyclic (reversed)
load test for shear resistance of vertical elements of the lateral force resisting systems for buildings.
ASTM E 2126. West Conshohocken, Pennsylvania: ASTM.
American Society of Civil Engineers (ASCE) 2000. Pre-standard and commentary for the seismic
rehabilitation of buildings. FEMA-356. Washington DC.
APEGBC. 2009. APEGBC Technical and Practice Bulletin: Structural, Fire Protection and Building
Envolope Professional Engineering Services for 5 and 6 Storey Wood Frame Residential Building
Projects (Mid-Rise Buildings).
APEGBC. 2011. Practice Bulletin. Structural, fire protection and building envelope professional
engineering services for 5 and 6 storey wood frame residential building projects (Mid-rise buildings).
Association of Professional Engineers and Geoscientist of BC.
ARA, 2003, Progressive Collapse Guidelines, prepared by Applied Research Associates for the General
Services Administration, Washington, D.C.
Archila, M. 2011. Nonlinear Response of High-Rise Buildings: Effect of Directionality of Ground
Motions. A thesis submitted in partial fulfillment of the requirements for the degree of Master of
applied science, University of British Columbia.
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Building Sound Insulation and Floor Vibration Control
4.4
Lead Author: Lin Hu and Stefan Schoenwald
Co-Authors: Y.H. Chui, Dan Dolan, Eric Karsh
Peer Reviewers: Brad Gover, Michael Green, Jean-Luc Kouyoumji, Peggy Lepper, Ciprian Pirvu
Abstract
Sound and annoying vibration mitigation are important serviceability considerations for the design of
multi-family or multi-party occupancies. While many of the methods for mitigating the two issues may
be similar, the actual mechanisms for mitigation may be different. This Section provides an overview of
each of these issues with general guidance to the designer on methods that might be employed to reduce
or eliminate the issue as detraction in the opinion of the occupants. It also recommends the best practices
for implementation of the design solutions in the real buildings to achieve the design goals and to ensure
the end users’ satisfaction.
The first part of the Section covers sound transmission and the design considerations associated with its
minimization. The requirements and recommendations for building sound insulation of the National
Building Code of Canada (NBCC) and other codes are provided, along with discussion of the
mechanisms available for wood construction to meet these requirements. The designer is provided with
general concepts of how the connections, structural form, and material combinations might be adjusted to
improve the mitigation effects on sound transmission. Examples of systems with good performance are
provided, along with architectural considerations that might render well design components ineffectual in
mitigating noise. The discussion illustrates to the designer that the overall building system has as large an
impact on the final sound mitigation effort as the design of the individual components.
The second part of the Section presents the annoying vibration topic, the mechanisms of why vibration
can be annoying, and design considerations on how to mitigate the excessive vibration response. Design
criteria previously proposed are provided, along with discussion of the mechanisms available in wood
construction to meet the response demands. The designer is provided with procedure to floor systems to
perform in an acceptable manner. The Section provides examples of how different types of wood floor
systems can be designed to meet the intent of NBC with respect to floor vibration serviceability in an
economical manner. Discussion is provided on how various structural configurations might affect the
vibration response as well as how simplifying assumptions might result in a conservative or nonconservative design.
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4.4.1
Building Sound Insulation
4.4.1.1
Scope
This section addresses the sound insulation of demising walls, partitions and floor/ceiling assemblies
between adjacent spaces, such as dwelling units, and between dwelling units and adjacent public areas
such as halls, corridors, stairs or service areas in buildings employing wood construction.
4.4.1.2
Terms and Definitions
For the purposes of this document, the following terms and definitions apply.
Apparent Sound Transmission Class (ASTC):
A single number rating of the apparent airborne
sound insulation performance of walls and floors
in buildings as perceived by the occupants. The
apparent airborne sound insulation accounts
direct transmission through the demising element
as well as flanking transmission. The ASTC is
determined according ASTM E 413 from data
measured according to ASTM E336. See the
scope for the definition of walls and floors.
Field Impact Insulation Class (FIIC):
A single number rating of the field impact sound
insulation performance of floors in buildings as
perceived by the occupants. The field impact
sound insulation accounts direct transmission
through the demising floor as well as flanking
transmission. The FIIC is determined according
to ASTM E 989 from data measured according to
ASTM E 1007. See the scope for the definition
of floors.
Flanking transmission:
The sound transmission along paths other than the
direct path through the common wall or floorceiling assembly.
Field Sound Transmission Class (FSTC):
A single number rating of the field airborne
sound insulation performance of walls and floors
in buildings. The FSTC is determined according
to ASTM E 413 from data measured according to
ASTM E 336. See the scope for the definition of
walls and floors.
Impact Insulation Class (IIC):
A single number rating of the impact insulation
performance of floors defined in the scope. It is
determined according to ASTM E 989 from data
measured according to ASTM E 492 in an
acoustic chamber where the flanking transmission
is eliminated.
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Normalized Impact Sound Rating (NISR):
A single number rating of the impact sound
insulation performance of floors in buildings.
Analogously to NNIC, NISR accounts direct and
flanking sound transmission and depends on the
room volume. It is determined according to
ASTM E 989 from data measured according to
ASTM E 1007. See the scope for the definition
of floors.
Normalized Noise Isolation Class (NNIC):
A single number rating of the apparent airborne
sound isolation performance of walls and floors
in buildings. NNIC is valid only for rooms of
150 m3 and accounts direct and flanking airborne
sound transmission. Contrarily to ASTC, it
depends on the room volume and area of the
demising floor or wall. NNIC is determined
according to ASTM E 413 from data measured
according to ASTM E 336. See the scope for the
definition of walls and floors.
Sound Transmission Class (STC):
A single number rating of the airborne sound
insulation performance of walls and floors
defined in the scope. It is determined according to
ASTM E 413 from data measured according to
ASTM E 90 in an acoustic chamber where the
flanking transmission is eliminated.
4.4.1.3
NBC and Other Code Requirements
The National Building Code of Canada (NBCC) requires STC rating of 50 for the airborne sound
insulation of walls and floors for dwelling units while an IIC rating of 50 is recommended for the impact
insulation of floors as defined in the scope (NRC 2010). A forthcoming change NBCC is to use ASTC
instead of STC as the measurement for airborne sound insulation. The change from STC to ASTC has
been accepted by standing committees responsible for Part 9 and Part 5 of the NBCC. The proposed
minimum ASTC rating is 47, when measured in accordance with ASTM E336. These changes are
necessary because ASTC accounts for the direct sound and flanking sound transmission paths, whereas
STC ignores the latter. At the time of preparing this report, this change has been sent for public review.
At this stage, no change has been proposed for impact sound insulation performance.
Note that FPInnovations and the Canadian Wood Council have been working with the National Research
Council (NRC) to develop information that can be used to design of construction built with various
materials, including light-frame wood assemblies and CLT, to meet target ASTC values. A
corresponding document is being developed by a multi-industry Special Interest Group – Guide to
Calculating Airborne Sound Transmission in Buildings – and includes guidance on calculating ASTC
values for light frame wood assemblies and CLT. This document is expected to be published this
summer. It is also anticipated that the latest research will incorporated into the SoundPATHS program,
making it easier for designer to specify appropriate building assemblies.
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The International Building Code (IBC 2009) provides the minimum requirements for sound insulation of
demising walls and floor/ceiling assemblies between adjacent dwelling units or between dwelling units
and public areas such as halls, corridors, stairs or service areas. Table 5 lists these requirements.
Table 5
IBC’s minimum requirements for sound insulation of demising walls and floor/ceiling
assemblies
Wall
Floor
Notes:
1.
2.
STC
FSTC
STC
FSTC
Airborne sound
50
45 (field measured1)
50
Structure-borne sound
N.A.
IIC
FIIC
50
When tested in accordance with ASTM E90 for STC and ASTM E336 for FSTC.
When tested in accordance with ASTM E492 for IIC and ASTM E1007 for FIIC.
The International Code Council (ICC 2010) recommends two grades of acoustical performance beyond
the current code minimum such as that shown in Table 5 – acceptable and preferred. Table 6 and Table 7
below list such recommendations.
Table 6
ICC grades of field acoustical performance recommendations
Field sound rating
Airborne noise, NNIC
Impact noise, NISR
Table 7
Preferred performance (Grade A)
57
57
ICC grades of laboratory acoustical performance recommendations
Laboratory sound rating
Airborne sound, STC
Impact sound, IIC
4.4.1.4
Acceptable performance (Grade B)
52
52
Acceptable performance (Grade B)
55
55
Preferred performance (Grade A)
60
60
Principles for Building Sound Insulation Design
Providing sufficient mass, decoupling building components, and discontinuing building components are
the basic principles for building sound insulation design. Specifically, the main factors affecting airborne
sound insulation of wall and floor-ceiling assemblies are (NRC 2002):
•
•
•
•
•
•
Total weight per unit area: The greater the weight, the better the sound insulation, especially for
low frequency sound;
Sound absorption: Sound absorbing material in the air space or the cavity between layers is
beneficial;
Stiffness: In general, for the “heavy” monolithic assemblies, such as CLT, concrete, etc., the
stiffer the assembly, the better the sound insulation. However, it cannot be generalized for the
light frame walls and floors. It has been observed that short-span very stiff wood-joist floorceiling assemblies result in poor low-frequency impact sound insulation; and that the stiff stud
walls with small stud spacing have also poor sound insulation;
Contacts between layers: The softer the contacts, the better the sound insulation;
Material porosity: The lesser the porosity, the better the sound insulation;
Multi-layers with air space: The larger the airspace, the better the sound insulation;
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•
Floor surface hardness: The harder the surface, the poorer the impact sound insulation, especially
for high-frequency impact sound.
However, the general design details that are effective in limiting sound transmission in wood floor
systems are: a) breaking of direct structural transmission of sound by separating the floor framing
between occupancy areas, b) providing a relatively high mass or topping, and c) providing soft materials
for floor covering or between the structural assemblies to damp the sound.
In general, to ensure acceptable sound insulation performance, the concept should be to a) contain the
sound in one room or occupancy area by planning traffic patterns and penetrations to avoid direct
transmission to the adjoining occupant’s area; b) break the sound transmission pathway by staggering
studs, or floor and ceiling joists; and c) providing materials that will either absorb (high mass) or damp
(soft) the sound in between the occupancy areas.
Designers should be aware that simply addressing the walls and floor construction details might not be
sufficient. Openings are very effective at transmitting sound. For example, a well-designed wall might
not transmit much sound, but if there are openings such as doors into a common hallway, or penetrations
to allow plumbing, electrical, etc. to pass from one room or floor to the next, the sound barrier will be
rendered ineffective. Penetrations and access patterns must be considered and additional methods for
insolating these locations must be employed.
Meanwhile, knowledge of human ear perception to noise is also important for a cost-effective sound
insulation design. Pope (2003) described how humans perceive change in sound levels (Table 8). This
knowledge is critical for developing cost-effective sound insulation solutions or improving existing sound
insulation strategies. Table 8 demonstrates that a change (reduction or increase) in sound level of less than
3 dB, will most likely not be perceived by a listener. However, a change of 3 dB or greater will most
likely be perceived by most people.
Table 8
Perceptible change due to the change in sound level (dB) (Pope 2003)
Change in sound level (dB)
3
6
10
15
20
4.4.1.5
Change in perceived loudness
Just perceptible
Noticeable difference
Twice as loud, or reduced to half of the loudness
Large change
Four times as loud, or reduced to one quarter of the loudness
Wood–Based Wall Sound Insulation
4.4.1.5.1 Light-Frame Wood Stud Walls
NBCC (NRC 2010) lists various light-frame wood stud walls with their STC ratings for typical low-rise
buildings. The list includes 38mm x 89mm wood stud loadbearing or non-loadbearing walls of single
row studs, two rows staggered studs on 38mm x 140mm plate, and two rows studs on separate plates.
The STC ratings measured on the walls vary from 32 to 62 depending on the construction details such as
stud spacing, wall sheathing panel type and thickness, finishing with gypsum boards and number of
gypsum boards layers, use of sound absorption materials in the wall cavity, and attachment detail of the
gypsum boards to the structure walls. The attachment detail of the gypsum boards to the structure walls
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includes directly attaching the boards to the walls, attaching the boards to the walls through furring, or
through resilient channels.
Tall wooden buildings require strengthened framing and shear bracing of the walls in the lower stories to
resist the increased axial and lateral loads due to the increased building height. Some of these structural
measures compromise the sound insulation performance of the walls and additional measures are
necessary to achieve sufficient sound insulation. Results of a parametric study that was conducted in
NRC Construction’s Wall Sound Transmission Test Facility on a set of typical load bearing wood stud
walls for midrise construction are published in (NRC 2013a). The study investigates in detail the effect of
framing variants (small stud spacing vs. doubled/tripled studs, built-up columns, etc.) and of shear
bracing (different materials, blocked, un-blocked) and lists STC data for a variety of walls with 38mm x
89mm and 38mm x 140mm staggered and single stud row.
FPInnovations’ report (Hu 2014a) contains additional information on ASTC and FSTC ratings of some
light-frame wood stud walls measured by FPInnovations in wood-frame buildings and the wall
construction details. The report includes single-row and double-row 38mm x 89mm wood stud walls
having FSTC ratings ranging from 50-56.
4.4.1.5.2 CLT Walls
Chapter 9 of the Canadian edition of the CLT Handbook (Gagnon and Kouyoumji 2011) provides
examples of CLT wall assemblies with their STC ratings. The examples include 3-ply of 95mm to
115mm thick single and double CLT walls with various construction details. The construction details
include finishing with gypsum boards, number of gypsum board layers, nature of attachment of the
gypsum boards to the CLT walls, and sound absorption materials in the wall cavity. The STC ratings of
the tested CLT wall assemblies ranged from 32 to 60 depending on the construction details, with the
highest rating being given to the assembly that is described in Table 9.
Table 9
STC 60 CLT wall assembly
Top view of cross-section of the wall
Assembly description of the drawing
from top to bottom
1. Gypsum board of 15 mm
2. 3-layer CLT panel of 95 mm ~ 115 mm
3. Sound insulation material (mineral or
rock wool) about 30 mm
4. Sound insulation material (mineral or
rock wool) about 30 mm
5. 3-layer CLT panel of 95 mm ~ 115 mm
STC
60
or above
depending
on CLT
thickness
Further STC data of 3-ply and 5-ply Canadian CLT walls with and without various gypsum board lining
is published in the NRC research report (NRC 2013b). This study also shows, that analogously to
masonry walls the sound insulation improvement of the wall-linings measured in one situation can be
used under certain conditions to predict the sound insulation performance when applied to other bare CLT
walls with known sound transmission loss. The study gives measured and predicted STC data for CLT
wall assemblies with a wide variety of generic wall linings.
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Additional construction details of field CLT wall assemblies with FSTC of 46-50 are provided in
Chapter 9 of the U.S. edition of the CLT Handbook (Hu and Adams 2012a). Field tested CLT wall
assemblies included single and double walls of 3-ply 78mm CLT and single wall of 5-ply 184mm CLT
with various construction details. Details of the CLT wall assemblies and the CLT building conditions
are described in a FPInnovations report (Hu 2014a). Besides the FSTC ratings, the report also provides
ASTC ratings of the field CLT wall assemblies. Table 10 provides the details of the field tested CLT wall
of FSTC /ASTC 50.
Table 10
CLT wall assembly of FSTC/ASTC 50
Top view of cross-section
Assembly description of the drawing
from top to bottom
1. 16mm Gypsum board about 11kg/m²
2. 38mm by 64mm wood studs at 400mm
O.C
3. 64mm rock wool in the wall cavity
4. 12mm air gap
5. 3-layer CLT of 105mm
6. 12mm air gap
7. 38mm by 64mm wood studs at 400mm
O.C
8. 64mm rock wool in the wall cavity
9. 16mm Gypsum board about 11kg/m²
FSTC/
ASTC
50
4.4.1.5.3 Other Wall Construction
For other wall construction, including other types of wall assemblies which are not covered by the
references above, their sound insulation design shall be verified with the measured STC, or FSTC, or
ASTC according with the ASTM E90 for STC and ASTM E336 for FSTC/ASTC.
4.4.1.6
Wood- Based Floors Sound Insulation
Usually, design details for Wood-Based Floors in tall buildings structurally do not differ very much from
typical designs that are used in low-rise buildings, because floors do not have to support additional loads
of the upper stories. However, there might be differences due to other requirements, e.g. fire safety, or
due to differences in the construction type (e.g. massive wood post and beam construction, hybrid
construction) that is employed for tall wooden buildings.
4.4.1.6.1 Light-Frame Joisted Wood Floors
NBCC (NRC 2010) lists various light-frame wood joisted floors given with their STC and IIC ratings.
The list includes solid sawn lumber joists minimum 38 x 235mm and wood I-joists minimum 38 x 38mm
flange with minimum 9.5mm OSB or plywood web, and minimum 241mm deep; and open web wood
trusses with wood framing members not less than 38mm x 89mm and minimum 235mm deep. The STC
and IIC ratings of the floor-ceiling assemblies vary from 31 to 70 and 19 to 51, respectively depending on
the construction details. Construction details include the topping material and thickness, and construction
details of the ceiling such as the number of gypsum board layers, thickness of gypsum boards, type of
attachment of the gypsum boards to joists, and the type of material used to fill in the ceiling cavity. It
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must be pointed that the topping material used in floor-ceiling assemblies is limited to wood panel such as
11mm OSB or plywood, 25mm gypsum-concrete and 38mm normal weight concrete. The concrete
should be directly poured on the subfloor, but not be floated. The floor-ceiling assemblies are assumed to
have no finishing.
Some producers of sound insulation materials and wood I-joists and open web trusses provide STC and
IIC ratings on various light-frame wood joisted floors in their product brochures.
FPInnovations report (Hu 2014a) contains additional information on ASTC, FSTC and FIIC ratings of
some field light-frame wood joisted floors measured by FPInnovations in various wood-frame buildings
and in a mock-up of a two-storey light frame wood building. The floor-ceiling assemblies included
finishing, and a dry or wet floating topping. The dry toppings consisted of a 20-30mm gypsum board raft
and cement-fiber board. The wet topping may be 25-50mm thick gypsum concrete, lightweight concrete
or normal weight concrete. The measured FSTC and FIIC ratings were from 45 to 60. The report also
includes a study of the effects of finishing, underlayment, topping on impact sound insulation of the lightframe wood floors. This study has been described in a paper by Hu et al (2013).
4.4.1.6.2 CLT Floors
STC and IIC ratings of selected assemblies with 5-ply 135-146 mm thick CLT panels and various
construction details are given in Chapter 9 of the Canadian edition of the CLT Handbook (Gagnon and
Kouyoumji 2011). The construction details cover solid and hollowed topping, underlayment for the
topping, dropped gypsum board ceiling, and its construction details such as gypsum boards and their
attachment to the CLT floors, and number of layers and thickness of gypsum boards. The measured STC
and IIC ratings of the CLT floor-ceiling assemblies were within the range of 39-67, and 24-72
respectively, depending on the construction details. Insulation ratings for additional field CLT floorceiling assemblies are provided in the U.S. edition of the CLT Handbook (Hu and Adams 2012a). These
field CLT floor-ceiling assemblies were measured by FPInnovations in various CLT buildings. The fieldtested CLT floor-ceiling assemblies with FSTC and FIIC ratings of 45 to 53 were made of 5-ply 175mm
CLT panels with various combinations of finishing, underlayment, a dropped gypsum board ceiling, and
a dry or a wet cement-based topping.
Further STC and IIC data of 5-ply and 7-ply Canadian CLT floor assemblies with and without floating
floor toppings and gypsum board ceilings is published in the NRC research report (NRC 2013b). This
study also shows, that analogously to concrete floors the sound insulation improvement of a floor topping
or hung gypsum board ceiling measured on one CLT floor can be used under certain conditions to predict
the sound insulation performance when applied to other bare CLT floors with known airborne and impact
sound insulation. The study gives measured and predicted STC and IIC data for generic CLT floor
assemblies with a variety of different floor and ceiling treatments.
Table 11 provides the details of the assemblies with STC of 67 and IIC of 72. Table 12 illustrates the
details of the 53 FSTC and FIIC field CLT floor assemblies.
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Table 11
STC 67 and IIC 72 CLT floor-ceiling assemblies
Assembly description from top to
bottom
End view of cross-section
Particleboard panel of 22 mm
Particleboard panel of 22 mm
Mineral wool about 40 mm
Lumber sleepers of 40 mm x 40 mm at
lease 400mm O.C. attached to
particleboard
5. REGUPOL underlayment
6. 5-layer CLT panel of 146 mm
7 Sound Isolation Clip of 100mm high
8. Metal hat channel
9. Glass fibre of 100 mm
10 .Gypsum board of 13 mm
STC
IIC
67
62
64
72
1.
2.
3.
4.
1. Prefabricated concrete topping of 20
mm
2. Kraft paper underlayment
3. Prefabricated concrete topping of 20
mm
4. Sub-floor ISOVER EP1 of 30 mm
5. Honeycomb acoustic infill
FERMACELL of 30 mm of 45kg/m²
6. Honeycomb acoustic infill
FERMACELL of 30 mm of 45kg/m²
7. Kraft paper underlayment
8. 5-layer CLT panel of 135 mm
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Table 12
FSTC and FIIC 53 field CLT floor-ceiling assemblies
Assembly description from top to bottom
1. Hardwood flooring attached to the
plywood
2. 18mm plywood
3. Around 10mm underlayment (rubber
mat, texture felt, etc.)
5. Sound Isolation Clips, at 100 mm high
8. Gypsum board of 12 mm type C about
9kg/m²
9. Gypsum board of 15 mm type X about
11kg/m²
Ceramic tile glue to
12mm plywood
18mm plywood
Around 10mm underlayment (rubber
mat, texture felt, etc.)
6. Sound Isolation Clips, at 100 mm high
9. Gypsum board of 12 mm type C about
9kg/m²
10. Gypsum board of 15 mm type-X
about 11kg/m²
FSTC
FIIC
53
53
53
53
1.
2.
3.
4.
A report to be published by FPInnovations (Hu 2014a) contains detailed information on the construction
details of the field 5-ply 175mm thick CLT floors in a building. The report also includes a study on the
sound insulation of 175mm and 130mm thick CLT floor-ceiling assemblies with various combinations of
finishing, underlayment, a dropped gypsum board ceiling, and a dry or a wet topping in a mock-up twostorey CLT building. The measured FSTC and FIIC ratings were from 40 to 60. The effects of thickness
of CLT, finishing, underlayment, topping, ceiling on CLT floor FSTC and FIIC ratings are discussed in
details.
4.4.1.6.3 Massive Timber Floors
Little information on the sound insulation of massive timber floors has been found. Table 13 provides the
field airborne sound and impact sound ratings of a massive timber floor found on internet at:
http://www.kineticsnoise.com/arch/tests/wood-framed.html.
A FPInnovations’ report (Hu 2014a) includes information on ASTC, FSTC, and FIIC ratings of field
massive timber floor systems consisting of glulam beam and thick wood deck measured by FPInnovations
in several post-and-beam office buildings and in a mock-up two-storey wood building. The glulam
floors had spans from 5.4m to 9m. The floors had no ceiling (i.e. the wood was exposed on the ceiling
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side) but with a gypsum board dry topping, or with a cement-based topping. The measured FSTC ratings
were from 40 to 50, and the FIIC ratings were from 33 to 49 depending on the finishing, and the
underlayment for the topping and the finishing. The system with the best field airborne sound insulation
had a heavy topping of 78kg/m², while the field floor with the best impact sound insulation had a carpet
finish over the heavy topping. The report also describes the effect of various underlayment and topping on
impact sound insulation of the glulam floors in the mock-up.
Table 13
FSTC and IIC of a massive timber floor
(http://www.kineticsnoise.com/arch/tests/wood-framed.html)
Assembly description from top to
bottom
75mm Lightweight Concrete (polished)
FSTC
FIIC
62
(NNIC)
54
13mm Plywood
50mm Kinetics® RIM L-2-16
89mm Wood Deck Subfloor
Steel Beam and Glulam Joist Support
4.4.1.6.4 Wood Concrete Composite Floors
There is no North American information on the sound insulation of wood concrete composite floors in
literature. Some information is available from Europe where this construction is more common, but
sound insulation ratings are developed according to ISO-standards (Lignum 20XX). FPInnovations
conducted a study on sound insulation of such a field floor. Information on ASTC, FSTC, and FIIC
ratings of the field wood concrete composite floor measured by FPInnovations in a post-and-beam
building can be found in a FPInnovations report (Hu 2014a). The floor was built with 100mm reinforced
concrete connected to an 89mm laminated-strand-lumber (LSL) through a 25mm thermal insulator using
shear connectors. The wood on the ceiling side of the floor system was exposed, and its top was covered
with a carpet. The span of the floor was 6.4m. The measured FSTC and FIIC of the floor was 53 and 63,
respectively.
4.4.1.6.5 Other Floor Construction
For other floor construction details not previously evaluated, their sound insulation performance should
be evaluated by testing to measure STC and IIC, or FSTC and FIIC or ASTC and AIIC in accordance
with ASTM E90 for STC and ASTM E336 for FSTC/ASTC; with ASTM E492 for IIC and ASTM
E1007 for FIIC/AIIC.
4.4.1.7
Wooden Building Sound Insulation System Performance
Apparent airborne and impact sound insulation performance of demising floor and wall elements in real
buildings is generally worse than when tested in a wall or floor sound transmission test facility due to
flanking sound transmission that is suppressed in the test facility. Flanking sound transmission occurs if
building elements - either the demising element or others that are connected to it - are excited by airborne
or impact sound. At the building junction structure-borne sound is transmitted from the excited element
to other connected building elements and radiated from the demising or the flanking elements into the
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receiving room. The principal of flanking transmission at the floor wall junction between two
horizontally adjacent rooms is depicted in Figure 110.
Figure 110 Direct and flanking sound transmission for the floor-wall junction between two side-byside rooms (Path naming convention according to ISO 15712: “D”, “d”: direct element; “F”, “f”:
flanking element; Source room: Capital letter; Receiving room: lowercase)
Other flanking sound transmission paths could also be leaks, e.g. due to building service installation that
penetrate the demising element, or duct work from one room to the other.
The sum of direct and flanking sound transmission between two rooms is the so-called apparent sound
transmission that is rated using ASTC, FIIC, NNIC and NISR.
A change of the requirement for airborne sound insulation from the element performance STC to the
building performance rating ASTC might be introduced in the National building Code Canada as early as
in 2015. FPInnovations and the Canadian Wood Council have been working with the National Research
Council (NRC) to develop information that can be used to design of construction built with various
materials, including light-frame wood assemblies and CLT, to meet target ASTC values. A
corresponding document is being developed by a multi-industry Special Interest Group – Guide to
Calculating Airborne Sound Transmission in Buildings – and includes guidance on calculating ASTC
values for light frame wood assemblies and CLT (NRC 2013c). It is also anticipated that the latest
research will incorporated into the SoundPATHS program that will be publicly available on the internet,
making it easier for designer to specify appropriate building assemblies.
4.4.1.7.1 Wood Frame Buildings
Currently, no established prediction method exists to completely predict the sound insulation of a
particular flanking path in wood frame buildings. Therefore, apparent sound transmission has to be
measured either in a real building or in a controlled laboratory environment in a special flanking sound
transmission test facility (Estabrooks et al. 2009). The latter has the advantage that flanking sound
transmission along a particular junction path can be isolated (King et al. 2009), effects design
modifications can be studied (NRC 2006a) and data from different junctions can be combined to predict
the apparent sound insulation between two rooms in a building. Flanking sound insulation data for
typical junctions of wood frame walls with 38 mm x 89 mm studs and wood joist floors is presented in
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NRC’s Guide for Sound Insulation in Wood Frame Construction (NRC 2006b). More recent data with
specific design details for taller wood frame buildings are expected to be published this fall (NRC 2014).
The report for example includes wall designs with staggered tripled 38 mm x 89 mm studs with and
without shear bracing, as well as the effect of build-up columns that are necessary in tall wood buildings
to support point loads or tie-downs. All the above mentioned data will be included in NRC’s
SoundPATHS software.
Besides laboratory testing the apparent sound insulation can also be measured in the field and the results
obtained at for an existing building can be used to demonstrate code compliance for another building with
nominally the same design details. Sources for airborne and impact apparent sound insulation results
from field tests are presented in Section 4.4.1.5.1 and Section 4.4.1.6.1.
4.4.1.7.2 CLT Buildings
CLT wall and floor assemblies are acoustically similar to homogenous monolithic construction like
concrete and masonry. Therefore, it is possible to predict flanking sound insulation from the direct
airborne and impact sound insulation data of the elements and data for coupling of structure-borne sound
at the building junction utilizing. The standardized prediction method is well established for concrete and
masonry buildings and it was demonstrated that it is also valid for the prediction of apparent sound
transmission in CLT buildings (Schramm et al. 2010). ISO 15712 is identical to the European Standard
EN 12354 that is usually referred in publications abroad. The application of the method for CLT
buildings is described in detail in the Guide to Calculating Airborne Sound Transmission in Buildings
(NRC 2013c). Necessary input data for the element performance and the junction coupling for some
typical building junctions is presented in a research report that is being prepared (NRC 2013b).
Unfortunately, the latter has to be measured at CLT junction mock-ups according to ISO 10848, since the
equations for simple line connections in ISO 15712 are not valid for CLT construction because the
elements are usually point connected with long wood screws or metal plates.
Further, it is also possible to obtain apparent sound transmission data in field tests in an existing building
or in the laboratory in a flanking sound transmission suite analogously to wood frame construction
described in Section 4.4.1.7.2. However, the effort for testing in a flanking sound transmission facility is
much greater than utilizing ISO 15712 methods and the apparent sound insulation measured in the field is
only representative for the particular combination of CLT wall and floor elements including the particular
surface treatments such as gypsum board wall and ceiling linings, as well as floor toppings and finishing.
Sources for airborne and impact apparent sound insulation results from field tests are presented in Section
4.4.1.5.2 and Section 4.4.1.6.2.
4.4.1.7.3 Massive Wood and Wood-Hybrid Buildings
For Massive Wood and Wood-Hybrid Buildings the methods described in Section 4.4.1.7.1 are valid and
in some cases for flanking transmission between rather homogenous monolithic building elements, like
wood composite floors, also predictions utilizing the ISO 15712 approach are applicable to obtain
estimates for the apparent sound insulation.
In Guide to Calculating Airborne Sound Transmission in Buildings (NRC 2013c) examples are presented
for the apparent sound transmission in wood hybrid buildings with concrete block walls and wood framed
floors. The examples are based on the results of a recent joint research project of the National Research
Council (NRC) and the Canadian Concrete Masonry Producers Association (CCMPA) reported in NRC
2013d.
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Further sources for apparent sound insulation data for massive timber floors and wood concrete composite
floors obtained in field measurements by FPInnovations are already listed in Section 4.4.1.6.3 and Section
4.4.1.6.4.
4.4.1.8
Best Practices for Ensuring End Users’ Satisfaction – Step by Step Guide
This section guides the readers, step by step, towards a satisfactory sound insulation for wood building
projects.
4.4.1.8.1 Step 1: Selecting Construction Solutions for FSTC and FIIC at Least 50
Experience in field surveys and investigations have shown that even meeting the minimum IBC
requirements (i.e. FSTC and FIIC of 45), or the proposed NBC requirement for ASTC 47 does not always
eliminate complaints from occupants. While not always possible, it is recommended to target FSTC and
FIIC ratings of 50 or use ICC recommendations, particularly in multiple residential dwelling units.
4.4.1.8.2 Step 2: Eliminating Avoidable Flanking Paths
To optimize the efficiency of the sound insulation solutions provided in codes and literatures, a qualitycontrolled installation protocol must be implemented in order to eliminate avoidable flanking paths.
There are two types of flanking transmission, i.e. sound leaking through openings, and vibration transfer
between the coupled surfaces or through the continuous structural elements. The basics of flanking
control are to seal openings, decouple surfaces, and discontinue structural elements if it does not affect
structural safety and serviceability. However, compromise will sometimes be necessary. Table 14
provides a flanking path checklist and treatment. The list includes the most obvious and crucial flanking
paths that must be controlled or eliminated. If the flanking paths can be controlled, then the recommended
solutions should provide satisfactory sound insulation for wood buildings.
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Table 14
Flanking path checklist and treatment
Flanking Path
Leaks around edges of partitions (ASTM E336).
Treatment
Seal leaks with tape, gaskets, or caulking compound
(ASTM E336). Plan the traffic patterns such that doors
do not open onto common areas where sound can be
easily transmitted around the dividing wall, floor etc.
Caulk joint between gypsum board and floor (NRC
2002).
Clean floor and caulk sill plate (NRC 2002).
Avoid back-to-back outlets by offsetting them 16” (400
mm) or at least one stud space from side to side (NRC
2002).
Cracks at wall/floor junctions.
Debris between floor and wall sill plates.
Leaks through electrical outlets.
If gypsum board is rigidly attached to studs or the wall
framing, the wall could contribute to flanking (NRC
2002).
Joint between the perimeter of flooring or topping and
the surrounding walls, especially if the flooring or
topping is floating or not rigidly attached to the subfloor.
Continuous subflooring, joists, and CLT elements
between two adjacent units.
Attach gypsum board on resilient channels (NRC
2002).
Leave a gap around the entire perimeter of flooring or
topping assembly and the walls. Fill it with resilient
perimeter isolation board or backer rod and seal the joint
with acoustical caulking.
Discontinue subflooring, joists, and CLT as much as
possible. Add floating topping and floating flooring if
the continuity is not avoidable. Connect CLT subelements of floor at a junctions with a wall.
4.4.1.8.3 Step 3: Measuring FSTC and FIIC after Finishing
To ensure proper airborne and impact sound insulation of the finished wall and floor assemblies, it is
advisable to measure the ASTC, FSTC, NNIC, FIIC and NISR to confirm that they meet or exceed target
design values. In the worst case scenario, if they do not meet expected ASTC, FSTC, NNIC, FIIC and
NISR values, it may be possible to remedy this situation before the buildings are occupied to ensure their
satisfactory sound insulation performance and avoid potential complaints.
4.4.1.8.4 Step 4: Subjective Evaluation by Architects, Designers, Builders and Contractors
It is recommended to conduct an informal subjective evaluation by builders, developers, architects,
designers, contractors and/or product manufacturers after the building is completed and before the
occupants move in so that they can obtain quick and easy feedback regarding the sound insulation
performance of a completed building. If they do not satisfy the sound insulation of the completed
building, then the situation should be remedied immediately.
FPInnovations has developed subjective evaluation protocols which are included in a report by Hu
(2014b).
4.4.2
Floor Vibration Control
4.4.2.1
Scope
This section addresses control of excessive transverse vibrations induced by footsteps from normal human
walking or machines in wood-based floors to ensure occupants’ comfort.
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4.4.2.2
Terms and Definitions
For the purposes of this document, the following terms and definitions apply.
Fundamental natural frequency:
The lowest frequency among the infinite number of natural
frequencies.
Floor vibration:
The oscillation perpendicular to floor plan and the vertical
oscillation of a floor about its neutral plane.
Damping:
Parameter relating to the dissipation of energy, or more precisely,
to the conversion of the mechanical energy associated with a
vibration to a form that is unavailable to the vibration (sound and
heat).
Vibration:
Vibration Frequency:
4.4.2.3
The oscillation of a system about its equilibrium position.
The number of oscillations per second (Hz). For a continuous
object, such as a floor, its vibration response to an excitation
usually contains an infinite number of frequencies.
Control of Vibration Induced by Footsteps for Occupant’ Comfort
4.4.2.3.1 Design Principles for Control of Floor Vibrations induced by Footsteps
Over 10 years of floor research at FPInnovations (Hu 2007) has found that for lightweight floors
characterized with fundamental natural frequency above 9 Hz, the vibrations induced by footsteps can be
controlled through design, which involves a proper combination of floor stiffness and mass. For heavy
floors characterized with fundamental natural frequency below 9 Hz, the vibrations induced by footsteps
can be controlled through controlling damping as well as stiffness and mass in the design. Generally,
wood-based floors are considered lightweight. Therefore, wood-based floor vibration can be controlled
with the proper combination of floor stiffness and mass.
The general cause of annoyance of occupants by vibration is that the vibration either interferes with some
part of the operation, or the vibration fundamental frequency falls within a window of frequencies that
correspond to the natural frequencies of different parts of the human body. Therefore, the objective of the
design is to determine the mass and stiffness of the floor system to provide a floor system with a
fundamental natural frequency significantly different from the equipment being affected (2-5 Hz), or the
annoyance range for the human body (7-12 Hz).
Some general rules that a designer should follow for improved floor vibration response are:
a) Separate the floor framing system between occupancy areas to prevent vibration transmission.
This can be accomplished by introducing a hinge at supports rather than using continuous multispan floor framing between occupancy areas (the main mechanism for transmission of vibration
is bending action); more discussion on the supporting beams can be found in Section 4.4.2.5.1
b) Plan the floor framing system such that the floor joists are not supported on other bending
framing members, or insure that the supporting framing is very stiff. The interaction of the
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flexible support with the floor joists often causes the vibrations to become annoying to the
occupants. More discussion on the supporting beams can be found in Section 4.4.2.5.1 and.
c) Isolate sources of noise associated with the vibration (such as china cabinets, etc.). The added
sense of hearing exasperates the perception of the vibration.
4.4.2.3.2 Light-Frame Joisted Floors
These are systems that are constructed with sawn lumber, wood I-joists or open-web parallel chord
trusses, and sheathed with structural wood panels or lumber boards. They may be overlaid with nonstructural topping made with concrete or gypcrete. There have been a number of research studies
conducted on this type of floor system, all with the objective of developing suitable design approaches to
prevent excessive human-induced floor vibration. Some of the design methods are summarized in Table
15.
Table 15
Summary of design methods for light-weight joisted floors in literatures
Design parameters
dpl
Performance criteria
dpl < 2mm, for span < 3m
where dpl = deflection
under a 1 kN load
f1
dpl < 8/span1.3mm, for span
≥ 3m
where f1 = fundamental
natural frequency
f1, dpl and Vpeak
f1 > 14Hz
where Vpeak = peak
velocity due to unit
impulse
f1, arms
f1 > 8 Hz
dpl < 1.5mm
Vpeak < 100(f1 ξ -1)
f1 > 8 Hz
arms ≤ 0.45 m/s2
Limitation
Not suitable for floors with heavy
topping, and not a dynamic-based
criterion which has limited its
application, see more discussion below
Appears quite restrictive especially for
long span floors and floors with a heavy
topping
Method & Reference
National Building Code of
Canada (NBC) (NRC
2010)
Limited validation and not validated
against floor with heavy topping
Swedish design guide
(Ohlsson 1988)
Limited validation and not intended for
floors with a heavy topping
Smith and Chui (1988)
Dolan et al (1999)
where arms – root-meansquare acceleration
The NBC method to establish vibration-controlled spans is limited for lumber joisted floors having spans
from 3.0 to 6.0m and with bridging/strapping/blocking. A table of vibration controlled spans of lumber
was derived from this method and provided in NBC. The method accounts for the effect of using glue
along with nails or screws to attach the subfloor to joists on floor vibration. Because the method allows
an increase in the floor span if a normal weight concrete topping is directly poured on a wood floor,
caution should be exercised in applying the method to floors with concrete topping. FPInnovations
research on the effect of concrete topping on light-frame joisted wood floors found, on the contrary, that
the floor spans should be reduced when concrete topping is added (Hu and Gagnon 2009).
The Canadian Construction Materials Centre (CCMC) published a method to determine the vibration
controlled spans for engineered wood members, e.g. wood-I joists or wood trusses (CCMC 1997). The
span range of the method is from 3.0 to 10m. The method was an extension of the original method for the
lumber joisted floors (NBC method). The CCMC method also accounts for the effects of glue, topping
and bridging/blocking/strapping on floor vibration. But it differs from the original method in the sense
that the CCMC method allows engineered wood floors without the use of bridging/strapping/blocking,
and the use of continuous multi-span joists. However, CCMC issued a document to caution users that for
concrete-topped floors and floors with bridging and/or blocking this method may lead to overestimation
of maximum spans (CCMC 2002). Caution should also be exercised when using this method to determine
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the maximum spans for continuous multi-span floors because this method also overestimates the
maximum spans of the floors using multi-span joists.
The limitation of the NBC and CCMC methods was thought to be that the method controls the floor
vibration through controlling only the floor stiffness.
To overcome the limitation of the NBC, CCMC methods, and other methods listed in Table 15 in
collaboration with the University of New Brunswick (UNB), FPInnovations has developed a generalized
design method to determine vibration controlled spans for light-frame joisted floors having spans from
3.0 to 13.0m (Hu 2007). This method controls the floor vibrations through controlling the floor stiffness
and mass. The new design approach developed consists of three elements: 1) design criterion; 2)
calculation method to determine the criterion variables; 3) proposed design properties for floor
component materials.
The design criterion was developed based on a field consumer survey and testing of more than one
hundred wood-framed floors across Canada. The design criterion is expressed in Equation [9]:
f
d
0.44
f
> 18.7 or d < (
18.7
) 2.27
[9]
where d and f are the calculated 1 kN static deflection at the floor center in mm, and the calculated
fundamental natural frequency of the wood-based floor in Hz, respectively. Equations [10] and [11] are
the equations to compute d and f, based on ribbed plate theory.
d =
4000 P
abπ 4
f =
1
∑
∑
π
1
 1 
1
Dx   + 4 Dxy   + Dy  
a
 ab 
b
m =1, 3, 5.. n =1, 3, 5..
2 ρ
4
2
4
m
 mn 
n
  Dx + 4 
 D xy +   D y
a
 ab 
b
4
2
in mm
[10]
4
in Hz
[11]
where,
𝐷𝑥 = Floor flexural rigidity in floor span direction;
𝐷𝑦 = Floor flexural rigidity in across span direction;
𝐷𝑥𝑦 = Shear rigidity of multi-layered floor deck + torsion rigidity of joist;
𝜌 = Area density of the floor system;
see reference (Chui 2002 ) for their units and calculations.
This design approach was validated using the database for in-situ floor testing. The predicted floor
vibration performance was well matched with the occupants’ expectations. The detailed comparison was
reported by Hu (2007).
This method accounts for all the construction details of light-frame wood joisted floors and overcomes
some of the problems found in the NBC, CCMC and other methods. The construction details include the
use of glue at the connections between subfloor and joists, bridging/strapping/blocking, topping, and
continuous multi-span joists.
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The application of equations [9] to [11]is an iterative process, which is cumbersome to apply in design.
FPInnovations further simplified the above to determine explicitly the vibration controlled spans, l as
shown in equation [12] (Hu 2010a). The simplified version of the original method has the same features
as the reference method, and was verified.
l≤
0.284
1 ( EI eff )
8.22 Fscl0.14 mL 0.15
[12]
where: EIeff = effective composite bending stiffness of the T-beam (joist plus subfloor);
mL = mass per length of the T-beam;
Fscl = factor related to lateral stiffness contribution from subfloor and topping
Additional background on the calculations may be found in Hu (2011a).
4.4.2.3.3 Light-Frame Joisted Floors with Heavy Topping
The FPInnovations and UNB method, and its simplified version (Equations [9] to [12]) can be applied to
light-frame joisted floors with heavy topping such as normal weight concrete topping, or other types of
cement-based topping.
4.4.2.3.4 CLT Floors
FPInnovations developed a vibration controlled design methods for a broad range of CLT floor systems
including CLT floors with or without topping, and with or without gypsum board ceiling (Hu 2011b and
2012b). A simplified method, equation [13] has been developed based on the same principles as the
reference method. The span range of the method is from 3.0 to 6.6m for 100 to 240mm thick CLT panels.
The method was verified through laboratory study and with the design method developed at the Graz
University of Technology, Austria (Schickhofer and Thiel 2010), which was based on the floor vibration
design method given in Eurocode 5 for conventional floor systems.
l≤
Where
1m 0.293
1 ( EI eff )
9.15 ( ρA) 0.123
[13]
1𝑚
𝐸𝐼𝑒𝑓𝑓
= effective apparent stiffness in the span direction and published by the producers for 1m
wide panel;
𝜌 = density of CLT,
A = area of cross-section of 1-m wide CLT panel
Additional background on the calculations may be found in Hu (2011b and 2012b).
4.4.2.3.5 Massive Timber Beam Floors
Massive timber beam floors are floor systems made of massive timber beams and a thick wood-based
deck. The beam could be glulam, sawn timber, or other innovative composite wood beams, etc. while the
wood deck can be constructed from glulam, LSL, PSL, LVL, CLT, timber plank and other innovative
composite thick wood panels.
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The FPInnovations and UNB generalized design method originally developed for light-frame joisted
floors, and equations [9] to [11] may be applied to massive timber floors, but further verification is likely
required before it can be used with confidence.
4.4.2.3.6 Hybrid Steel Truss and Thick Wood Deck Floors
Although the generalized design method developed by FPInnovations and UNB was originally developed
for light-frame joisted floors, equations [9] to [11] also has potential to be applied to hybrid steel truss and
thick wood deck floors. If the method is used for the design, further review and verification is required
(see Chapter 9 for guidance on collecting information for model verification).
FPInnovations conducted a study on the vibration performance of a 9m span steel truss-glulam deck
hybrid floor system (Hu and Gagnon 2010). It was found that for occupants’ satisfaction, hybrid steelwood floor should have fundamental natural frequency above 10 Hz.
The American Institute of Steel Construction (AISC) design guide written by Murray et al (1997) was
developed for heavy steel beam or steel truss floors with a heavy concrete deck. Murray et al (1997)
recommend a limit on the peak acceleration, ap, to control floor vibrations due to walking. A formula was
proposed for calculating the peak acceleration using equation [14]:
ap
g
=
P0 exp(−0.35 f1 )
βW
[14]
where
𝑎𝑝 = peak acceleration of a floor, g;
g = acceleration due to gravity = 9.81 m/s2;
𝑃𝑜 = a constant force equal to 0.29 kN (65 lb.);
𝑓1 = fundamental natural frequency of the floor structure, Hz;
𝛽 = modal damping ratio;
𝑊 = effective weight of a floor, kN,
See the reference (Murray et al 1997) for information on the acceleration limit and the modal damping
ratio, and the method for the calculating the effective weight of a floor.
Such heavy steel-concrete floors typically have fundamental natural frequencies below 8 Hz, and the
vibration induced by human walking on such floors is mostly resonance-based. Applying this method to
hybrid steel truss and thick wood deck floors requires caution, especially in assigning the damping values
to the hybrid steel-wood floors and the calculations of the effective weights of the steel-wood floors. A
review and verification of the floor design is required. A report by Hu and Gagnon (2010) reviewed the
application of the AISC method for hybrid steel truss and thick wood deck floors.
4.4.2.3.7 Wood-Concrete Composite Floors
Wood-concrete composite floors are systems comprising a thick reinforced concrete slab mechanically
connected to a thick wood deck using shear connectors. Different types of proprietary shear connectors
are available in the market such as HBV or SFSintec shear connectors. Technical information on these
connectors are available from the manufacturers, however, there are currently no standardized design
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methods or evaluation procedures to evaluate these product. In the absence of a properly validated design
method, laboratory tests and subjective evaluation of the floors are advised to assess the vibration
performance of these heavy floor systems. Guidance on measuring the performance of such systems are
included in Chapter 9. FPInnovations can also provide assistance in designing the floors and in
conducting the required testing to assess performance and validate the floor design, including subjective
evaluation.
4.4.2.3.8 Other Innovative Wood-Based Floors
For any type of innovative wood-based floors not discussed above, existing design methods may not be
applicable. Testing of field and laboratory-fabricated assemblies may be necessary to assess the validity
of a floor design. Such a test should include subjective evaluation. However, FPInnovations can provide
assistance in designing the floors and in conducting the required testing to assess performance and
validate the floor design, including subjective evaluation.
4.4.2.4
Control of Vibration Induced by Machine for Occupant’ Comfort
NBCC recommends that the undesirable effects of continuous vibration caused by machines can be
minimized by special design provisions such as locating machinery away from sensitive occupancies,
vibration isolation, or altering the natural frequencies of the structure (NRC 2010).
Altering the natural frequencies of a floor is generally conducted to ensure that the first few natural
frequencies of the floor do not coincide with the vibrating frequency of the equipment. Check with the
manufactures of the machine and its specifications to obtain the relevant information on the operation
frequency of the machine. Equation [15] can be used for calculating the natural frequencies of most floor
systems including light frame joisted wood floor system, timber beam or hybrid steel truss and thick
wood deck floors, CLT and wood-concrete composite floors with four edges simply supported. For
orthotropic plate with only two edges simply supported, there is no exact formula to calculate the natural
frequency. However, for the floor in a building, equation 7 can be used by assuming the floor width is the
building width, which is usually greater than the span. In effect, the supports at other two edges have
little effect on the frequency calculated.
The effect of topping can also be incorporated in the appropriate input properties, as has been discussed
by Hu 2007).
𝑓𝑚𝑛 =
𝜋
1
𝑎 2
𝑎 4
∙ � �𝐷𝑥 ∙ 𝑚4 + 2 ∙ 𝐷𝑥𝑦 ∙ 𝑚2 ∙ 𝑛2 ∙ � � + 𝐷𝑦 ∙ 𝑛4 � �
2
2𝑎
𝜌
𝑏
𝑏
[15]
Where
𝑚 = number of half sine waves in the x direction
𝑛 = number of half sine waves in the y direction
𝑎 = floor span
𝑏 = floor width
ρ = Area density of the floor system;
𝐷𝑥 = Floor flexural rigidity in floor span direction;
𝐷𝑦 = Floor flexural rigidity in across span direction;
𝐷𝑥𝑦 = Shear rigidity.
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Currently, the input values for some of the rigidities of CLT or wood-composite floors are not available.
Therefore, laboratory tests are recommended to measure these properties and / or analytical software that
appropriately model the anisotropic and non-homogeneous behaviour of CLT panels. Designers are
advised to validate the any models used with tests on the components and assemblies.
4.4.2.5
Best Practices
4.4.2.5.1 Proper Supports
Support conditions for floor joists, beams and plates influence the stiffness properties of floor systems,
therefore their response to dynamic excitation. The support conditions also determine whether the floor
vibration will be transmitted to other areas of the building. The bending action of the beams is the
primary mechanism of transmission of annoying vibration. In platform construction where the joists or
plates rest on a supporting wall below, the support condition approaches that of a simple support which is
the underlying assumption for all vibration controlled design methods proposed to date to use the
estimated either natural frequencies or deflection or acceleration to control floor vibration.
Deviation from the simple support condition can occur in several ways. When floor components are
supported on non-rigid supports such as joist hangers, secondary floor beams or steel angle brackets, the
floor joists or plates may be resting on an elastic foundation, in which case the actual natural frequencies
will be significantly lower and actual deflection under an applied load will be higher than the respective
model predictions. This could lead to falsely accepting an unsatisfactory floor. When this is suspected, a
more in-depth analysis of the influence of the support condition is necessary. The AISC design guide
(Murray et al 1997) provides an approach to calculate fundamental natural frequency of a floor system
with one end supported on a secondary beam, equation [16].
𝑓 2𝑓𝑙𝑜𝑜𝑟
𝑓𝑠𝑦𝑠𝑡𝑒𝑚 =
� 𝑓𝑓𝑙𝑜𝑜𝑟 2
�
� +1
𝑓𝑏𝑒𝑎𝑚
[16]
Where 𝑓𝑓𝑙𝑜𝑜𝑟 can be determined using the method described in Section 4.4.2.4. The frequency of the
supporting beam 𝑓𝑏𝑒𝑎𝑚 can be determined using equation [17] assuming the supporting beam is simply
supported.
𝑓𝑏𝑒𝑎𝑚 =
𝜋 𝐸𝐼
� in 𝐻𝑧
2𝑙 2 𝑚𝑙
[17]
where
𝑙 = span of the supporting beam, m;
𝐸𝐼 = apparent bending stiffness of the beam, Nm²;
𝑚𝑙 = mass of the beam per unit length, kg/m.
Currently, there is no design guidance on minimum supporting beam stiffness required to ensure it is rigid
enough that it does not affect the natural frequency of the floor system. However, one “rule of thumb” is
that the ratio of the floor natural frequency to the beam natural frequency should be so small that the
calculated system natural frequency is almost equal to the floor natural frequency.
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In contrast, when floor joists span over an intermediate support or the ends of floor joists or CLT plates
are ‘clamped’ between upper and lower storeys it creates an end fixity condition. In that situation, the
assumption of simple support leads to a conservative prediction of floor performance, which should be
acceptable. However, note that this does provide a flanking path for sound transmissions.
4.4.2.5.2 Subjective Evaluation on Floors by Architects, Designers, Builders and Contractors
For systems where there is little field experience, it is recommended to conduct an informal subjective
evaluation by builders, developers, architects, designers, contractors and/or product manufacturers before
the building is occupied. Where this is not expected to be feasible, some evaluation may be performed on
mock-ups and on the floor system as construction progresses so that possible adjustments to details can be
made.
Nevertheless, valuable data can be obtained from existing structures through informal subjective
evaluation carried out by builders, developers, architects, designers, contractors and/or product
manufacturers.
We encourage designers and builders to record their observations through the subjective evaluation
protocols and evaluation questionnaire developed by FP Innovations as described in (Hu 2014b).
4.4.2.5.3 Field Measurement Before and After Finishing
The field tests measure the floor maximum deflection under a concentrated load applied at the floor
centre, floor acceleration or velocity response, and the floor natural frequencies. Conducting field
measurements on floor systems before and after finishing provides useful information on the floor’s
stiffness, indicated by the deflection under a concentrated load, and on the floor’s natural frequencies.
Test protocols for field measurements can be found in a report by (Hu 2014b).
4.4.3
References
Canadian Construction Materials Center (CCMC). 1997. Development of design procedures for vibration
controlled spans using engineered wood members. Concluding report prepared for CCMC and
Consortium of manufactures of engineered wood products used in repetitive member floor systems by
Canadian Wood Council, DMO Associates, Quaile Engineering Ltd., Forintek Canada Corp. Ottawa.
Canadian Construction Materials Center (CCMC), 2002. Industry update: CCMC involved in efforts to
make engineered wood joists easier to use. Construction Innovation, December 2002, Institute for
Research in Construction. Ottawa.
Chui Y. H. 2002. Application of ribbed-plate theory to predict vibrational serviceability of timber floor
systems. Proceedings of 7th World Conference in Timber Engineering, Shah Alam, Malaysia,
August 12-15, 2002, paper No. 9.3.1, Vol.4, pp.87-93.
Dolan J. D. et al. 1999. Preventing annoying wood floor vibrations. Journal of Structural Engineering,
Vol.125, No. 1, p. 19-24.
Gagnon, S. and Kouyoumji, J-L. 2011. Acoustics: Acoustic performance of cross-laminated timber
assemblies, Chapter 9 in Cross-Laminated Timber Handbook, Canadian Edition, FPInnovations
Hu, L. J. 2007. Design guide for wood-framed floor systems. Canadian Forest Service Report No. 32.
Quebec: FPInnovations. 60 p. + appendices.
Hu, L. J. and Gagnon, S. 2009. Verification of 2005 NBCC maximum spans for concrete-topped lumber
joist floors. Report of Canadian Forest Service No. 2, FPInnovations project No. 5756, Quebec.
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Hu, L. J. and Gagnon, S. 2010. Construction solutions for wood-based floors in hybrid building systems.
Report of Canadian Forest Service No. 1, FPInnovations project No. 201000299, Quebec.
Hu, L. J. 2011a. New design method for determining vibration controlled spans of wood joisted floors.
FPInnovations report of projects 201005024 and 201005105. Revised in 2013, FPInnovations,
Quebec.
Hu, L. J. 2011b. Vibration: Vibration performance of cross-laminated timber assemblies, Chapter 7 in
Cross-Laminated Timber Handbook, Canadian Edition, FPInnovations
Hu, L. J. and Adams, D. L. 2012a. Sound: Sound insulation of cross-laminated timber assemblies,
Chapter 9 in Cross-Laminated Timber Handbook, U.S. Edition, FPInnovations.
Hu, L. J. 2012b. Vibration: Vibration performance of cross-laminated timber assemblies, Chapter 7 in
Cross-Laminated Timber Handbook, U.S. Edition, FPInnovations
Hu, L. J., Omeranovic, A. and Dufour, R. 2013. Effects of flooring, topping and underlayment on impact
sound insulation of wood-joisted floor-ceiling assemblies. Proceedings of 21th International Congress
on Acoustics, Montreal, June 2-7 2013.
Hu, L. J. 2014a. Serviceability of next generation wood buildings: Sound insulation performance of wood
buildings. Report for projects 301007970, 301006715 and 301006866, FPInnovations, Quebec. In
progress.
Hu, L. J. 2014b. Vibration performance of wood floors and staircases, and sound insulation performance
of wood/ceiling and wall assemblies: Protocols for field evaluation. Report for projects 301007970
and 301006715, FPInnovations, Quebec. In progress.
International Building Code (IBC), 2009. Section 1207: Sound Transmission.
International Code Council (ICC), 2010. Guideline for acoustics, ICC G2-2010.
Kouyoumji J.L., Gagnon S., Experimental approach on sound transmission loss of, Cross Laminated
Timber floors for building, 39th Internoise International Congress, June 13-16, 2010, Lisbon,
Portugal.
Kouyoumji J.L., Gagnon S., Sound transmission loss of Cross Laminated Timber ‘CLT’floors,
measurements and modelling using SEA. 38th Internoise International Congress, 23-26 August 2009,
Ottawa, Canada.
Lignum, 20xx, Sound insulation data of wood composite floors
Murray, T. M., Allen, D. E. and Ungar, E. E. 1997. Floor vibrations due to human activity. Steel Design
Guide Series No. 11, American Institute of Steel Construction and Canadian Institute of Steel
Construction.
National Research Council (NRC) 2002. Leaks and flanking sound transmission. Presentation at “Sound
Isolation and Fire Containment – Details That Work” seminar, Building Science Insight Seminar
Series, Institute for Research in Construction, National Research Council, Ottawa, ON.
National Research Council (NRC), 2006a. "Flanking Transmission in Multi-Family Dwellings: Phase
IV," Nightingale, Quirt, King and Halliwell, Research Report RR-218, Ottawa, Ontario.
National Research Council (NRC), 2006b. “Guide for Sound Insulation in Wood Frame Construction”,
Quirt, Nightingale and King, Research Report RR-219, Ottawa, Ontario
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Council of Canada, Ottawa.
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National Research Council (NRC). 2013b. Apparent Sound Insulation in Cross-Laminated Timber
buildings. Research Report RR-335, Ottawa, Canada, In progress, December 2013
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Buildings. Research Report RR-331, Ottawa, Canada, September 2013
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Research Report RR-336, Ottawa, Canada, In progress, February2014Ohlsson S. 1988. Springiness
and human induced floor vibration – A design guide. Swedish
Council for Building Research
Document D12, Stockholm.
Pope, J. 2003. Principles of Acoustics. Principles of acoustics and the measurement of sound,
Presentation at B&K (Brüel and Kjær ) Seminar, April 10-11, 2003 in Norcross, GA.
Schickhofer, G. and Thiel, A. 2010. Comments on FPInnovations new design method for CLT floor
vibration control. E-mail message to author, July 1st, 2010.
Schramm, Dolezal, Rabold and Schanda, 2010. “Stossstellen im Holzbau – Planung Prognose und
Ausführung“, Proceedings of the DAGA 2010, Berlin, Germany.
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Fire Safety and Protection
CHAPTER 5 Fire Safety and Protection
Lead Authors:
Co-Authors:
Peer Reviewers:
Andrew Harmsworth and Christian Dagenais
Gary Chen, Robert Heikkila, Gary Lougheed, Jim Mehaffey
George Hadjisophocleous, Angela Lai, Tim Ryce
Peter Senez, Joseph Su, Geoff Triggs,
Abstract
Chapter 5 addresses concerns about fire safety that are often considered to be impediments to the use of
wood elements in tall buildings. Historically the building code’s acceptable solutions have restricted the
use of combustible construction to 3 or 4 storeys due in part to concerns about fire safety. These
acceptable solutions are largely prescriptive and do not reflect state-of-the-art fire safety engineering nor
the advantage of mass timber over traditional light frame. Applying these provisions to all combustible
construction effectively penalises the superior performance of mass timber construction. In this Chapter, a
path is outlined for developing an alternative solution that demonstrates a tall mass timber building can
meet or exceed the level of fire performance provided by the acceptable solutions for tall buildings of
non-combustible construction.
The prescriptive acceptable fire safety solution for a tall building can be quite complex. In this Chapter, it
is assumed that the proposed tall wood building will comply with most of the prescriptive requirements.
The most significant alternative is that the structural elements will be of 2-hour rated mass timber
construction as opposed to 2-hour rated non-combustible construction.
A straightforward approach could be to encapsulate combustible structural elements. Encapsulation would
initially delay the onset of a contents fire affecting structural elements, and would delay combustible
structural elements from contributing to the fire. The pros and cons of three levels of encapsulation are
considered: complete, limited, and no encapsulation.
For tall buildings, a 2-hour fire-resistance rating is prescribed for structural elements. The Chapter
describes methods for calculating the structural fire resistance ratings of mass timber elements whether
they are exposed or encapsulated including the protection of connections. The fire-resistance integrity of
fire separations is given attention including methods for the protection of service penetrations and of
joints between mass timber panels.
It is acknowledged that some void spaces will occur and that these spaces could contribute to fire spread
through a building. The recommendation is that all exposed timber within concealed spaces be protected
such as with encapsulation, but other alternatives are also discussed.
For sprinklered tall-wood buildings in which complete encapsulation has been employed, it is assumed
that the building will perform as well as if it were a sprinklered building of non-combustible construction
for all fire protection considerations. If partial encapsulation is employed, the fire intensity will not likely
be greater than in a non-combustible building, but the potential fire duration could be longer, and water
supply duration available for controlling exposures may need to be increased, such as with on-site water.
The Chapter also recommends fire safety measures during the construction of a tall mass timber building.
The chapter demonstrates that development of sound alternative solutions for tall mass timber building is
feasible and practical and will provide the level of fire safety required by traditional concrete and steel
high rise buildings.
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5.1
Introduction
5.1.1
Acceptable Solutions for Fire Safety
The acceptable solutions addressing fire protection in buildings are found in Part 3 of Division B of the
NBCC (NRCC, 2010). Historically the NBCC has restricted the use of combustible construction to 2, 3 or
4 storeys due in part to concerns about fire safety. These acceptable solutions anticipate construction with
the lowest level of fire performance, which for combustible construction means light-frame wood
construction. As a consequence, the current treatment of wood construction in general and mass timber
construction in particular is often disproportionately conservative. Furthermore, as the acceptable
solutions are largely prescriptive in nature, they do not reflect the state-of-the-art fire engineering design
methodologies available today. While it is acknowledged that some credit is given to certain mass timber
with unprotected steel connections in interconnected floor spaces and in roofs of 2 storey buildings,
generally there is minimal recognition of the advantages of mass timber. Consequently the NBCC
effectively penalizes the superior performance of mass timber construction.
5.1.2
Alternative Solutions for Fire Safety
The objective-based NBCC is structured such that a designer can adopt an acceptable solution explicitly
spelled out in Division B or implement an alternative solution that demonstrates an equivalent level of
performance in the areas identified by the objectives and functional statements attributed to the
acceptable solution it is replacing (Figure 111).
As noted by Buchanan et al. (2006), prescriptive Codes are written in a manner that sounds like “Do this,
do that, don’t ask any questions” and are more concerned with how the building is built, rather than how
it will actually perform. The main advantage, to some extent, of complying with the prescriptive
provisions is that it is easier and faster for designers and authorities having jurisdiction to develop, apply,
review and approve a design. However, it also presumes that there is only one way of providing a given
level of fire safety in a building (Hadjisophocleous, Benichou, & Tamin, 1998).
Unlike performance-based codes, the objective-based NBCC does not explicitly establish or express
explicit performance levels to be achieved regarding fire safety. Instead, it provides objectives that
explain the intent behind the prescriptive provisions. Under this framework, the acceptable solutions in
Division B establish the minimum acceptable level of performance for the specific objectives attributed to
the acceptable solutions.
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Building Code
Compliance
Objectives and
Functional Statements
Acceptable Solutions
(Division B)
Alternative Solution
• Meet the objectives and functional
statements
• Provide the same level of
performance relative to objectives
and functional statements
• Deemed-to-satisfy solutions
• Establish level of performance
Figure 111 Summary of the two compliance paths in the NBCC
To demonstrate compliance with the fire safety provisions of the NBCC using an alternative solution, one
should carry out a qualitative or quantitative fire risk assessment to establish the level of fire risk
associated with the acceptable solution, and then carry out the same assessment for the alternative
solution, so that the level of performance between the two designs can be compared. If it is shown in this
comparative risk analysis that the alternative solution provides at least the same level of fire performance
as the Division B acceptable solution, then the alternative solution can be accepted as also complying
with the building code. Guidance on undertaking a fire risk assessment is provided in Appendix 5A at the
end of this Chapter.
It is the intent of this Chapter to outline a path for developing an alternative solution that demonstrates
that a tall mass timber building can meet or even exceed the level of fire performance currently provided
by the NBCC acceptable solutions for tall buildings of noncombustible construction.
5.1.3
Acceptance by Authority
An alternative solution requires agreement by the authority having jurisdiction that the solution provides
the requisite level of performance, although the process for review varies by jurisdiction.
An alternative solution for a tall timber building is inherently complex and it may be appropriate for the
applicant and the authority to agree to delegate the review process to third-party or peer reviewers with
qualifications in timber engineering and fire science. It is recommended that the review process and
selection of peer reviewers be agreed upon very early in the process and that reviewers and proponents
establish a good dialogue on the project. Furthermore, it is recommended that for an effective peer
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review process, that peer reviewers be tasked with assisting in finding a solution rather than just
identifying errors and omissions. Further guidance with respect to peer review is found in some of the
SFPE Guides (SFPE, 2000; 2004; 2009).
Experience has shown that for complex alternative solutions it is important for the proponent and
authority to meet early and often and that an effective dialogue between the applicant, the authority and
peer reviewers, if any, is essential for all parties to be satisfied with the outcome.
5.1.4
The objectives and functional statements attributed to a particular acceptable solution identify the risk
areas that the NBCC is addressing by that provision. Risks that are not addressed by the objectives are
outside the NBCC framework and are therefore not considered (i.e. the risk of failure due to terrorist
attack is currently not a risk area recognized by the NBCC).
The fire safety, health and accessibility provisions set forth in the NBCC interrelate to four main
objectives. They describe, in very broad and qualitative terms, the overall goals that the NBCC's
provisions are intended to achieve, namely:
1.
2.
3.
4.
OS – Safety;
OH – Health;
OA – Accessibility for persons with disabilities, and;
OP – Fire and structural protection of buildings.
The objectives describe undesirable situations and their consequences, which the NBCC aims to limit the
probability of occurrence in buildings. Each objective is further refined through the establishment of subobjectives, which can be found in Parts 2 and 3 of Division A of the NBCC. It is recognized that the
provisions of the NBCC cannot entirely prevent all undesirable events from happening or eliminate all
risks. Therefore, the objectives are intended to “limit the probability” of “unacceptable risk” of injury or
damage caused by exposure to different hazards. It is thus assumed that an undesirable situation can occur
and means shall be provided to limit its consequences. Moreover, it is further understood that an
“acceptable risk” is the level of risk remaining once compliance with the NBCC prescriptive (acceptable)
solutions has been achieved (NRCC, 2010).
Each provision (i.e. acceptable solution) prescribed in Division B of the NBCC is linked to one or more
objectives and sub-objectives, and to one or more functional statements. An example of a sub-objective is
OP1.3, aiming at limiting the probability that, as a result of its design or construction, the building will be
exposed to an unacceptable risk of damage due to collapse of physical elements due to a fire or explosion.
This particular sub-objective is directly linked to structural fire-resistance requirements for load-bearing
elements. A functional statement describes a function of the building, or a part of the building, that a
particular requirement helps achieve. They are more detailed than the objectives and, similarly, are
entirely qualitative. Examples of functional statements related to fire safety provisions that can be found
in Part 3 of Division B of the NBCC are:
•
•
•
•
F01 – to minimize the risk of accidental ignition;
F02 – to limit the severity and effects of fire or explosions;
F03 – to retard the effects of fire on areas beyond its point of origin;
F04 – to retard failure or collapse due to the effects of fire;
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•
•
F05 – to retard the effects of fire on emergency egress facilities;
F10 – to facilitate the timely movement of persons to a safe place in an emergency.
Additional information on objectives and functional statements can be found respectively in Parts 2 and 3
of Division A of the NBCC.
5.1.5
Level of Performance
In the objective-based NBCC, the performance targets for the acceptable solutions are implicit in the
provisions themselves; the performance attained by the acceptable solutions in Division B constitutes the
minimum level of performance required. For example, Sentence 3.4.2.5.(1) prescribes that the maximum
travel distance to an exit in a sprinklered office (Group D) floor area be 45 m. The objective and
functional statement attributed to Sentence 3.4.2.5.(1) is [F10-OS3.7], which is to facilitate the timely
movement of persons to a safe place in an emergency in order to limit the risk of injury due to persons
being delayed in or impeded from moving to a safe place during an emergency. The performance target is
the measure of time for occupants to reach an exit within the 45 m maximum distance relative to the onset
of unsafe conditions (i.e. un-tenability conditions for occupants). If an alternative solution is proposed,
one would need to demonstrate that the resultant travel distance to exit meets or exceeds the performance
attained by the 45 m travel distance scenario with respect to [F10-OS3.7], assuming all other factors
remain unchanged.
5.1.6
Fire Dynamics and Engineering Design
The general hazard associated with buildings constructed of wood is that as a combustible material, wood
may be exposed to the fire and subsequently support the spread and/or growth of the fire. Therefore, not
only would the structural integrity of the combustible construction be affected by fire, the construction
material itself may also become the fuel. However, to scientifically understand and discuss both the
inherent, as well as explicit, risk of combustible construction in fire, it is important to first discuss some
basic physics concepts related to compartment fires and the general strategies that have been implemented
in the NBCC for buildings to address the risks posed by compartment fires. This would allow for a more
systematic approach to examining the relevant fire risks.
Fire is the exothermic chemical reaction of a fuel with oxygen in air. Common solid combustibles found
in buildings must first be heated to their ignition temperatures before they become involved in fire. The
most important products of combustion released during fire include significant heat, soot (smoke), carbon
dioxide and carbon monoxide (Drysdale, 1998). In buildings, compartment fires are generally the fire of
concern (as opposed to exterior fires for example). The walls, floors and ceilings in a building can be
designed to create physical and thermal boundaries that confine the fire to an enclosure (the
compartment). Accordingly, the behaviour of the fire can be understood in terms of a set of unique
physics commonly referred to as compartment fire dynamics. The progression of a compartment fire can
generally be depicted by the heat release rate versus time curve shown in Figure 112.
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Figure 112 Typical stage of fire development (Buchanan A. H., 2002)
During the pre-ignition and growth stages, the fire is a localized phenomenon. Heated gas and products
of combustion rise and form a hot upper smoke layer under the ceiling. As the fire progresses, the smoke
layer thickens and begins to descend along walls. Its temperature also increases steadily. During this
stage, the building’s construction materials may have a significant influence on the advancement of the
fire. This includes interior finishes and any exposed construction material of the building. Combustible
building materials, particularly on the ceiling or upper walls may ignite and hence contribute to the
temperature and rate of descent of the smoke layer. If the temperature of the smoke layer climbs above
550°C, the radiant heat emitted by the smoke layer causes the temperatures of combustibles below the
smoke layer to reach their ignition temperatures almost simultaneously and flames will engulf the entire
compartment. This transition from a localized fire involving a few combustibles to full room involvement
is referred to as “flashover”. It is generally seen as an important fire safety objective to prevent or, at least
delay, the time to flashover.
The general strategy employed to address the life safety and property protection risks during the preflashover stage are to limit the spread and growth of fire, to initiate and foster the evacuation of occupants
for life safety, and to facilitate firefighting and/or automatic suppression for life safety and property
protection. This strategy generally includes: a) providing automatic fire detection through heat (e.g.
sprinklers) and smoke detectors to notify occupants of the fire, and to notify emergency responders; b)
limiting flame spread ratings of interior finishes in certain parts / assemblies of the building to limit fire
spread and growth; c) providing automatic sprinklers to limit and control fire growth and smoke spread;
d) using fire-rated separations (i.e. walls and floors) to control the spread of fire and its effects, which are
principally heat and smoke; and e) using fire-resistance rated and/or non-combustible materials to limit
the involvement of building materials.
In regard to the unlikely scenario of a fire progressing to reach flashover and becoming fully developed,
which is a rare event in sprinklered buildings, the fire protection strategy shifts towards preventing fire
spread outside the fire compartment (room of fire origin) and preventing partial failure or collapse of the
building’s structural elements within a given timeframe. Although survival within the fire compartment is
not possible in a post-flashover environment (at temperatures above 600°C), the building’s fire endurance
in post-flashover environments is important as it provides time for movement/evacuation of occupants
outside the compartment of fire origin (e.g., in public corridors, at different floor levels or in exits), as
well as time for firefighters to carry out their operations in those areas.
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Structural fire endurance in a fully developed fire is generally achieved by requiring a minimum fireresistance rating for the building’s key assemblies including all floors and those walls and structural
members that carry gravity loads. It is important to note that when a fire in a room or compartment
reaches the fully developed stage, the heat release rate of the fire is typically governed by the available
ventilation to the room or space. In the transitioning of a fire from the pre-flashover stage to the postflashover, the fire transitions from being fuel-controlled to being ventilation-controlled. Consequently, the
post-flashover burning rate is governed by the amount of oxygen available, not the fuel load itself. As
such, the temperature within the compartment climbs very slowly during the post-flashover stage until the
decay phase is initiated, provided that fire-rated boundary elements enclosing the fire compartment are
still fulfilling their duty. It should be noted that the NBCC currently does not address the decay phase, as
previously shown in Figure 112.
Understanding compartment fire dynamics allows one to strategically design buildings to perform
structurally in an acceptable manner during the different stages of a fire. In the context of allowing greater
building height for mass timber buildings, the following key questions need to be considered:
1.
2.
3.
4.
5.
Are mass timber buildings designed to limit the involvement of structural wood elements during
the pre-flashover stage?
Are mass timber buildings designed to limit spread of fire and smoke beyond the room or
compartment of fire origin?
Are mass timber buildings designed to provide an acceptable environment for emergency
responders to conduct their operations both within and outside the compartment of fire origin
during the fully developed stage?
Are mass timber buildings designed to remain structurally sound should a compartment fire
become fully developed?
Are mass timber buildings designed to limit the spread of fire to neighbouring buildings should a
compartment fire reach flashover?
With an objective-based code, it is not always clear how to identify the required minimum level of
performance that the fire engineering design strategy, such as outlined by the five items listed above,
needs to achieve. What is the acceptable level of performance to building officials and designers, and
thereby, presumably tolerable to building occupants? As mentioned in Subsection 5.1.4 of this chapter,
the objectives and functional statements of the NBCC aim to “limit the probability” of “unacceptable
risk” of injury or damage caused by exposure to different hazards, which is a qualitative statement for the
most part. In order to develop an “acceptable” alternative solution to the prescriptive fire safety
provisions, an understanding of risk assessment is needed. As mentioned earlier, guidance on undertaking
a fire risk assessment is provided in Appendix 5A at the end of this Chapter.
5.2
Development of a Fire Safe Alternative Solution
5.2.1
Approach to an Alternative Solution for Fire Safe Tall Wood Buildings
In theory development of an alternative solution related to fire safety is a simple matter of developing a
method of assessing the relative fire risks of the proposed building (in this case one containing mass
timber elements as the primary structural elements) and a building that conforms with the traditional
construction methodology reflected in the Division B acceptable solutions.
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The prescriptive acceptable solutions of Division B of the NBCC have historically restricted the use of
wood in building construction to 2, 3 or 4 storeys above grade. As building size increases, the occupant
load and the value of the property generally increase proportionally. Commensurate with this, the
provisions for fire safety in the NBCC become more stringent to reflect the higher risk (higher probability
of injury / loss of life due to fire and/or greater financial loss due to failure of the building). In larger and
higher buildings, the NBCC generally prescribes lower flame spread ratings for the interior finishes and
also the use of higher fire-resistance rated and/or non-combustible construction materials. The objective
is to minimize the probability of combustible materials contributing to the pre-flashover fire growth (i.e.
in the early stage of fire development), thereby increasing the chance of the fire protection strategy being
successful. It is significant that the NBCC does not and cannot regulate building contents, which usually
pose a much larger fire load and greater fire hazard than combustible interior finishes or structural
elements.
Although the NBCC does not explicitly discuss the fundamental fire protection strategies, the inherent
concern regarding use of combustible construction materials is noted in the intent statements of the
NBCC. For example, the objectives and functional statements attributed to requirement for noncombustible construction within a storey are [F02-OS1.2, OP1.2]. The intent statement reads as follows
(NRCC, 2012):
“To limit the probability that combustible construction materials within a
storey of a building will be involved in a fire, which could lead to the growth
of fire, which could lead to the spread of fire within the storey during the
time required to achieve occupant safety and for emergency responders to
perform their duties, which could lead to harm to persons and damage to the
building”.
It is the intent of this chapter to outline a path for developing a fire safe alternative solution that will
demonstrate that mass timber structural systems in a tall building can meet at least the same level of
performance related to fire safety currently provided by the existing deemed-to-satisfy solution (i.e.
noncombustible construction). It is however not intended that the approach discussed in this guide is the
only suitable approach: Development of other approaches is encouraged. This guide does however
highlight issues and concerns that these other approaches may need to address.
There are a number of methods for approaching a fire safe alternative solution for a tall wood building.
Theoretically it is possible to address the building as a combustible building, and analyze the risks that
need mitigating, using a risk assessment methodology. Then develop an assessment that demonstrates the
overall level of safety (level of risks) is equivalent to or better than that afforded by the deemed-to-satisfy
noncombustible construction permitted by Division B for the same building scenario.
With a tall wood building as an alternative solution, it is necessary to start from the premise that the
building will conform to the provisions for a noncombustible building and assess the impact on the level
of risk arising from the introduction of combustible components. That is, as mass timber components are
introduced in the building, the performance of these components, complete with their protection
methodology must be compared to the performance of traditional components and their representative
protection methodology. The approach is essentially iterative. The steps that need to be taken are as
follows:
1.
Review the specific code provisions – in this case the requirement for non-combustibility.
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2.
3.
4.
5.
6.
7.
Assume the fire protection elements/features of the building other than the proposed combustible
elements are in conformance with the acceptable solutions of the NBCC (to be reviewed after the
development of the risk analysis);
Perform the risk analysis on the direct risks envisioned by the objectives and functional
statements and demonstrate that comparatively the proposed solution results in a level of risk that
is equal to or lower than the acceptable solution, or if not of lower risk, what compensating
measures are available to lower the level of risk to an acceptable level;
Review other critical elements of the building compliant with Division B to establish if the
alternative solution (here the use of combustible construction) would negatively impact the
effectiveness of the provisions;
Review the impact on the specific alternative solution on other alternative solutions envisioned
for the building and the impact of these other alternative solutions on the specific alternative
solutions;
If more refined alternative solutions to Division B solutions are required, perform the risk
analysis on these alternative solutions and establish if compensating measures are required; and
Repeat the review of the critical elements, if any, noting specifically that some alternative
solutions may have impacts on other alternative solutions.
Because of the extent of the proposed alternative solution, it is appropriate to review basic fundamentals
of building fire protection, measures for high buildings, including measures for firefighting, automatic
sprinkler and fire alarm provisions, and measures to protect from fire spread on the exterior of the
building.
5.2.1.1
Other Fire Safety Objectives and Unknown Fire Risks
The objective-based NBCC provides objectives for each provision given in Division B. However it is
recognized that the NBCC does not address all fire risks; nor can the attribution of objectives for each
provision in the acceptable solutions necessarily be considered comprehensive and complete. In practice
the state of knowledge is not at a point where a pure quantitative fire risk analysis can be performed; in
essence, it is not possible to predict all the possible events and failures that might occur. It is also
important to be cognizant of “unknown unknowns”; that is, potential areas of concern that do not occur
with conventional construction that we therefore have not envisioned.
On the other hand it is important to note that the solutions of Division B do not address all possible events
and failures that may occur. There are many known risks in existing non-combustible construction, as
well as “unknown unknowns”.
It is the intent of this guide, through involving a wide range of experts with varied experience, to resolve
most of these issues, and unearth enough of the ‘unknown unknowns’ to provide confidence that in its
entirety, a tall timber building will provide the level of fire safety that we have grown to expect in modern
non-combustible buildings.
5.2.2
Level of Performance in the Areas Defined by Objectives and Functional Statements
As noted above, it is necessary to limit the scope of the alternative solution to a manageable level. As
such, for the purpose of this chapter, and to simplify the alternative solution it is proposed that the
building will conform to the Division B prescriptive solutions for noncombustible construction with the
exception that the structural elements will be of 2 hour fire-rated mass timber construction. Section 5.4 of
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this chapter details how to design mass timber structural elements to provide the specified 2 hour fireresistance rating.
5.2.2.1
Objectives and Functional Statements related to Noncombustible Construction
The objectives and functional statements of the NBCC attributed to the requirement for noncombustible
construction are [F02-OS1.2] and [F02-OP1.2], which can be summarised as follows:
•
•
[F02-OS1.2] – to minimize the severity and effects of fire or explosions so as to limit the
probability that, as a result of the design or construction, a person in or adjacent to the building
will be exposed to an unacceptable risk of injury due to fire or explosion impacting areas beyond
its point of origin; and
[F02-OP1.2] – to minimize the severity and effects of fire or explosions so as to limit the
probability that, as a result of the design or construction, a building will be exposed to an
unacceptable risk of damage due to fire or explosion impacting areas beyond its point of origin.
In summary, the concern is that the combustible material may contribute to the intensity, severity or
spread of fire (including products of combustion) beyond its point of origin (i.e. the room of fire origin).
In developing an alternative solution it is necessary to either 1) control the intensity, severity and spread
of fire and smoke to the levels anticipated in a building of noncombustible construction, or 2) to provide
features that compensate for the increased intensity, severity and potential spread of fire and smoke.
The intent statement also notes a time frame during which these objectives and functional statements must
be satisfied; namely, “during the time required to achieve occupant safety and for emergency responders
to perform their duties”.
It is noted that for a complex alternative solution a detailed analysis of life safety of the whole building is
necessary in recognition of the fact that many other elements of the solution in Division B are predicated
on the assumption of noncombustible construction.
5.2.2.2
Scope of Proposed Alternative Solution
To simplify further the analysis in this guide, it is assumed that floor to floor heights are relatively
standard, and that fire loads (quantities of combustibles) are limited and similar to the fire loads found in
typical assembly, residential or office occupancies. Where fire loads are unusually high, it may be
appropriate to perform a fire modeling study to establish that the increased fire loads will not lead to
potential collapse of the building. These comments apply equally to any type of construction; however
given that a tall mass timber building may undergo increased scrutiny and is frequently perceived to be of
a higher risk, such an analysis will be more critical for early mass timber tall buildings. The proponents
of the alternative solution will need to establish that their assumptions are consistent with the occupancy
and characteristics of the proposed building.
It is noted that this guide assumes the proposed building is defined as a high building. In some cases
office buildings between 18 and 36 m in height with sufficient cumulative exiting are not defined as
“high” building by the NBCC, and it may be that not all tall building measures are applicable. Similarly
there may be cases where a proposed mass timber building is of lesser height, yet of sufficient area that
the acceptable solutions of Division B prescribe noncombustible construction to have only a 1h fire
rating. In such scenarios, recommendations in this guide should be appropriately modified.
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5.2.2.3
Combustible Components Explicitly Permitted by Division B of the NBCC
Within a conventional building required to be of noncombustible construction numerous elements, both
minor and in some cases more significant, are permitted to be combustible.
Subsection 3.1.5 of the NBCC provides an outline of combustible components permitted in a building
required to be of noncombustible construction. For the purposes of this chapter it is assumed that
elements permitted by Subsection 3.1.5 are also acceptable in a building of mass timber construction. Of
specific note is that of gypsum board: The paper covering the gypsum board is combustible; however,
gypsum board is specifically permitted in Article 3.1.5.1 of Division B of the NBCC.
Provisions described in Subsection 3.1.5 provide a significant recognition of the advantages of sprinkler
protection, permitting significant additional combustible components when a building is sprinklered,
including combustible partitions and combustible wall finishes. Essentially, any interior fire separation,
except for vertical shafts, that has a prescribed fire-resistance rating less than the floor rating is permitted
to be of combustible construction (i.e. fire-rated light-frame wood partitions).
Provisions for combustible cladding and roofs are discussed later in this section.
5.2.3
Assessment of Performance Level (Division B vs. Alternative Solution)
5.2.3.1
Encapsulation
Encapsulation is a fundamental approach to fire protection of all structural materials. Steel is traditionally
protected in large buildings by fibrous or cementitious coatings, board type materials such as gypsum
board or special paints. Reinforced concrete is a composite material of steel and concrete and is usually
protected by a non-loadbearing layer of concrete, referred to as “cover” of 20 to 35 mm thickness that
protects the load-bearing composite structure (Figure 113). The definition for encapsulation used in this
guide is:
“Encapsulation relates to the use of materials for protecting the structural
elements to mitigate the effects of the fire on the structural elements. In this
way, any effects of the combustible structural elements on the fire severity
can be delayed.”
a) Encapsulation of steel components (CEN, 2005)
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c) Encapsulation of structural steel in concrete
components (Buchanan A. H., 2002)
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b) Encapsulation of steel components with Type X
gypsum board (NRCC, 2010)
d) Partial encapsulation of a composite steel-concrete
assembly (Lennon, 2011)
Figure 113 Examples of encapsulation methods for structural steel and concrete components
Encapsulation will initially delay the onset of a contents fire affecting the structural elements, and delay
the combustible structural elements from contributing to the fire.
For clarity, we introduce three levels of encapsulation: complete, limited and fully exposed (no
encapsulation).
5.2.3.2
Complete Encapsulation
As an initial conservative approach the duration of delay of initiation of charring or ignition for
encapsulation may be taken as that of the specified two-hour fire-resistance rating, which is the prescribed
fire-resistance rating for the tall building of non-combustible construction within the acceptable solutions
of the NBCC.
The prescribed minimum fire-resistance ratings in the NBCC are used to meet two separate objectives: 1)
structural integrity and 2) fire separation requirements. Basing the encapsulation time on the 2 hours
required for structural fire-resistance will provide a very conservative result. This method is referred to as
“complete encapsulation”.
Protecting all beams, columns and structural floor and roof panels with multiple layers of Type X gypsum
board can be demonstrated to provide this “complete” protection. During a 2 hour fire exposure, the mass
timber elements would not be expected to be affected by a 2 hour standard fire nor would the timber
elements be expected to ignite and contribute to the fire intensity, severity or spread of fire or smoke.
Therefore, a building with the mass timber structural elements fully encapsulated so as not to contribute
fuel to the fire, nor have the structure affected by fire can be demonstrated to provide an equal or likely
higher level of fire performance than a deemed-to-satisfy noncombustible building on a component basis.
However, given that mass timber has significant inherent fire-resistance without encapsulation, this level
of protection of typical mass timber assemblies will provide an approximately 4 hour fire-resistance
rating.
This level of protection is not only costly, but may be unnecessarily conservative for certain elements.
Subsection 3.1.5 of the NBCC already recognizes that a certain quantity of wood elements used in a large
building is acceptable, and this includes permission for solid wood walls, wood framing in many fire
separations, and wood linings for walls, floors and ceilings. In practice a lesser level of encapsulation
would likely be sufficient. A real-scale fire test of a representative apartment suite conducted in April
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2013 at the National Research Council of Canada laboratory in Ottawa indicated that using 2 layers of
12.7 mm (½ in.) Type X gypsum board delayed the effect of the fire on CLT structural elements. The fire
separations on the floor of fire origin remained intact, limiting fire spread for in excess of 2 hours (Taber,
Lougheed, Su, & Bénichou, 2013).
It should be noted that complete encapsulation has been used in tall timber buildings in the United
Kingdom (UK), as shown in Figure 114. UK regulations however require fire-resistance ratings less than
the 2 hours in the NBCC, typically 90 min. It is significant that more recent buildings in the UK have
used lesser encapsulation and fully exposed approaches, as discussed in the next section.
Figure 114 Complete timber encapsulation used in London, England (credit: Karakusevic Carson
Architects.)
5.2.3.3
Fully Exposed
At the other extreme of using the concept of encapsulation for an alternative solution for tall mass timber
buildings, the mass timber members could be left fully exposed, with a sacrificial layer of timber
providing the encapsulation while also providing the requisite fire-resistance; that is, a sacrificial layer of
70 to 90 mm of timber on all structural members as described in Section 5.4 of this chapter. This
approach can be considered a form of encapsulation, in that the structural integrity is encapsulated by a
sacrificial layer. In this case the exposed timber, both that exposed in the occupied space and that exposed
in void spaces, service spaces and other cavities may contribute to the intensity of fire and may
substantially increase the production of smoke. Consequently compensating measures to control the
spread of fire and especially the spread of smoke will be required.
Development of an alternative solution for a fully exposed timber building was beyond the resources and
time available during the development of this guide; however it may be feasible to demonstrate that a
fully exposed mass timber approach can provide the required level of performance.
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5.2.3.4
Limited Encapsulation
In principle it is believed that a more economic approach to complete encapsulation can be developed
with a significantly reduced level of encapsulation. As discussed in Subsection 5.1.6 of this chapter, a fire
can be divided into two fundamental stages, ignition and growth (i.e. pre-flashover), and fully-developed
(i.e. post-flashover). After flashover the fire is primarily controlled by ventilation factors, so the presence
of mass timber elements or additional combustibles will not increase the temperature or impact on fire
separations, although they may increase the duration of the fire until burnout. It is thereby considered by
the authors that a reasonable approach would be to limit the involvement of the mass timber elements
during the period of fire growth, and potentially some portion of the fully-developed stage.
Providing encapsulation sufficient to delay involvement of the wood to the point at which a compartment
fire in noncombustible building would achieve flashover can be demonstrated as providing equivalent
performance to that provided by a steel or concrete building as the burning rate at flashover is ventilationcontrolled (i.e. limited by the available oxygen and contents, along with combustible finishes permitted in
a noncombustible building). When a single layer of 15.9 mm (⅝ in.) Type X gypsum board is directly
attached to mass timber elements, the contribution of the timber to the fire, and the impact of the fire on
the mass timber may be delayed as discussed in Subsection 5.4.7. This delay can also be compared to, for
example, the time for occupants of the fire compartment and adjacent compartments to evacuate. This
approach is in agreement with the intent statement detailed in Subsection 5.1.6 of this chapter.
Another measure in the NBCC used to limit fire spread within floor areas is the use of walls as fire
separations. As indicated in Section 1.6, a primary concern regarding the use of combustible structural
elements is the potential for increased spread of fire within the storey of fire origin. As such, for most
occupancies, the NBCC requires a one-hour fire resistance rating or less for fire separations between
suites (see NBCC Section 3.3). Providing encapsulation sufficient to delay the involvement of the
combustible structural elements for a similar time duration would limit the effect of a fire involving the
structural elements on the fire separations and thus on fire spread within the storey of fire origin.
While it is generally agreed that a single layer of gypsum board directly applied to the mass timber
elements can be demonstrated to be sufficient to provide the level of performance specified, it is likely
prudent to use two layers of either 12.7 mm (½ in.) or 15.9 mm (⅝ in.) Type X gypsum board in
demonstrating the expected level of fire performance.
In using a limited encapsulation approach, it is necessary to identify and appropriately address the
potential for an extended duration of fire. In part this can likely be addressed with enhanced fire
protection systems, such as enhancing reliability through an on-site or gravity fed water supply to the
automatic sprinklers, and improved provisions for responding fire-fighters that may have to perform
extended mop-up operations. These items are further addressed in Subsection 5.2.3.7 of this chapter.
In conclusion, direct encapsulation such that the mass timber elements are protected for a period of 2
hours for all combustible elements of a building is a conservative approach that can be demonstrated as
providing equivalent or better fire performance than that of directly prescribed noncombustible
construction of the same fire-resistance rating. A lesser level of encapsulation can be shown to similarly
provide the level of performance required. It is the responsibility of the designers of the alternative
solution to establish a balance of encapsulation that achieves the level of protection necessary.
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Further, the levels of encapsulation discussed are based on compartment fires. Fire modelling and
analysis can be used to demonstrate that less encapsulation may be sufficient for areas with limited fire
loads such as concealed spaces, exit and elevator shafts service spaces and exterior walls.
5.2.3.5
Suspended Membrane Type Encapsulation
In lieu of applying the encapsulation directly to the structural members a technique commonly used in
steel buildings is to provide a membrane fire separation at the ceiling, with unprotected steel members
supporting a steel deck ceiling. A similar approach can be used to protect mass timber; that is with a
membrane ceiling suspended below a ceiling cavity. In this case it will be necessary to demonstrate two
things:
•
•
The assembly provides the requisite fire-resistance rating; and
That a fire in the cavity will not spread excessively, nor affect the structural fire-resistance rating
of the assembly.
However, further work is required to establish how the level of performance provided in a
noncombustible building can be provided in a combustible building with exposed mass timber in large
void spaces. As an initial approach it is recommended that all exposed timber in concealed spaces be
protected by direct encapsulation sufficient to protect it from a fire that might occur in the concealed
space.
It may be feasible to demonstrate that an appropriate level of protection can be provided by sprinkler
protection of void spaces with exposed combustibles. If such an approach is taken, research may be
necessary to demonstrate that the level of sprinkler protection is appropriate. Specifically, if such an
approach is taken, it cannot be assumed that the exemptions in NFPA 13 (2013) for omitting sprinklers
from concealed spaces are appropriate.
Alternatively a high level of fire blocking may also be acceptable, especially fire blocking that may be
inherent in the design of the assembly or provided by mass timber elements, and result in only limited
void volumes or areas not used for services or connected to other voids. The degree to which such fire
blocking is acceptable is a matter the individual designer must assess and demonstrate. Some examples of
suitable approaches are shown in Figure 115.
Figure 115 Approaches to encapsulation creating concealed spaces
It should be noted that in many cases conformance with the sprinkler standard requires sprinklers be
placed in the cavity or alternatively that the flame spread rating of exposed elements within cavities is
required to be below 25 employing a method acceptable to the NFPA 13 standard.
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Consideration may be made of surface treatment of void space by such readily available products such as
paint to reduce flame spread. However it is not considered certain that the current method of surface
testing of such products to demonstrate a flame spread rating adequately demonstrates their effectiveness
in addressing the concerns with fire spread unique to narrow or small void spaces. Additional testing is
needed to validate these approaches.
Furthermore where timber is exposed, it will be necessary to review the impact of such timber in cavities
on development of smoke and smoke movement within the building. Use of paint or other treatment to
reduce flame spread in concealed spaces may increase smoke production and toxicity. Additional
discussion on concealed spaces is given in Section 5.8 of this chapter.
5.2.3.6
Exposed Mass Timber within Occupied Spaces
The acceptable solutions of the NBCC already provide for significant use of combustible finishes within
the interior of a building otherwise required to be of noncombustible construction. These include:
•
•
•
•
Wall and ceiling finishes up to 25 mm in thickness;
Floor finishes of any thickness;
Solid wood partitions that are not a part of floor to floor separations or exit separations;
Light wood framing in partitions that are not a part of floor to floor separations or exit
separations.
These provisions provide a basis for allowing exposed mass timber within these spaces. As discussed in
Chapter 5, mass timber has a limited flame spread rating, and during the growth stages of a fire will
contribute to the growth of the fire less than the permitted wood finishes. Similar to the discussion earlier
on limited encapsulation, mass timber may however lead to significantly longer fire durations and longer
firefighting operations, and appropriate measures to compensate for this may be appropriate.
Floors are a special case, in that the involvement of floor finishes is usually minimal in a fire
compartment due to the location below the hot upper smoke layer, and it is considered that exposed mass
timber floors can be reasonably considered to be acceptable based on the permission in the NBCC for
wood floors of any thicknesses as it is unlikely that a greater depth of wood floor would be involved in a
compartment fire when exposed from above. However, due to buoyancy of hot gases and combustion
products, ceilings are typically more of a concern as they will heat up and potentially ignite faster than
floor finishes. This is one of the reasons Division B requires ceilings to exhibit a flame spread rating not
more than 25 when used in noncombustible buildings.
It is incumbent upon the proponent of the alternative solution to assess each area where exposure of mass
timber is proposed and establish, through fire dynamics and comparative risk analysis with directly
permitted timber finishes, that acceptable performance is provided. In many cases, it can be ascertained
that the NBCC permits wood finishes based on a reliance on sprinklers and this may be equally applicable
to exposed mass timber elements.
5.2.3.7
Automatic Sprinklers
The acceptable solutions of Division B of the NBCC specify that all high buildings be provided with
sprinkler protection in accordance with NFPA 13 ‘Standard for the Installation of Sprinkler Systems’.
Sprinklers are highly effective in the control of fires. While it is not within the scope of this document to
debate measures of reliability or specifics of sprinkler systems various data put the effectiveness of
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sprinklers in preventing significant fires at between 95% and 99%. It is useful to note that studies at the
lower end of this range usually incorporate high challenge occupancies, older systems and systems that
are not supervised by a modern fire alarm system complete with automatic signals to the fire department.
The state of sprinkler systems is perhaps best described in an article in Canadian Building Digest in 1985
(Richardson, 1985) , and is still supported by more recent data. This article indicated that systems in use
in Canada at that time had a 96% reliability and that this could reasonably be improved to 99% with
measures to improve reliability of such systems.
The solutions in Division B currently attribute some benefits to sprinkler systems, such as halving
required limiting distances and significantly increasing allowable building areas, but perhaps do not fully
recognize the value of sprinkler systems. This guide compares a sprinklered noncombustible tall building
with a sprinklered mass timber tall building. Not within the scope of this guide, but perhaps providing an
alternative approach is comparing the level of safety provided in a sprinklered mass timber building to a 3
storey building of unsprinklered combustible construction as permitted in the acceptable solutions of
Division B of the NBC.
Options for enhancing the reliability of sprinkler systems include:
• Provision of an onsite secondary water supply,
• Provision of redundant fire pumps and/or power sources,
• Provision of redundant risers or piping systems to the floor area, and
• Provision of redundant systems.
As discussed elsewhere in this chapter, depending on the level of encapsulation provided, in consideration
of the possible extended duration of fires, or in areas with limited firefighting capability, or areas subject
to seismic risk, an onsite water supply should be provided for a tall timber building. The size of such a
supply will be based on an analysis of fire department response times, evacuation times, presence of
persons with mobility impairments and other factors.
5.2.3.8
Non-Standard Fire Exposure
The preceding discussion relates to elements that may be subjected to a standard fire exposure (i.e.
CAN/ULC-S101). There are many elements of a building that may be exposed to a lesser, or in some
cases, greater fire exposure. For example in shafts and concealed spaces and exterior faces of the
building, fire exposure may be significantly less. Some of these conditions are discussed elsewhere in this
Chapter, including Subsection 5.9.2 on exterior cladding.
Alternatively, as discussed earlier, for higher hazards such as high piled storage, large retail occupancies
or industrial occupancies it may be necessary to perform a fire modelling exercise to establish fire
exposure and design the encapsulation or adjust charring rates as appropriate for the fire exposure.
5.2.3.9
Protection in Depth
Not well addressed in the NBCC discussions on alternative solutions is the need to ensure redundancy.
For a traditional noncombustible building, the approach to fire safety includes redundant measures such
as noncombustible construction, conservative fire ratings, sprinkler protection, and smoke control
systems. Therefore an alternative solution must provide similar redundancy. For example it is probably
not sufficient to rely on mechanical smoke control systems to control the spread of smoke alone.
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One concern that may be raised in encapsulating mass timber is the potential for damage to the
encapsulation. While the encapsulation provided for steel and concrete systems are also subject to
damage, both before and during a fire event, it can be argued that those materials, even if exposed, do not
provide fuel to a fire. However, with direct encapsulation of the mass timber elements by multiple layers
of directly applied gypsum wallboard or concrete, the contribution to a fire of mass timber elements
would be minimal, certainly likely to be less than that of the minor combustible components such as 25
mm thick wood panelling currently permitted in a building required to be of noncombustible construction.
Protection in depth can be provided with other systems, such as enhancing effectiveness, coverage or
reliability of sprinkler systems and smoke control system or enhancing fire alarm and detection systems
and exit systems.
5.2.3.10 Practical Considerations
While it is up to the proponent to develop the details of the alternative solution, it is strongly
recommended that certain features be addressed to simplify the alternative solution, in recognition of the
lack of extensive experience with these types of buildings:
•
Protected exit stair shafts and elevator shafts: Stair shafts and elevator shafts provide both
occupant egress and fire fighter access and in a full building evacuation may be occupied for
extended durations. It is suggested that the enclosing walls and underside of stairs should be lined
with gypsum wallboard to maintain low flame spread and tightness of smoke separations and
minimize the probability of fire spread within the exit shaft. Intumescent coatings on mass timber
elements are not yet considered to provide the requisite durability.
•
Service shafts are similarly a concern. Because of the propensity for rapid fire spread in a vertical
shaft and rapid smoke spread driven by stack effect, vertical shafts, whether for services or other
purposes should similarly be lined with gypsum wallboard or metal.
5.3
Provisions for High Buildings (Part 3 of Division B)
The NBCC acceptable solutions in Division B contains prescriptive and some performance-based
solutions for high buildings. The principal concerns for high buildings are 1) movement of smoke in tall
shafts due to the effects of stack action and 2) more challenging firefighting conditions due to the
limitation of exterior firefighting capabilities. Based on the occupancy classification and physical height
of the building, Division B of the NBCC provides a formula to determine when a building is considered
to be a high building.
High building provisions in the NBCC are found in Subsection 3.2.6. They can be categorized into smoke
control measures and measures that facilitate firefighting and are summarized below:
a) Limits to smoke movement between stairways that serve below and above grade storeys;
b) Limits to smoke movement between connected buildings of which at least one is a high building;
c) Emergency operation and design of elevators for firefighters;
d) Venting to aid firefighting;
e) Central alarm and control facility; and
f) Voice communication system.
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5.3.1
What is Stack Effect?
There is airflow within building shafts such as stair shafts, elevator shafts, service shafts, and mail and
linen chutes that is driven by the difference in temperature between the exterior and the interior of the
building. In winter, there is an upward flow in shafts caused by the buoyancy of warm air inside the
building relative to the cold outdoor air. This is similar to the upward flow in smoke stacks and is known
as normal stack effect. In air-conditioned buildings in the summer, the temperatures can be lower inside
the building producing a downward airflow within the shaft known as reverse stack effect.
Figure 116 shows the general airflow in a building for normal stack effect. Air flows into the building
below the neutral plane (N.P.), flows up the shafts and out of the building above the neutral plane. The
neutral plane is a horizontal plane where the pressure difference inside the shaft equals the outdoor
pressure.
The severity of stack effect is dependent on the height of the building and the temperature gradient
between the outdoor and indoor temperatures. In Canada, winter temperatures can be very low, resulting
in significant stack effect. For fire safety engineering design, it is generally assumed in Canada that
reverse stack effect in summer is minimal and can be ignored.
Windows
Shaft
Shaft
N.P.
N.P.
Windows
and doors
Negative
Positive
Figure 116 Normal stack effect in high buildings
5.3.2
Design of Tall Shafts to Resist Movement of Smoke to an Acceptable Level
Naturally, there may be a desire to leave some elements of the wood construction exposed. However,
when designing tall shafts such as stair shafts, elevator shafts, service shafts, and chutes for mail, garbage
and linen, it is recommended that the wood construction be lined. The goal of this recommendation is
twofold. First, because modern mass timber construction typically consists of large wood panels (such as
CLT), while the panels themselves may be fairly air-tight; the joints may not be as tight as concrete or
framed assemblies. As a result, shafts constructed of exposed wood panels may be subject to greater risk
of smoke migration. Where there are contiguous (scissor) exit stairways, exposed wood panels may also
permit migration of smoke from one stairway to another. Second, exposed wood panels in a tall shaft
could pose the risk of rapid flame spread within the shaft. For these two reasons, it is recommended,
unless significant further analysis, testing or modeling demonstrates otherwise, that shafts be lined with
noncombustible material, such as sheet steel, or a layer of gypsum board.
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With respect to the level of encapsulation, if all mass timber in an essentially otherwise noncombustible
shaft is covered with sheet steel or with gypsum board so as to prevent a significant sized fire initiating; it
is considered that no further encapsulation is required.
We note that garbage and linen chutes may contain significant combustible loads and may require special
consideration, however the solutions in Division B of the NBCC already provide for these shafts to be
lined with steel and to be provided with significant additional protection in the form of sprinklers at
alternate floors and protection of the shaft openings.
Short shafts in tall wood buildings may not be subject to the same risk; however, the engineer must
exercise sound engineering judgment in determining whether certain shafts can have exposed wood.
Stair shafts require particular consideration as they may contain significant additional combustible loads,
are subject to damage, yet there is a heightened need to keep them free of fire and smoke. It is therefore
recommended that stair shafts be lined with gypsum board. Further, where stair shafts are back to back or
scissor stair configurations, special attention to maintaining the fire and smoke separations between the
stair shafts is required.
When designing a tall wood building, it is expected that the measures required by Division B will be met
either directly or through alternative solutions to limit smoke movement through shafts to an acceptable
level. Additional smoke control methods can be found in NFPA 92 (2012) and Klote et al. (2012) as well
as in the Canadian developed methods for unsprinklered buildings found in the Supplement to the 1990
NBCC (NRCC, 1990).
5.4
Fire-Resistance of Assemblies and Components
Building regulations require that key building assemblies exhibit sufficient fire-resistance to allow time
for occupants to escape and to minimize property losses as well as for emergency responders to carry out
their duties. The strategy is to limit the possible of structural collapse and to subdivide a building into
fire-rated compartments. The goal of the compartmentalization concept is to limit fire spread beyond its
point of origin by using boundary elements (e.g. walls, ceilings, floors, partitions, etc.) having a fireresistance rating not less than the minimum ratings prescribed by the NBCC. Fire-resistance ratings are
usually assigned in whole numbers of hours (e.g. 1-h and 2-hrs) or parts of hours (e.g. ½-h or 30 min and
¾-h or 45 min). In the case of tall buildings, a 2-hours fire-resistance rating is typically the minimum
required for structural elements. Moreover, each suite in other than business and personal services
occupancies (Group D) are to be separated from adjoining suites by fire separation having a fireresistance rating not less than 1 hour.
5.4.1
What is Fire-Resistance
Structural fire performance of building assemblies are typically assessed by conducting standard fire
resistance tests in accordance with CAN/ULC-S101 (2007). There are other sources for deriving what are
considered generic fire-resistance ratings, such as those contained in Appendix D of the NBCC.
The ULC S101 fire test method is essentially a means of comparing the fire performance (such as
restriction of fire spread and structural response capabilities) of one building component or assembly with
another in relation to its performance in a standard fire. ULC S101 is a performance-based fire endurance
test method with particular performance criteria that are used to assign fire-resistance ratings. Such testing
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is not related to standard fire testing conducted to determine the noncombustibility of materials, which is
determined on the basis of fire testing conducted in accordance with CAN/ULC S114 (2005). When
assigning a fire-resistance rating to floor, roof or ceiling assemblies, the assemblies must be fire-rated for
exposure to fire from the underside. Firewalls and interior vertical fire separations must be fire-rated for
exposure to fire from either side, while exterior walls must be fire-rated for exposure to fire from inside
the building.
The standard fire-resistance test method entails three performance criteria used to establish the fireresistance ratings (Figure 117). The time at which the assembly can no longer satisfy any one of the
following three criteria establishes the fire endurance period from the fire test, which is then used to
define the assembly’s fire-resistance rating:
1.
2.
Separating function
3.
Insulation: the assembly must prevent the temperature rise on the unexposed surface from being
greater than 180°C at any location or an average of 140°C measured from 9 locations, above the
initial temperature;
Integrity: the assembly must prevent the passage of flame or gases hot enough to ignite a cotton
pad;
Structural resistance: the assembly must support the applied load, if any, for the duration of the
test.
a) Insulation
b) Integrity
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Load-bearing (structural) function
c) Structural resistance
Figure 117 Fire-resistance criteria per ULC S101
The structural fire resistance criterion applies to the load-bearing function of assemblies and components
(floors/ceilings, beams and columns) while the integrity and insulation criteria relate to the separating
function of assemblies such as partitions, doors, walls, roofs and floors/ceilings. In the case where an
assembly is acting as a load-bearing and a separating element (e.g. roofs, floors and many walls), all three
criteria need to be fulfilled.
5.4.2
Standard Fire vs. Design Fire Scenarios
The ULC S101 fire test method requires a wall, floor or roof assembly, or a structural element such as a
column or beam to be exposed to a post-flashover fire in which the temperature of the fire gases increases
over time following a standardized time-temperature curve (Figure 118). As mentioned previously, the
standard fire test allows for comparison of the performance of one building component or assembly with
another in relation to its performance in a fire. Although the standard time-temperature curve does do not
represent “real” fire scenarios, it has been developed in an attempt to replicate the post-flashover
conditions of real fires with a standard (nominal) time-temperature curve, which is the stage of a fire that
challenges a building structural system the most. They do not however consider the potential decay phase
or change in fire regime (i.e. a fire would most likely be ventilation-controlled during the fully-developed
phase while being fuel-controlled during the growth and decay phases).
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Figure 118 ULC S101 standard time-temperature curve
A design fire, now being commonly used in fire safety engineering design, allows for a better
representation of an actual fire and is based on the compartment configuration, the boundary conditions
(including their thermal inertia), the compartment geometry, the available ventilation (oxygen to sustain
combustion) and the furniture (fuel load) arrangement. It usually entails a rapid peak temperature and then
reduces rapidly during the decay phase when compared to a standard fire (Figure 119). When conducting
a performance-based fire design, a fire may be assumed to decay when 80% of the fuel has been
consumed and the duration of the decay phase can be estimated by a linear decay rate (ABCB, 2005).
It can generally be stated that for a contents fire in a typical noncombustible fire compartment or one
lined with noncombustible material, a real fire is hotter but significantly shorter than a typical test run in
compliance with ULC S101.
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Figure 119 Observed fire time-temperature curves in several fire experiments compared to a one-hour
fire exposure in compliance with ULC S101 (Bwalya, Gibbs, Lougheed, & Kashef, 2013)
5.4.3
Behaviour of Wood at High Temperatures (Charring)
In elevated temperatures conditions, such as those associated with building fires, the behaviour of a
structural component is dependent, primarily, on the thermal, mechanical and chemical properties of the
material of which the component is composed.
When exposed to elevated temperatures, wood’s chemical components undergo thermal degradation that
affects wood performance. This material-specific property, called pyrolysis, begins at approximately
200°C while the remaining wood converts afterwards to char at temperatures ranging from 280 to 300°C
(Figure 120). Charring is influenced by various factors such as density, moisture content, wood
contraction as well as the exposure conditions (fire severity). As reported by White (2008), the charring
rate has been found to be proportional to the ratio of the external heat flux over the wood’s density. In
experiments conducted on spruce specimens, charring rates were found to be 0.56, 0.80 and 1.02 mm/min
′′
when exposed to heat fluxes of 25, 50 and 75 kW/m² respectively. A critical heat flux (𝑞𝑐𝑟
) of 12.5
kW/m² is commonly accepted for piloted ignition of wood, while 33 kW/m² is recognized for autoignition of wood.
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Figure 120 Char layer formed during a small-scale flame test per CSA O177 (2011)
The charred layer formed around the exposed surface acts as a thermal protection to the inner core
thereby reducing the rate of burning. It protects the inner core from thermal and strength degradation as
shown in Figure 121. The base of the char layer is at approximately 300°C, with a heated layer about 35
to 40 mm thick below the char front. The effective thermal protection provided by the char layer requires
a minimum char depth of approximately 25 mm.
Figure 121 Wood properties below the char layer (Buchanan A. H., 2002)
Most design codes specify a constant charring rate throughout the fire exposure, depending on the wood
density. Charring rates for softwood and hardwood as specified in the Eurocode 5: Part 1-2 (CEN, 2004)
can be found in Table 16. The same charring rates would apply to Canadian timber, provided the
characteristic densities are similar. The one-dimensional charring rate, βo, for standard fire exposure
represents the rate expected for thermally thick slabs of wood (i.e. thicker than 35 mm, as detailed in
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Subsection 5.4.6) and βn is the notional charring rate which is increased to offset the loss of cross section
at corners and the opening of fissures. Corners of rectangular members become rounded during fire
exposure. As such, most codes specify a radius of rounding equal to the depth of the charred layer.
Table 16
Design charring rates of timber as specified in Eurocode 5: Part 1-2
Timber
Softwood and
beech
Hardwood
LVL
Characteristic
Glulam with a ρ ≥ 290 kg/m3
Solid timber with a ρ ≥ 290 kg/m3
Solid or glulam hardwood with a ρ ≥ 290 kg/m3
Solid or glulam hardwood with a ρ ≥ 450 kg/m3
LVL with a ρ ≥ 480 kg/m3
βo
mm/min
0.65
0.65
0.65
0.50
0.65
Note: ρ is the characteristic density value (5th percentile), not the mean value as given in CSA O86.
βn
mm/min
0.7
0.8
0.7
0.55
0.7
The most critical information for determining fire-resistance of assemblies and components is the
temperature in the room or compartment and the heat flux impinging on the assemblies or components,
both of which can be assessed on the basis of exposure to either a standard or design fires.
Figure 122 illustrates a flow chart depicting the various steps typically used for determining the fireresistance of structural elements from advanced calculations methods such as used in a performancebased design. It can be seen that three (3) fundamental models are required:
1.
2.
3.
Fire model used to determine the exposure from either a standard, real or design fire;
Heat transfer model used to evaluate the temperature rise within the element or assembly; and
Structural model used to determine the structural resistance of a component/assembly at elevated
temperatures from simple or advanced calculation models.
Different calculation models can be found in the literature that allow for calculating the temperature
profile for standard and design fire scenarios (Drysdale, 1998; SFPE, 2008; Buchanan A. H., 2002;
Lennon, 2011; SP Trätek, 2010).
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Figure 122 Flow chart for advanced calculations for structural fire-resistance of elements
Heat transfer occurs from regions of high temperature to regions of cooler temperature within solids (e.g.
from the room of fire origin to adjacent compartments through a wall or floor assembly). Such heat
transfer in solid materials is called conduction and is a well-known mechanism that satisfies Fourier’s law
of conduction, as shown from the transient heat transfer partial differential equation [18].
∂ 
∂T  ∂ 
∂T  ∂ 
∂T  
∂T
 k y
 +
k x
+
k z
 + Q = ρc
∂x 
∂x  ∂y 
∂y  ∂z 
∂z 
∂t
[18]
where T is the temperature (K), kx,y,z are thermal conductivities in x, y, z directions (W / m·K ), 𝑄̇ is the
internally generated heat by the rate of heat (heat of reaction) per unit volume due to chemical reaction
(pyrolysis of wood) and the heat to evaporate water per unit volume (W / m³), ρ is the density (kg / m³), c
is the specific heat (J / kg·K), and t is the time (sec).
Heat transfer through a material that exhibits charring behaviour is slightly more complicated than that of
other materials such as steel and concrete. The rate of heat per unit volume due to chemical reaction
′′′
consists of two parts: 1) the pyrolysis of the wood (𝑄̇𝑝𝑤
) expressed by an Arrhenius function and 2) the
′′′
̇
heat of evaporation of water per unit volume (𝑄𝑤 ). More information in regards to the rate of heating,
pyrolysis of the wood and heat of evaporation of water can be found in (SFPE, 2008; Lu, 2012; Craft,
2009). Steel and concrete are considered noncombustible materials, thus do not generate heat when
exposed to fire, and their contribution to 𝑄̇ can be ignored considerably simplifying equation [18].
Charring of wood is a complex process and defining thermal properties for every stage of pyrolysis can be
onerous. As such, commercially available finite-element software packages are normally used for solving
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the differential equations. The heat transfer and structural capacity are calculated using various
parameters and sub-routines. However, use of such a software package can be challenging in regards to
convergence of models, while being sometimes cumbersome for designers due to long computational
time. Fire-resistance calculation models are therefore usually simplified for reducing computational time
and facilitating broad acceptance in the design community.
5.4.4
Fire-Resistance of Timber Structure – Structural Criteria
The inherent fire resistance of massive timber such as CLT can, in many cases, be comparable to that of
other building materials, e.g. concrete, masonry, and steel. Calculating the fire-resistance of timber
structures can be relatively simple because of the essentially constant rate of charring during the fire
exposure. Figure 123 illustrates a cross-section of a timber component exposed to fire. It can be seen that
from any given fire exposure duration (t), the reduced cross-section can easily be calculated based on the
charring rate (β), by subtracting the char layer (c = β·t) from the initial dimensions (b and d). Charred
wood is assumed to provide no strength and no rigidity; therefore the remaining (reduced) cross-section
must be capable of carrying the applied design load in fire design for fulfilling the structural resistance
criterion.
Figure 123 Charred timber cross-section exposed to fire from 3 sides (left) and 4 sides (right)
5.4.4.1
Massive and Glued-Laminated Timber
The acceptable solutions of the NBCC enable the fire-resistance of massive and glued-laminated timber to
be determined on the basis of full-scale fire-resistance tests conforming to ULC S101 or, in the case of
glued-laminated timber beams and columns, from equations provided in Section D-2.11 of Appendix D of
the NBCC. Provisions for assigning fire-resistance of massive (solid) wood walls, floors and roofs can
also be found in Section D-2.4. It should be noted that there are currently no specific provisions or
guidelines in NBCC for assessing fire-resistance of fasteners and connections for mass timber systems.
Section D-2.11, originally introduced in 1977, provides design equations for beams and columns based on
their stress ratio and column slenderness. The stress ratio should be determined from factored load effect
as provided, for example, from load case no 2 of Table 4.1.3.2.A from Division B of the NBCC. The
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factored resistance shall be determined from relevant sections of CSA O86 Engineering Design in Wood
(2009).
Other approaches are permitted by the NBCC under the alternative solution process.
5.4.4.2
Structural Composite Lumber
Structural composite lumber (SCL) comprises proprietary engineered wood products manufactured and
tested in accordance with ASTM D5456 (2009) and evaluated for conformance by the Canadian
Construction Materials Center (CCMC). Parallel strand lumber (PSL), laminated veneer lumber (LVL)
and laminated strand lumber (LSL) are the most commercially available SCL products in North America.
A study conducted by the US Forest Products Laboratory and proprietary fire-resistance test data show
that SCL has a similar charring rate as that of massive timber (White R. H., 2006; iLevel, 2008). As such,
the charring rate shown for LVL in Table 16 could be used for SCL, unless the manufacturers have
specific data for some other charring rate that applies. Moreover, tests conducted by O’Neil et al. (2001)
showed that build-up SCL elements (nailed, screwed and bolted) do not exhibit the same fire behaviour as
that of single SCL element of similar initial dimensions. It is anticipated that glued build-up SCL
members may also not behave similarly to a solid single SCL element exposed to fire, unless proven
otherwise.
Guidance on some SCL products can be found in product evaluation reports by CCMC for verifying
whether the equations in the current Section D-2.11 of the NBCC is applicable or not to these proprietary
products.
5.4.4.3
Cross-Laminated Timber
The fire-resistance of CLT elements may be determined from Chapter 8 of the latest edition of the CLT
Handbook – Canadian edition published by FPInnovations (Gagnon & Pirvu, 2011). The wood and
adhesive used in the manufacturing of CLT need to conform to ANSI/APA PRG 320 (2012). Full-scale
fire-resistance tests on CLT assemblies conducted by FPInnovations in close collaboration with the
National Research Council Canada have demonstrated close to a three hour fire-resistance has been
achieved with unprotected CLT floors elements tested under full loading conditions (Osborne, Dagenais,
& Bénichou, 2012). Fire-resistance ratings greater than 3 hours can easily be achieved under actual
specified loading conditions (i.e. lower stress ratio than those from the test series).
Similarly to the equations from Section D-2.11 of the NBCC, the fire-resistance calculation method of
CLT is largely influenced by the induced stress ratio. The structural capacity of the residual (reduced)
cross-section of a CLT assembly exposed to fire must be determined from the classical laminated wood
composites theory whereas the cross plies (i.e. minor strength axis) are not taken into account in the
calculation of the design resistive moment for floors nor the resisting wall compression capacity (i.e. E90
= G0 = G90 = 0). Also, the one-dimensional charring rate of CLT (βo) is influenced by the thickness of the
laminates (d). Therefore, for use in Canada, the charring rate should be determined from equation [19], in
addition to the heated (zero-strength) char layer (Dagenais, Osborne, & Benichou, 2013):
when d ≥ 35 mm
0.65
β0 (mm/min ) = 
(d/35)
when d < 35 mm
0.65
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Lastly, as opposed to a beam or a column, CLT assemblies are used as load-bearing and separating
elements. Therefore, integrity and insulation criteria as set forth in ULC S101 need to be evaluated in
addition to the structural fire-resistance described herein. Subsections 5.4.5 and 5.4.6 of this chapter
address the integrity and insulation criteria of massive timber assemblies.
5.4.4.4
Timber-Concrete Composite Structure
Composite structures made from timber-concrete are gaining in popularity, especially for long span floor
systems as well as to provide enhanced serviceability and acoustic performance. Traditionally, the
concrete would mainly be stressed in compression while the timber would be stressed in tension. The
composite structure can be either a concrete slab connected to a massive timber slab (i.e. a “sandwich”
assembly) or a concrete slab connected to timber beams (e.g. a T-shaped cross-section). Typically, the
composite action between the materials is provided by shear connectors plates or discrete fasteners such
as bolts and screws (Figure 124).
a) Shear connector plates (HBV, 2011)
b) Discrete fasteners (screws) at 45° angle
Figure 124 Examples of timber-concrete composite systems
Full-scale experiments on such assemblies by Fontana & Frangi (1999) and O’Neil et al. (2001) showed
that their structural fire-resistance can easily be calculated using the reduced cross-section of the timber
components as shown in Figure 125, provided that the concrete slab is fire-resistance rated accordingly
for the entire duration required in the NBCC. As the behaviour of composite timber-concrete structure is
governed by the shear connection between the materials, it is also fundamental that heat transfer through
the assembly is sufficiently limited so as not to affect the shear connectors (including adhesive, if used).
Further research is required to better understand the fire behaviour of these hybrid systems. Also, as with
structural design under normal conditions, calculations of new sectional properties (due to an upward shift
of the neutral axis) and applied stress are required.
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Figure 125 Reduced cross-section of a timber-concrete composite structure
5.4.4.5
Fasteners and Connections
Connections in mass timber construction play an essential role in providing strength, stiffness, stability,
ductility and structural fire-resistance. Moreover, connections using metallic fasteners such as bolts,
dowels and steel plates or brackets are widely used to assemble mass timber components or CLT panels
and to provide an adequate load path for gravity and/or lateral loads. Consequently, these connections
require attention by designers to ensure that connections are not the weak link in mass timber buildings
exposed to fire.
Performance of mass timber connections exposed to fire can be quite complex due to the influence of
numerous parameters such as the type of fasteners, the geometry of the connection, different failure
modes as well as different thermal conductivity properties of steel, wood and char layer components. As
such, as mentioned previously, the NBCC does not provide a specific fire design methodology for
determining the fire performance of connections in mass timber construction specified to provide fireresistance of 1 hour or greater.
Due to the high thermal conductivity of steel, metallic fasteners and plates directly exposed to fire may
heat up and not only lose strength but also conduct heat into the wood members. The wood components
may then experience charring on the exposed surface and around the fastener. As structural connections
form an integral part of the structural system and recent research has revealed the significant importance
of their role in assembled frameworks in fire, it is important that good fire protection engineering practice
be exercised to ensure connections are designed or protected for exposure to fire. Typically, a connection
in which the steel is located within the reduced cross-section of the wood element is considered to be
properly protected from thermo-mechanical degradation.
Specific caution is noted with respect to intumescent and spray applied coatings. Recent testing at
Carleton University has indicated that intumescent coating may not always perform as well as expected
when used for protection of steel components in mass timber connections. Further research is required to
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assess the appropriate fire-resistance durability of intumescent coatings used on exposed metallic
fasteners.
It is also noted that the historic good fire performance with unprotected steel or cast-iron connections in
traditional heavy timber buildings may not be applicable to modern connections, as historically such
connections were significantly more massive than modern connections, and generally consisted of column
caps providing load distribution rather than connections relying on tensile and/or shear strength. Some
historic connections do however prove informative. Figure 126 shows a connection found in the 100 year
old 9-storey Leckie building in Vancouver that demonstrates an effective design of a connection that,
while unprotected, would most likely not be a weak structural link in the event of a fire.
Figure 126 Connection in historical Leckie Building in Vancouver
Some connections are not vulnerable to the damaging impact of fire. For example, a wall-to-floor
connection used to resist wind or seismic load from a platform-framed CLT construction would most
likely not be significantly impacted by fire because the steel components are protected by wood.
However, connections used to resist gravity loads may require some special considerations for increasing
their fire-resistance to exposure from underneath. Examples of connections in mass timber construction
that may require some special considerations for increasing their fire-resistance can be found in Dagenais
et al. (2013).
To improve aesthetics, designers often prefer to conceal connection systems (Figure 127 and Figure 128).
Hidden metal plates can be used, but they require machining to produce the grooves in the timber
elements to conceal the metal plates. When the connections are used in fire-retardant or preservative
treated wood, recommendations with regard to types of metal fasteners need to be obtained from the
chemical manufacturer since some treatments cause corrosion of certain metals.
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a) Fire-resistance test conducted on concealed plate (credit:
L. Peng (Peng, Hadjisophocleous, Mehaffey, & Mohammad,
2010))
b) Connection covered with wood paneling
Figure 127 Protected connections for enhanced fire performance
a) Internal steel plate
b) Internal plate and concealed fasteners
Figure 128 Concealed connections for enhanced fire performance
It is advisable to review the recommendations provided in Chapter 4 of this guide with respect to proper
detailing of connections in timber construction.
5.4.4.6
Structural Adhesive
Various structural engineered wood products are manufactured with adhesives. When exposed to fire, the
adhesive needs to perform in such way that the glued product behaves similarly to a solid timber element.
Traditionally, thermosetting adhesives such as phenolic-based adhesives have proven to perform well in
fire conditions. Phenolic-based adhesives need to be evaluated in accordance with CSA O112.6 (2006) or
CSA O112.7 (2006).
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Relatively new adhesives are now gaining popularity. In particular the isocyanate-based adhesives which
can be observed from their clear or white color are now in use. These new adhesives must be evaluated in
accordance with CSA O112.9 (2010) or CSA O112.10 (2008), depending on the type of adhesive and its
intended use (wet- and/or dry-service conditions). These new standards provide an elevated temperature
creep test, called the condition B2 creep test, intended to simulate temperatures that framing members in a
fire-protected assembly can be exposed to at some point during fire exposure. The B2 test is used to
provide information to determine whether fire-resistance ratings obtained from assemblies framed with
solid wood can be maintained when framed with glued wood members (e.g., finger-joined lumber). The
results of the tests in the standards are however not intended to be used as the sole basis for replacing fullscale fire-resistance tests.
Moreover, the CLT performance standard ANSI/APA PRG 320 requires that, for use in Canada, adhesive
be evaluated in accordance with CSA O112.10 as well as Sections 2.1.3 and 3.3 of AITC 405. In addition,
adhesives need to be evaluated for heat performance conforming to Section 6.1.3.4 of DOC PS1. The
intent of the heat performance evaluation is to determine whether an adhesive exhibits delamination
characteristics, which may increase the char rate of CLT when exposed to fire. During the full-scale fire
research on CLT manufactured with a structural polyurethane (PUR) adhesive conforming to the
standards (Osborne, Dagenais, & Bénichou, 2012), localized pieces of the charred layers were observed to
fall off when the temperature at the CLT lamination interface (i.e. glue lines) approached 300°C,
indicating an adhesive failure. As such, an adapted charring rate model applicable to CLT assemblies has
been developed to account for potential delamination and is a function of the thermal penetration depth of
the laminates, thus their thickness. Additional research is needed to better evaluate adhesive performance
in CLT assemblies exposed to fire.
5.4.5
Fire-Resistance of Timber Structure – Integrity Criteria
As described previously, integrity is one of the two performance expectations of the separating function
of building assemblies. The time at which an assembly can no longer prevent the passage of flame or
gases hot enough to ignite a cotton pad defines the integrity fire-resistance. This requirement is essential
in limiting the risk of fire spread to fire compartments beyond the fire compartment of fire origin.
The junction details between components can affect the integrity performance of timber assemblies. The
sides of individual components (planks, decking, CLT, etc.) are shielded from full fire exposure by
adjacent panels collectively acting as a joint. Partial exposure may occur as panels shrink and joints
between panels open. Traditionally, floor integrity performance is deemed to be fulfilled in heavy timber
construction by requiring the use of tongued and grooved flooring not less than 19 mm thick laid
crosswise or diagonally, or tongued and grooved phenolic-bonded plywood, strandboard or waferboard
not less than 12.5 mm thick.
The integrity of building assemblies is also regulated in the NBCC by the provisions that throughpenetrations (i.e. service penetrations) in assemblies be fire-rated (refer to Section 5.7 of this chapter for
more details). Additional guidance for evaluating the integrity performance if timber assemblies can also
be found in Eurocode 5: Part 1-2 and in Chapter 8 of the US edition of the CLT handbook from
FPInnovations.
5.4.6
Fire-Resistance of Timber Structure – Insulation Criteria
Insulation is the second performance expectation of the separating function of building assemblies. The
time at which an assembly can no longer prevent the temperature on the unexposed surface from rising
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above 180°C at any location, or an average of 140°C measured at 9 locations, above the initial
temperature, defines the insulation fire resistance. This requirement is essential in limiting the risk of fire
spread to fire compartments beyond the fire compartment of fire origin as well as allowing safe egress
within the space located on the side of the assembly away from the fire (unexposed side).
As reported by White (2008), when a wood member is charring, the temperature gradient is steep in the
remaining uncharred wood section. The temperature at the innermost zone of the char layer is assumed to
be 300°C. As wood has a low thermal conductivity, the temperature located 6 mm inward from the base
of the char layer is about 180°C once a quasi-steady-state charring rate has been obtained. Equation [20],
also represented graphically in Figure 121, allows evaluation of the temperature gradient within the
remaining uncharred wood section.
𝑥 2
𝑇 = 𝑇𝑖 + �𝑇𝑝 − 𝑇𝑖 � �1 − �
𝑎
[20]
Where 𝑇 is the temperature (°C), 𝑇𝑖 is the initial temperature (°C), 𝑇𝑝 is the char front temperature
(usually taken as 300°C), 𝑥 is the distance from the char front (mm) and 𝑎 is the thermal penetration
depth (mm). Based on data from White (1992) conducted on eight species, the best fit values for the
thermal penetration depth (a) were 34 mm for spruce, 33 mm for western red cedar and southern pine, and
35 mm for the redwood. From this equation it can be seen that no temperature rise on the back surface is
calculated to occur until the residual timber thickness is reduced to 35 mm, which also correlates with the
temperature profile shown in Figure 121.
5.4.7
Fire-Resistance of Gypsum Board Membranes
As discussed previously, the char layer provides effective thermal protection against heat effects. If the
components are protected by additional protective membranes (e.g. gypsum board, wood-based
sheathings, insulation or concrete) the start of charring (ignition) is delayed and, where the protective
membrane remains in place after the charring is initiated, the charring rate is reduced when compared to
that of initially unprotected timber components.
In some areas, the use of gypsum boards may be required for addressing other fire-related performance
attributes such as flame spread rating. If gypsum board is applied on the fire-exposed sides, the following
time can be added to the unprotected member failure time calculated from the models presented in subSection 5.4.4 of this guide:
15 minutes when one layer of 12.7 mm (½”) type X gypsum board;
b) 30 minutes when one layer of 15.9 mm (⅝”) type X gypsum board is used;
c) 40 minutes when two layers of 12.7 mm (½”) type X gypsum board are used;
d) 60 minutes when two layers of 15.9 mm (⅝”) type X gypsum boards are used;
e) 90 minutes when three layers of 15.9 mm (⅝”) type X gypsum board are used, and;
f) 120 minutes when four layers of 15.9 mm (⅝”) type X gypsum boards are used.
a)
These additional times (15 to 60 minutes) are based on experiments completed on beams in tension at the
US Forest Products by White (2009), CLT assemblies protected by Type X gypsum boards at
FPInnovations by Osborne et al. (2012) as well as in Appendix D-2.4 of the NBCC and should only be
applicable to mass timber assemblies. The 90 and 120 minutes duration are extrapolated from testing and
based on Harmathy’s “Ten Rules of Fire Endurance Rating” (1965).
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These numbers are based on the following attachment methods. The gypsum board protective membranes
should be attached directly to the massive timber members using 57 mm (2¼ in.) Type S drywall screws
spaced at 305 mm (12 in.) on center along the perimeter and throughout. Screws must be kept at least 38
mm (1½ in.) from the edges of boards. When using a single thermal protective membrane, the gypsum
board joints should be covered with tape and coated with joint compound. End joints must be staggered
from end joints of adjacent gypsum boards. When using two layers of thermal protective membranes, the
face layer joints should be covered with tape and coated with joint compound. In all cases, the screw
heads of the exposed layer should also be covered with joint compound. End joints of the face layer must
be staggered from end joints of adjacent gypsum boards and end joints of the base (1st) layer.
The above six solutions are simple additive components that provide conservative values. However,
advanced calculations using finite element modeling, for example, may help expand these thermal
protection times of gypsum boards. Heat transfer models can usually be adapted to various material
thermal properties at elevated temperatures (specific heat, density and thermal conductivity), provided
that the latter are available and validated from tests or literature. It can also be demonstrated from such
models that the encapsulation by multiple Type X gypsum boards can maintain the temperature at the
gypsum board-timber interface below 300°C. Such encapsulation would permit, as one of many other
alternative solutions, meeting the NBCC objectives by limiting structural combustible materials from
being involved in a fire and by limiting their contribution to fire growth beyond the required time period.
5.5
Flame Spread Rating of Exposed Timber
The NBCC limits the allowable flame spread rating (FSR) and smoke development class (SDC) of
interior finishes based on the location, building occupancy and availability of an automatic fire
suppression system. The prescriptive provisions for interior finishes in noncombustible construction are
set forth in several sections of Division B of the NBCC, including Subsection 3.1.13 (in general), Article
3.1.13.7 (for high buildings) and Article 3.1.5.10. These provisions are intended to limit the spread of fire
and products of combustion through a building in a manner that allows safe egress of the occupants and
limits the damage to the building in which the fire originated.
In Canada, the FSR of a material, assembly or structural member is determined on the basis of not less
than three standard fire tests conducted in conformance with CAN/ULC S102 (2010). Some construction
materials that can be assigned in generic terms, such as gypsum board and most softwood lumber species,
have been assigned FSRs based on historical data, which are specified in Section D-3 of the NBCC.
Results of FSR testing on proprietary materials are usually available from accredited fire testing
laboratories.
5.5.1
What is Flame Spread?
Surface flame spread is a process of a moving flame in the surroundings of a pyrolyzing region on the
surface of a solid (or liquid) that acts as a fuel source (Hasemi, 2008). In the case of a massive timber
panel, the spread of flame occurs as a result of the heating of the surface ahead of the flame by direct or
distant heating by the flame generated by the burning of the thick timber panel. Flame spread may also
occur in different configurations, which are governed by the orientation of the fuel and the direction of the
main flow of gases relative to that of flame spread (White & Dietenberger, 2010).
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The growth of a fire depends on the time it takes a flame to spread from the point of origin (i.e. ignition)
to involve an increasingly large area of combustible material (Drysdale, 1998). However, the material’s
thermal conductivity, heat capacity, thickness, and blackbody surface reflectivity influence the material’s
thermal response, and an increase in the values of these properties corresponds to a decrease in the rate of
flame spread (White & Dietenberger, 2010).
The primary purpose of the ULC S102 test is to determine the comparative burning characteristics of a
material or an assembly under test by evaluating the flame spread over its surface when exposed to a
standardized test fire. The test method attempts to establish a basis on which surface burning
characteristics of different materials or assemblies may be compared, without specific considerations of
all the end use parameters that might affect these characteristics. Flame spread rating and smoke
developed classification are recorded as dimensionless numbers in this test, while there is not necessarily
a relationship between these two measurements.
5.5.2
Fire Safety Strategies in a Pre-Flashover Compartment
In general, the NBCC requires that interior wall and ceiling finishes have a FSR no greater than 150,
which can easily be met by most wood products. In unsprinklered buildings, in public corridors, the
maximum FSR is set to 25 for ceilings and 75 for walls, but the NBCC allows for materials having a FSR
up to 150 in the lower half of corridor walls, provided the top half has a FSR of not more than 25. In
sprinklered buildings, the FSR for wall and ceiling finishes in public corridors is permitted to be 150.
Moreover, when a building is required to be of noncombustible construction, such as tall buildings (as per
Subsection 3.2.6 of Division B from NBCC), combustible interior wall and ceiling finishes are generally
required to have a FSR not more than 150 and 25 respectively, and should not be more than 25 mm in
thickness. The NBCC also allows ceilings to have materials with a FSR of 150 or less, provided they do
not cover more than 10% of the ceiling area within a fire compartment.
Traditionally, combustible interior wall and ceiling finishes have been evaluated for flame spread using
lumber specimens of 19 mm in thickness. In order to evaluate the surface burning characteristics of
massive timber assemblies such as cross-laminated timber (CLT) and structural composite lumber (SCL),
flame spread tests on massive timber assemblies have been conducted in accordance with ULC S102.
Fully exposed CLT (105 mm thick) as well as parallel strand lumber and laminated strand lumber
specimens (89 mm thick) exhibited very low flame spread ratings ranging from 35 to 75 when compared
to thinner similar products, usually having FSR in the order of 100 (Table 17).
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Table 17
Flame spread rating of massive timber assemblies
Flame
Spread
Rating
Wood and wood-based products
Lumber, not less than 19 mm in thickness (1)
Western Red
73
Cedar
Pacific Coast Yellow
78
Fir
Amabilis (Pacific Silver)
69
Hemlock
Western
60-75
Maple
Flooring
104
Red
Oak
100
White
Eastern White
85
Lodgepole
93
Ponderosa
105-230
Pine
Red
142
Southern Yellow
130-195
Western White
75
Poplar
170-185
White
65
Spruce
Sitka
74
Western
100
Shakes
Western Red Cedar
69
Shingles
Western Red Cedar
49
Structural composite lumber
PSL
Parallam® PSL (min. 89 mm) (2)
35
LVL
Brisco Mfg. LVL (min. 140 mm) (3)
35
LSL
TimberStrand® LSL (min. 89 mm) (2)
75
Cross-laminated timber
CLT
3-ply SPF E1 stress grade (min. 105 mm) (2)
35
Structural sheathing
Douglas Fir, 11 mm thickness
Plywood (4)
Poplar plywood, 11 mm thickness
150
Spruce face veneer plywood, 11 mm thickness
n.d. = not determined due to insufficient test information.
(1) Taken from Fire Safety Design in Buildings (CWC, 1997)
(2) Taken from Dagenais (2013).
(3) Taken from QAI Laboratories test report (2012).
(4) Taken from Table D-3.1.1.A of the National Building Code of Canada (NRCC, 2010).
5.5.3
Smoke
Development
Classification
98
90
58
n.d.
n.d.
100
122
210
n.d.
229
n.d.
n.d.
n.d.
n.d.
74
n.d.
n.d.
n.d.
25
30
85
40
300
Impact of Exposed Timber on these Fire Safety Strategies
Previous studies and results presented in White et al. (1999) suggest that there may also be a relationship
between the time to reach flashover conditions in an ISO 9705 (1993) room/corner fire test and the
ASTM E84 (2012) flame spread ratings of materials. Materials exhibiting low FSR provide longer time
before flashover conditions are reached. It should be noted that while both ASTM E84 and ULC S102
methods generate flame spread and smoke developed data, the sample mounting procedures and test
sampling are different. As such, the results may not be exactly the same, although the standards generally
provide very similar flame spread values, within a 10% range.
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The use of materials that exhibit lower flame spread ratings than typical combustible interior finish
materials could result in a reduced “risk” of ignition and potentially a reduced time to flashover
conditions depending on the configuration and ventilation of the room of fire origin. By doing so, such
reduced risk may permit achievement of the NBCC objectives when developing an alternative solution
although further work on this approach is likely required.
Should they become ignited; exposed massive timber assemblies can have a significant effect on the fire
growth. Recent studies conducted at Carleton University in Ottawa (Ontario) showed an increase in the
fire growth rate in unprotected (fully exposed) CLT room fires, leading to flashover conditions being
reached sooner when compared to those where CLT is initially protected by gypsum board (McGregor,
2013). In rooms with initially protected CLT (using directly applied gypsum board), the fires selfextinguished when all combustibles had been consumed and the CLT remained unaffected by the fire
with no noticeable contribution to fire growth, duration or intensity. This research highlighted fire hazards
associated with construction using exposed mass timber in situations where no active fire protection is
provided and where the fire is burning over extended periods without response. However, as required for
tall buildings in NBCC, automatic sprinklers would provide an active protection against fire growth as
their activation is expected to occur before significant fire growth and fire involvement of exposed CLT
panels. A review of the acceptable solutions permitting combustible room finishes will provide the
engineer with guidance on where reliance on sprinklers to limit fire growth and flashover is appropriate.
Similar room fire studies were conducted by Hakkarainen (2002) in an attempt to evaluate the
temperature development and charring behaviour in compartments built from light and heavy timber
construction. Results showed that the temperature rise and peak in the compartment constructed with
unprotected timber were lower than those from the protected timber rooms (using directly applied gypsum
board). Based on visual observations and energy balance considerations, it was concluded that a larger
part of the pyrolysis gases burnt outside the fire compartment in the case of unprotected timber rooms
compared to protected rooms.
As such, even if timber is considered a combustible material, the change of the boundary conditions of a
compartment (i.e. thermal inertia and flame spread rating of exposed surfaces) influences the fire
development and dynamics.
5.6
Fire Separation
5.6.1
What is a Fire Separation?
As mentioned previously, the NBCC requires that buildings be sub-divided into fire-rated compartments
in order to limit the risk of fire spread beyond the compartment of origin. In order to effectively provide
fire-rated boundaries to a compartment, these boundaries need to be built as fire separations. It is noted
that the acceptable solutions of Division B already permit wood framing and solid wood walls in some
fire separations within floor areas in buildings otherwise prescribed to be of noncombustible building.
Fire separations are also used to subdivide the area of a building façade into fire-rated compartments
when determining the exposing building face and spatial separation provisions.
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5.6.2
Integrity of Fire Separations
Walls, partitions and floors required to be designed as fire separations need to be constructed in such way
as to provide a continuous element and, often, to have a prescribed fire-resistance rating. However, due to
service penetrations and gaps in fire separation are inevitable. As such, in order to maintain the integrity
of a fire separation, closures, shafts or other means must be provided for maintaining the integrity and the
fire-resistance, when required, of a fire separation. Also, smoke-tight joints must be provided where fire
separations abut on or intersect a floor, a wall or a roof.
Subsections 3.1.8 and 3.1.9 of Division B of the NBCC detail the specific provisions for enclosures and
penetrations in fire-rated separations.
5.7
Fire Protection of Service Penetrations and Construction Joints
5.7.1
Fire Stopping
Fire stop systems are intended to ensure the integrity of a fire compartment by maintaining the fireresistance rating of the floor and/or wall assemblies that they penetrate or at construction joints.
Subsection 3.1.9 of Division B of the NBCC prescribes penetrations of an assembly to be sealed by a fire
stop system that has been tested in accordance with ULC S115 (2011). A minimum F-rating of 90
minutes is required for fire stops used in assemblies of 2 hours fire-resistance rating for service
penetrations and 2 hours F-rating for construction joints. An F-rating can be defined as the time period
where the through penetration fire stop system limits the spread of fire through the penetration. It is noted
that T-ratings (based on temperatures on the unexposed side of the penetration) are prescribed for
firewalls and other separations between buildings.
5.7.2
Availability of Fire Stop for Mass Timber Assemblies
Very little information is available on the fire performance of fire stops used in mass timber assemblies
with partial or full penetrations. Further research needs to be carried out in order to adequately investigate
the fire performance of fire stop systems in massive timber construction. Engineering judgment can also
be requested from fire stop manufacturers on a project-by-project basis.
However, there are numerous fire stop systems that are already approved for use with concrete and/or
light-frame construction. Both of these types of construction have similarities to mass timber assemblies,
where concrete is massive and typically does not have void cavities, and light-frame contains wood
elements. Provided that material compatibility is confirmed and the fire stop is not located in the potential
char layer during fire exposure, it can be demonstrated that many concrete slab fire stop system are
suitable for timber slab penetrations. On this matter, a literature review made by CHM Fire Consultants
Ltd. on behalf of FPInnovations on research conducted in Europe demonstrates that fire stop systems
currently used in reinforced concrete and light-frame construction may be used with success in solid
wood construction (Dagenais, 2013).
Additional information on firestops, service installations and detailing in timber structures can be found in
Chapter 8 of a document entitled “Fire Safety in Timber Buildings – Technical Guideline for Europe” (SP
Trätek, 2010) and in the literature review from FPInnovations (Dagenais, 2013).
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It is noted that a specific concern has been identified with CLT construction joints which may require
special fire stopping practices, in that the potential gaps between the boards can create channels around
the fire stops allowing smoke to bypass traditional fire stops, as shown in Figure 129.
Figure 129 CLT smoke leakage paths (credit: RDH Consulting)
5.8
Concealed Spaces
5.8.1
What are Concealed Spaces and the Concern with Them?
Inherent in any building are concealed spaces, some that are used for specific purposes, such as service
spaces, and some that are simply inherent in the building construction. These may be large, such as above
a ceiling suspended a distance below the structure, or small, simply formed by the attachment of
protective membranes, or as part of a wall or floor assembly.
In developing an alternative solution, it
is essential to acknowledge that some void spaces will occur, and establish a methodology to address the
risk. The concern with concealed spaces is simply that these spaces may allow for fire growth, or may
contribute to fire spread through a building, possibly bypassing fire separations.
The premise of the approach in this document is that all exposed timber within concealed spaces must be
protected, unless it is shown that protection is not required in the alternative solution.
Inherently, the method of encapsulation will lead to some concealed spaces with exposed combustibles
within. For example, in enclosing connections or uneven profiles, small cavities will exist and should be
acceptable; however, the impact of these concealed spaces should be assessed and addressed in the
alternative solution for combustible construction. Examples of concealed spaces are illustrated in Figure
115.
Additional void spaces may occur around connections or other elements where it is not possible to attach
the encapsulation directly to the connection and the encapsulation needs to be attached to a supporting
frame.
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5.8.2
Performance of Combustible Concealed Spaces of Mass Timber
It is significant that the traditional concern with combustible concealed spaces in light wood frame is not
applicable to mass timber due to the inherent fire performance of the mass timber elements. If an
alternative solution is developed to include significant void spaces in the encapsulation methodology, it is
useful to understand that, unlike light wood frame construction, fire spread in mass timber in a void space
may have limited consequence.
In light wood framing, fire spread through concealed spaces has been known to lead to premature, sudden
and unexpected collapse of structures, such as floors, away from the fire incident. This is particularly
prevalent in older (pre 1970) structures that did not have appropriate fire blocking. This is unlikely to
happen for mass timber because the mass timber members have a significant inherent fire-resistance such
that a fire in a small void space is unlikely to do significant damage to the building structure or metal
connections. Furthermore, it is likely that the inherent reduced flame spread of mass timber will result in
reduced flame spread within concealed spaces; however, at this time there is no known research to
demonstrate this.
5.8.3
Building Code and NFPA 13 Provisions for Concealed Spaces
Within a noncombustible building, void spaces are generally not expected to contain significant exposed
combustible construction materials. Subsection 3.1.5 of Division B lists small quantities of combustible
materials permitted to be used within non-combustible buildings. Subsection 3.1.11 from Division B of
the NBCC describes specific provisions for fire blocking in concealed spaces, including those found in
buildings required to be non-combustible. Such provisions do not permit large amounts of exposed
timber, unless the space is found within wood elements otherwise permitted.
The standard on automatic sprinklers referenced in the NBCC, NFPA 13, provides guidance for concealed
spaces within buildings of both noncombustible and combustible construction where sprinkler protection
may be either required or omitted. It is not considered that these provisions are appropriate for anything
other than the design of a sprinkler system. They should not be used as justification to omit protection of
exposed timber in concealed spaces within a tall timber building required to provide the level of
performance of a noncombustible building without further investigation.
5.8.4
Methods of Protecting Concealed Spaces
The premise of complete or partial encapsulation is that the mass timber structure is protected from fire.
In developing the alternative solution, it must be acknowledged that some void spaces will occur and a
strategy may need to be developed to address such spaces.
One approach would be to fill all such void spaces with noncombustible insulation, or apply a spray type
or other form fitting protection to all exposed timber. If this approach is taken, materials such as spray
applied fire resistance materials typically used for providing fire-resistance to structural steel elements
may be an acceptable approach if material compatibility and adhesion is acceptable. However it is likely
that this approach is excessive and unacceptably expensive for all void spaces, and also that some smaller
void spaces are acceptable.
If the void space is sealed and small enough (or fireblocked to be small enough) to preclude significant
circulation of air within these spaces, it will have an insignificant impact on the encapsulation approach.
Fire is limited by the available oxygen. If the space is sealed and calculations show that the impact of fire
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within the void space is unlikely to threaten the integrity of either steel connections or mass timber
elements such unprotected void spaces can be demonstrated to be acceptable.
It may be appropriate to rely on provision of sprinkler protection of these void spaces, especially in cases
where the sprinkler system is provided with enhanced reliability, such as on-site with backup water
supply.
Alternative approaches to protection of void spaces with exposed heavy timber include coating all
surfaces within the void space. Consideration may be made of surface treatment of void space by such
readily available products such as paint to reduce surface burning characteristics of materials such as
flame spread. However it is not considered that current surface testing of such products to demonstrate a
flame spread rating of adequately demonstrated their effectiveness in addressing the concerns with fire
spread unique to narrow or small void spaces. Additional testing is needed to validate these approaches.
It is noted that, unless otherwise specifically addressed in the alternative solution, the permission for
outlet boxes or combustible services to penetrate the membrane provided in the acceptable solution of
Division B should not be applied to encapsulation membranes unless the effects of such penetrations are
specifically considered as this may allow for increased air circulation within void spaces aiding
combustion.
5.9
Spatial Separation and Exposure Protection
A fire in a building can pose a threat to neighbouring buildings. Flames issuing through windows or other
unprotected openings in the exterior wall can cause combustible materials on a nearby building to ignite
by either direct flame impingement or through excessive thermal radiation. The potential for fire spread
from building to building was studied by the National Research Council of Canada (NRCC) in 1958
during experiments called the “St Lawrence Burns”. While the St. Lawrence Seaway was being
constructed several towns had to be demolished and NRCC conducted a number of experiments in those
towns to establish the nature of flame extension through openings and the intensity of thermal radiation
emitted by such flames.
5.9.1
Assumptions behind the Current Spatial Separation Provisions
It is well known that when a typical combustible material, such as wood, is exposed to a radiant heat flux
of 12.5 kW/m² for about 15 minutes, the surface of the material reaches a sufficiently high temperature
that a small pilot (spark or flame) will cause the material to ignite. Using the values of thermal radiation
emitted by flames measured during the St. Lawrence Burns and some geometrical analyses, it was
possible to predict appropriate distances from openings of an arbitrary size at which the radiant heat flux
received at a neighbouring building would remain lower than 12.5 kW/m². Such analyses form the basis
of the fire separation and exposure protection requirements in Subsection 3.2.3 of Division B of the
NBCC, and in similar codes in the UK and the USA, where radiant heat fluxes of 180 kW/m² and 360
kW/m² are assumed to be emitted from openings of normal and severe hazard respectively (Torvi, Kashef,
& Benichou, 2005).
In an attempt to limit the risk of fire spread from one building to another, the following measures have
been introduced in Subsection 3.2.3 of the NBCC:
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The size of unprotected openings in an exposing building face (i.e., the exterior wall of a fire
compartment) is limited; thereby reducing the surface area and hence overall intensity of the
radiating flames.
b) A minimum permitted distance from the exposing building face to the property line is prescribed.
This minimizes the risk of direct impingement, but also limits the radiant heat flux on the face of
the neighbouring building as the radiant heat flux decreases significantly as a function of the
distance from the radiating surface.
c) The combustibility and fire resistance of the exposing building face and the combustibility of its
cladding are regulated. This ensures that the size of the radiating surface does not increase to an
unacceptable level during a fire due to burn through of the exterior wall or due to flame spread
along the surface of the cladding.
a)
The precise nature of these measures depends upon the occupancy of the exposing building (which
impacts the potential fire severity) and whether or not the exposing building is sprinklered (which also
impacts the potential fire severity). Of course the tall wood buildings considered in this guide must be
entirely protected by automatic sprinklers and hence fire severity can be expected to be limited.
For sprinklered tall-wood buildings in which the “complete encapsulation” strategy has been employed, it
can be assumed that the building will perform as well as if it were a sprinklered building of
noncombustible construction for all fire protection considerations including those related to the spatial
separation and exposure protection requirements of Subsection 3.2.3.
If a “partial encapsulation” method is employed, it is noted that while fire intensity will not likely be
greater than in a noncombustible building, the potential fire duration could be longer, and water supply
duration available for controlling exposures should commensurately be increased. Again a properly
designed automatic sprinkler system will significantly mitigate this concern.
In suburban or rural areas where rapid fire fighter response is not available (defined in Subsection 3.2.3 as
less than 10 minutes from the time the alarm is received by the fire department), consideration should be
made to provide additional reliability measures to the sprinkler system including on-site water supplies
with sufficient volume to maintain sprinkler operation until fire fighting forces can respond to reinforce
the water supply and initiate suppression activity.
Lastly, as discussed below in Subsection 5.5.2 of this chapter, the tall wood building envisioned in this
guide would conform to the same performance standard for cladding systems as a noncombustible
building, that is noncombustible cladding, or exterior wall and cladding systems that conform to 3.1.5.5
of Division B. As such the cladding and exterior wall system will not increase the fire exposure to
adjacent buildings, nor the propensity to ignite from an adjacent fire over that envisioned in the
prescriptive provisions of Division B of the NBCC.
5.9.2
Exterior Cladding
The premise of the approach is that all timber is fully encapsulated except where it is demonstrated that a
lesser or no encapsulation is sufficient. For exterior cladding the NBCC already permits combustible
cladding, and cladding systems containing combustible components, provided they conform to specific
performance criteria found in Article 3.1.5.5 when tested to CAN/ULC S134. The latter is a test with a
fire plume extending from a window (Figure 130), and is considered to represent an appropriate design
fire for exposure to both a window plume and to be conservative relative to exterior fire impingement. It
is essentially a test to confirm that a cladding system containing combustible components will not support
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unacceptable flame propagation up the face of a building any more that that allowed when using noncombustible cladding.
Figure 130 CAN/ULC S134 Full-scale test of exterior window plume
While the criteria of Article 3.1.5.5 refer to the cladding system of a noncombustible assembly in a
noncombustible building, for a building containing combustible elements in or on an exterior wall, it is
appropriate to assess the entire wall system for conformance to Article 3.1.5.5.
Exposed untreated wood claddings will not meet the performance criteria established by the NBCC. Any
treatment of wood would also have to be a permanent treatment, not subject to degradation due to
weathering, as identified in Sentence 3.1.5.5.(5) and at this time availability of suitable treatment is
limited and costly.
Therefore any mass timber exterior walls will likely need to be encapsulated to protect them from
exposure to fire on the exterior. A level of encapsulation based on CAN/ULC S101 fire tests as discussed
earlier for interior spaces would be acceptable, but likely excessive for the exterior face of exterior
components with any form of encapsulation. Encapsulation of exterior face by a single layer of 15.9mm
exterior gypsum wallboard or gypsum sheathing may be considered to provide conformance with Article
3.1.5.5 as confirmed by testing by NRC IRC in the 1990’s (Oleszkiewicz, 1990).
It is also probable that various forms of noncombustible exterior cladding systems and cladding systems
in combination with noncombustible exterior insulation can be shown to conform to Article 3.1.5.5 when
installed over mass timber wall elements; however, this will require testing or analysis to demonstrate
conformance. It is noted that while full-scale testing is expensive and time consuming, in some cases it is
possible to develop hand-based calculations and models to demonstrate conformance.
Similarly, use of minor combustible components in these exterior walls can be demonstrated as
acceptable based on testing or analysis to show conformance with Article 3.1.5.5; however, the authors
are unaware of any such systems that have been tested.
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5.9.3
Wildfire
The majority of tall buildings are expected to be in urban or suburban environments where wildfires are
not a significant hazard. Where a wood building may be exposed to a potential wildfire, as the exterior
cladding would conform to the exterior cladding provisions for a building of noncombustible
construction, the risk of wildfire, both of the building being affected by wildfire, or wooded areas being
affected by the building would be no different than from a conventional building. As a tall wood building
would be sprinklered, the risk would be significantly less than the risk imposed by a much smaller
unsprinklered building, whether combustible or noncombustible. It should be noted however that wildfire
considerations are outside the scope of the NBCC.
5.9.4
Roof Construction
The acceptable solutions of the NBCC permit significant combustible elements in roof construction when
constructed above a concrete roof deck and exterior parapets, and permit conventional heavy timber roofs
of unlimited area in larger buildings not exceeding two storeys in height. The approach to design of the
roof system should address not only firefighter safety but also the propensity for the construction to
contribute to flying brands during a fire. Use of appropriate roof covering materials in conjunction with
mass timber elements having an inherent rating can address this issue. It is noted that Subsection 3.1.5 of
the NBCC permits combustible roof framing and sheathing in a noncombustible building of unlimited
height when located above a concrete roof deck.
5.10
Firefighting Assumptions
5.10.1
Firefighting Considerations in Tall Wood Buildings
The safety of firefighters and other emergency responders is addressed under the objectives and
functional statements of the NBCC because they are treated as occupants’ of the building. Such
consideration is directly tied to NBCC requirements that specifically call for either passive fire protection,
through the provision of fire-resistance rated construction, or active fire protection, through the provision
of automatic sprinkler protection.
Traditionally, unsprinklered construction relied on exterior firefighting operations. With the advent of
buildings protected with monitored and supervised sprinkler systems and related firefighting practices, the
NBCC has shifted to reliance on sprinkler systems and interior firefighting access.
In buildings entirely protected by sprinklers, firefighting operations can be conducted from the interior of
the building due to the reduced internal fire risk in comparison to unsprinklered buildings. A sprinkler
system is the single most effective aid for firefighting, as such systems have been shown to be reliable,
effective and typically operate automatically well before firefighters arrive at the fire scene. Essentially,
there are few known major fires in fully sprinklered high buildings, except for cases when the specific
event (terrorism or explosion or poor maintenance) has disabled the systems.
This relationship implies that the NBCC considers sprinkler protection as an important asset and
significantly more beneficial than other measures such as facing multiple streets. It also indicates the
expectation is for interior firefighting in a sprinklered building as a sprinklered building is only required
to face 1 street, which results in access openings for firefighting being required to be provided in the
exterior wall facing only that single street.
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In principle, firefighters’ safety in a completed building of combustible construction, with all required
passive and active fire safety systems complete, will be similar to that in a noncombustible building also
designed with the minimum passive and active fire protection. The main difference between these two
scenarios is that in a combustible building there is a possibility of the structure becoming involved in a
fire, which may be more likely in a building with an exposed wood structure instead of an encapsulated
structure.
Safety of both firefighters and occupants in a tall wood building depends primarily on the reliability of the
fire suppression system. All tall wood buildings will be sprinklered, and as in noncombustible buildings
the sprinkler system will relied upon to control or extinguish a fire. As stated elsewhere in this technical
guide, a backup independent water supply should be provided for tall wood buildings to mitigate the risk
of failure of the municipal water supply. This will typically require onsite water storage and a fire pump.
Historically, the greatest operational factor contributing to firefighter deaths or injuries is incomplete
situational awareness (IAFC, 2012). In other words, lack of knowledge of the building and its contents,
and of the fire location and characteristics, is the major factor in increased risk for individual firefighters.
The majority of tall wood buildings are expected to be in urban environments where there are professional
Fire Departments with the capability of pre-planning fire responses, and where tall buildings will have
fire safety plans. It is also important for a project design team to involve the local Fire Department in the
design stages of a project both to ensure that firefighters are aware of any special characteristics of a tall
wood building and to address Fire Department concerns at an early stage in the design process.
It is important to understand that a critical risk to firefighters in light frame construction is fire attacking
the lightweight elements and also, in combustible buildings, the potential for spread within voids
attacking structural elements and resulting in premature and unexpected collapse of floor assemblies. As
mass timber has an inherent fire-resistance, this level of risk related to buildings of light frame
construction is significantly mitigated in mass timber buildings (noting specifically that not only is the
timber fire rated, but connections will require inherent or applied fire protection). As noted in Subsection
5.4.1 of this chapter, the probability of fire spread within void spaces of mass timber buildings is
significantly less than within light-frame combustible assemblies. Furthermore, the consequence of a fire
challenging the fire-resistance of any type of light frame structure or fire spread within void spaces in
light wood frame building is the potential for collapse, whereas the consequence of fire challenging the
fire-resistance of the mass timber or fire spread within void spaces of a mass timber building will rarely
include the probability of collapse.
Many tall wood buildings will be high buildings as defined by the National Building Code. However,
particularly for non-residential buildings, there may be wood buildings of a height where an alternative
solution approach is required but that are below the threshold to be considered as a high building. For
those buildings, provision of firefighter communication features required for high buildings should be
considered, including firefighter’s telephones at each exit stair on each floor, and voice communication
throughout the floor areas. These systems can assist communication between firefighters but also allow
firefighters to provide instructions to occupants and thereby improve the efficiency of Fire Department
operations.
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5.11
Provisions for Mobility Impaired Occupants
Provisions for mobility impaired occupants in a tall timber building would be no different than for a
conventional building, as the building will be designed to perform in a manner equal if not superior to a
noncombustible building.
Provisions for fire separations and smoke control systems, along with emergency communication
systems, will protect occupants from the fire event and provide information to occupants unable to
evacuate on their own. While there should be no need to provide additional features for a mass timber
building, as the building will be designed to provide the same level of safety as a noncombustible
building, to provide additional comfort, firefighter’s telephones or panic buttons could be provided at
elevators to enable occupants with disabilities to call for assistance.
In the unlikely event that a building is designed to house a large number of mobility impaired occupants,
it may be appropriate to undertake evacuation modeling to confirm that the mobility impaired occupants
can be evacuated within the prescribed 2h fire duration of Division B. However this concern would
equally applicable to a noncombustible building with a large number of mobility impaired occupants.
Alternatively, and again, in excess of the level of performance provided for the permitted noncombustible
buildings, it is possible to design elevator systems as egress elevators. Guidance can be found in the
International Building Code (ICC, 2009) which permits unsprinklered high-rise buildings (not permitted
in Canada) and requires such buildings to have elevator systems available for egress for persons with
disabilities.
An
informative
discussion
article
with
references
is
available
at
http://www.mrsc.org/artdocmisc/elevators.pdf.
5.12
Consideration for Major Natural Disasters
While Part 4 of the NBCC (Structural Design) specifically addresses natural disasters such as earthquakes
and wind storms, Part 3 of the NBCC does not address this well. However, for a tall timber building, it
may be necessary to address disasters that may impact emergency response and infrastructure. In terms of
risk, this is a low probability event with potentially high consequences.
If the structure of a tall timber building is protected by complete encapsulation, it can be expected to
perform the same as a noncombustible building; however, if it is protected by partial encapsulation or has
significant exposed mass timber elements, it must be recognized that the building might continue to
char/smoulder following a fire, and could eventually collapse if inherent fire-resistance is limited and
firefighting operations are not available to clean up and extinguish the fire.
In a major disaster, such as an earthquake, it must be assumed that emergency response may be
overwhelmed and not available, and that municipal water supplies may be cut off, and that furthermore,
the disruption of a building and its systems and contents significantly increases the probability of a fire
incident following an earthquake. Robertson (1998) and Harmsworth (2000) provide further guidance on
the concerns of fire following earthquake. Robertson was at the time the Chief Building Official for the
City of Vancouver and his report notes the following with regard to the City of Vancouver’s water
supply:
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"An internal report of the City of Vancouver concludes that, at present, an
M-7 earthquake would render the Greater Vancouver Water District supply
system completely dysfunctional with 1000 water main breaks and 1000
service breaks."
It is noted that the NBCC already requires an emergency power supply for 2 hours. It is therefore
suggested that tall timber building also have a secondary water supply, such that the sprinkler system can
remain operational and assist firefighters or civilians performing firefighting duties in extinguishing the
fire.
Guidance on design requirements may be found in NFPA 13 and the International Building Code which
requires all high buildings to have a 30 minute water supply for sprinkler systems onsite. In assessing the
required duration for the water supply, it is noted that with an operating sprinkler system, although such
systems frequently do not fully extinguish fires and require human intervention, particularly with fires
located in spaces shielded from sprinkler spray, such as under tables, sprinklers have been shown to
maintain tenable conditions and facilitate non-professional fire response. Therefore with occupants alerted
to concerns by the seismic event and the building alarms, it is reasonable to assume that in the absence of
civilian firefighters, occupants can and will respond to a fire incident within a reasonable time frame. As
such, provision of a 30 minute or 60 minutes water supply should be sufficient.
5.13
Fire Safety during Construction
5.13.1
Fire Risk Factors and the Fire Problem during Construction
Many combustible buildings are at their highest risk and potential of fire during construction due to
hazards and conditions at construction sites that are different from those for completed buildings. These
include:
•
•
•
•
•
•
•
Incomplete fire separations,
Lack of functional fire suppression and fire alarm systems,
Inadequate water supply for manual firefighting,
Fire Department response inefficiencies due to the state of construction,
Increased risk of incendiary fires during periods when the site is unoccupied,
Hazardous operations that occur on construction sites such as hot work (for example, cutting,
grinding, welding, soldering, and torch applied roofing), and
Significant onsite stored and erected unprotected combustible materials.
Fire exposure to or from adjacent buildings may also differ from conditions for a completed building
because spatial separation provisions of Building Codes are based on completed buildings with protective
systems and fire separations that are not available to partially constructed buildings.
5.13.2
Management of Risk
In a completed building, the emphasis is on providing a level of fire safety that meets the needs of the
occupants during the life of the building. Fire Safety during construction differs from fire safety in a
completed building in that it is generally addressed throught management of the risk, with acceptance that
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a certain degree of risk of fire is acceptable and addressed through insurance provisions. There are many
different factors that can be controlled to limit the risk of fire during construction as discussed below.
5.13.3
Considerations in Fire Safety during Construction and Renovations
Fire safety on a construction site is primarily the responsibility of the contractor(s) and possibly also the
owner or developer. For major projects, the approach to construction fire safety should involve several
members of the project team, including general contractor, trades contractors, owner or developer, project
consultants, possibly a specialty consultant dealing with fire protection issues, as well as the local Fire
Department. The projce construction insurer may also be involved. The National Fire Code of Canada
(NFCC) (2010), Canadian Wood Council (CWC, 2012; 2012) and NFPA 241 (2013) can provide
additional guidance.
Items and features that should be considered in developing an approach to fire safety on construction sites
include, among other things, a construction fire safety plan, coordinator, pre-construction meeting, fire
watch during off-hours, firefighting water supply and fire compartmentalization.
5.13.4
Construction Fire Safety Plan
A detailed construction fire safety plan should be developed to include information such as designation
and organization of site personnel responsible for carrying out fire safety duties, emergency procedures to
be followed, measures for controlling fire hazards, and maintenance procedures for on-site firefighting
facilities. Development of the construction fire safety plan should include an analysis of the fire risk
including any unique hazards associated with the particular construction site, and proposed actions to
mitigate the risk. The type and severity of the risk will vary depending on factors such as the project size,
type of construction, scheduling, complexity, and proximity to other buildings. Due to the changing
nature of the typical construction site, the construction fire safety plan should be updated at regular
intervals to address hazards associated with particular construction phases.
The construction fire safety plan will have to address various stages of construction and may require
frequent modification to reflect site conditions.
The NFCC provides further guidance on construction fire safety plans. The construction fire safety plan
should be developed in communication with the responding Fire Department, and the completed
construction fire safety plan should be submitted to the Fire Department for their review and comment
prior to commencement of construction. Implementation and adherence to a construction safety plan can
greatly assist in limiting the risk of fire on a construction site.
5.13.5
Construction Fire Safety Coordinator
For tall buildings the contractor should assign a fire safety coordinator, with overall responsibility for
coordinating fire protection and risk mitigating issues, from commencement of construction through to
building occupancy. This coordinator should keep workers up to date with emergency procedures on a
regular basis, monitor the site relative to ongoing fire hazards and the fire safety plan, be the main contact
with the local Fire Department, and be trained to identify and respond to fire hazards. The construction
fire safety coordinator should also implement and manage a system for control of hot works including
monitoring of areas where hot work has occurred for several hours after such hot works
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5.13.6
Pre-Construction Meeting
A pre-construction site meeting should be held between the general contractor, key members of the
project team, and the local Fire Department. This meeting can allow all participants to clarify their
expectations and provide an opportunity to review the fire risk conditions for the project. It may be
advantageous to involve construction workers performing hot work, and it may be advantageous to repeat
such meetings throught the construction period as phases of construction change.
5.13.7
Fire Watch during Off-Hours
Construction sites face a risk of incendiary fire, primarily during non-working hours. Consideration
should be given to a fire watch system when the site is not actively occupied by construction activity.
This may be a manned fire watch system with regular surveillance of all areas of the site, or active
detection systems, or a combination thereof, and may also include other features such as security cameras.
5.13.8
Firefighting Water Supply
In many projects, connection to the municipal water supply is not completed until fairly late in the
construction period. This means that there is minimal possibility of interior firefighting during most of
the construction.
Tall buildings will typically require a standpipe system with hose connections for use by the Fire
Department. To allow manual firefighting during construction, the standpipe system installation must
progress with the building construction, and be connected to the municipal water supply. The National
Fire Code of Canada requires the standpipe to be not more than one floor below the highest forms, staging
and similar combustible elements at all times, with the fire department pumper connection to be
accessible from the street. Early coordination is required between the local municipality, project
consultants, and applicable contractors to provide a water supply for firefighting as soon as significant
quantities of combustible material arrive on site. It is noted that standpipe systems can be either
temporary or the final permanent standpipe system.
Where practical, the sprinkler system should be charged and in an operational state. This will vary
depending on the stage of construction, and may include the partially completed system being operational
during off-hours but temporarily deactivated for specific zones while work is in progress in that area.
5.13.9
Early Fire Compartmentalization
Without completed fire separations within the floor area, or without completed exit stairs, there may be
few if any physical barriers to fire spread within a building, nor safe routes out of the building.
Consideration should be given to provision of an unobstructed and completed (with respect to fire
separations) exit stair to each floor level, which discharges at ground level. Consideration should also be
given to construction sequencing to allow fire compartments, firewalls and closures in fire separations to
be given priority, with closures kept closed when practical.
It has become common in some areas for “top down” interior completion of low-rise and mid-rise
buildings, particularly multi-unit residential, where interiors of the uppermost levels are completed first
and then lower levels progressively. This approach can expose the entire building to the effects of a fire
at a lower level because the structure at the lower levels supports the upper levels, and also makes it more
difficult to progressively install fire protection systems. For a tall timber building it will be necessary to
limit the fire risk by providing at least temporary fire protection systems to lower floors as construction
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progresses. Consideration should be given for “bottom up” interior completion of the building, to allow
fire protection systems to be progressively installed and to minimize the volume of affected combustible
construction.
5.13.10 Exposure Protection from Wildfires
The majority of tall buildings are expected to be in urban or suburban environments where wildfires are
not a hazard. Where a wood building may be exposed to a potential wildfire, the fire risk analysis should
consider protective measures against fire spread from the exterior. This may include establishment of a
clear buffer zone or temporary barriers around the building, and provision of an exterior water supply to
allow protection by exterior fire hose. Where practical, construction sequencing should prioritize
completion of the building face(s) most exposed to potential wildfire.
5.13.11 Exterior Exposure
A combustible building under construction has the possibility of exposing existing adjacent buildings to
effects of fire, beyond that contemplated by spatial separation provisions of the NBCC. The analysis of
fire risks during construction should take into account location and size of existing buildings and
structures, along with any other exterior items that may spread fire such as adjacent trees or shrubs, and
consider appropriate mitigating features. These may include exterior fire barriers, control of combustibles
adjacent to other buildings, temporary exterior fire suppression or standpipe systems, and prioritizing
completion of building faces adjacent to other buildings.
5.13.12 Fire Safety during Renovations
Renovations cannot easily be categorized because the variation of the scope of work for different types of
renovations can differ widely. A renovation is assumed to occur in a building that was previously
completed. As such, the various fire safety systems and features of the building should have been in a
completed condition prior to commencement of the renovation. During the renovation, some or all of the
risk factors noted above for a building under construction may also be applicable to a renovation
depending on the scope of the renovation.
In addition, a renovation may occur in a portion of the building while the remainder of the building
remains occupied. In that situation, fire safety planning for the renovation should take into account the
safety of occupants in adjacent areas of the building.
5.14
Conclusion
Development of sound alternative solutions for a tall mass timber building is, as presented in this chapter,
both feasible and practical given the current knowledge of mass timber buildings and building elements.
At a most conservative level, complete encapsulation (e.g. 4 layers of fire-resistance rated gypsum board
directly attached) of all mass timber elements will provide an equal or better level of fire performance
than that provided by concrete or steel buildings. A lesser level of encapsulation, and exposure of certain
mass timber elements, can also be demonstrated as providing an equivalent level of safety as a traditional
steel or concrete building.
Effectively the approach recommended is akin to peeling an onion, wrap all mass timber elements in
sufficient gypsum board such that they are not affected by a fire, nor do they contribute to the fire. Then
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by analyzing each condition, peel back the layers where a prudent fire analysis can demonstrate that lesser
protection continues to provide the level of fire safety that we have grown to rely on in modern tall
buildings.
A recent study by Koo (2013) confirmed that a substantial number of historical tall wood buildings built
in Toronto and Vancouver in the beginning of the 20th century continue to provide excellent service.
These buildings are up to 9-storeys and 30 m in height. The floor areas of these buildings are also worthy
of mention as total floor space can be up to 29,000 m². Similar buildings have also been identified in
Montreal.
These building have not only served for over 100 years, but many have been renovated, and redeveloped,
including vertical additions, and are the foundation of some of the most popular entertainment, office and
residential districts of Vancouver’s Gastown and Yaletown, Montreals’ Vieux Port and Toronto’s
downtown.
The experience with the redevelopment of these buildings confirms the conclusion of this study that a tall
mass timber building can be designed to meet and exceed the level of fire safety we currently enjoy in
modern tall buildings.
5.15
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Appendix 5A
Fire Risk Assessment
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5A.1 What is risk?
In the most basic terms, risk is the product of the probability (or likelihood) and the consequence of an
undesirable event occurring. For fire engineering, the risks in consideration are generally related to fire,
and hence are referred to as fire risks.
In conducting a fire risk assessment, the designer is assessing both the likelihood and the consequence of
fire events occurring using available fire risk assessment methods. A fire risk analysis is the detailed
examination carried out to understand the negative threat of fire to human life and property, which
includes fire risk assessment and management of alternatives.
5A.2 Fire Risk Assessment Methods
There are a number of methods that can be used to assess fire risks. These methods can generally be
categorized into qualitative, semi-quantitative and quantitative methods. A class of risk assessment
methods called risk indexing has also emerged as a useful tool in assessing non-specific fire risks.
5A.2.1 Qualitative methods
Qualitative methods do not assess risk in any quantitative way; they are generally limited to identifying
what the risks are. Qualitative methods include what-if analysis, checklists and logic-tree analysis. An
example of qualitative risk assessment is an engineering judgment on “what if the fire plume spills out the
window?”
5A.2.2 Semi-quantitative methods
Semi-quantitative likelihood methods assess either the likelihood or consequence component of the risk
quantitatively. They can be further classified into semi-quantitative likelihood methods and semiquantitative consequence methods. Semi-quantitative likelihood methods such as an event tree analysis
quantitatively assess the likelihood of a fire event and subsequent events occurring, while assessing the
consequence qualitatively or not at all. An example of this may be an assessment that “based on statistics,
there is a 5% chance sprinklers do not operate as expected in a fire”, but without assessing the
consequence of this particular fire scenario any further. While this type of assessment is useful in ruling
out low probability events, it could potentially miss certain low probability but high consequence events,
such as the failure of the sprinkler system during a major disaster when other resources such as fire
department and exit paths may also be disrupted.
Semi-quantitative consequence methods, on the other hand, focus on the consequence of a fire event and
give little or no consideration for the likelihood of the event occurring. In fire engineering, they are
typically deterministic fire models such as zone and computational fluid dynamics (CFD) models and
evacuation models. For example, a fire modelling assessment may be used to show that a steel column in
a shopping mall could fail when the surface temperature reaches 600°C; however, this assessment
provides no indication of the likelihood of the event occurring. While an engineering assessment of this
nature generally produces conservative answers, it could result in overly onerous provisions when the
event in question is very rare.
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5A.2.3 Quantitative methods
Quantitative methods estimate the likelihood of an event quantitatively. They can be a combination of
semi-quantitative likelihood and semi-quantitative consequence methods as illustrated in Figure 5.A.1. In
this hypothetical example, three potential events are identified as A, B and C with likelihoods of X, Y and
Z, respectively. A fire modelling assessment was carried out to determined that the sprinkler activation
times were 2 min, 1 min and 8 min for Events A, B and C, respectively. As a result following Event A fire
damage would be limited to nearby objects, following Event B fire damage would be limited to the object
of fire origin, and following Event C there would be extensive fire damage within the room. The same
assessment would be carried out for both the Division B solution and the alternative solution such that a
decision can then be made by the engineer whether the events, their likelihoods meet the level of
performance required by the NBCC. This can be extended to assess the impact on property protection and
life safety.
Fire Initiating Event
Event A
Probability X
Event B
Probability Y
Event C
Probability Z
Sprinkler activation time
= 2 min (determined using
CFD modelling)
= 1 min (determined
using CFD modelling)
= 8 min (determined using
CFD modelling)
Fire damage limited to
nearby objects
Fire damage limited to
object of fire origin
Extensive fire damage
Figure 5A.1 Example of an event tree assessment
There are quantitative risk assessment software packages that have been created, including CESARE-Risk
from Australia, FiRECAM, FIERAsystem and CURisk from Canada, CRISP from the UK and QRA from
Sweden. It is important that in using software packages, the designer is fully familiar with the fire risk
assessment principles and assumptions made during development of the software so that an informed
engineering judgment can be made regarding the results.
5A.2.4 Indexing method and the Delphi panel
A class of fire risk assessment methods known as fire risk indexing has been used in recent years to study
risks relating to more fundamental fire safety issues of a building such as compartment area, compartment
height and construction provisions [Hultquist, H., 2000]. The indexing approach examines risk in a more
implicit manner in that it is not based on any specific fire event; rather, it determines how well a building
might perform in any fire event as a whole based on its design features (or parameters) such as the type of
construction, number of exits, firefighting response time and occupancy classification. Indexing involves
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a score system that assigns points to the relevant parameters, which are weighted based on the importance
of each parameter:
n
Index = ∑ s iw i
[21]
i =1
where si is the score for parameter i and wi is the weighting for a parameter i. When comparing the
performance of two building designs, for example a Division B compliant design and an alternative
solution design, the design with the greater overall index score would be regarded as having a superior
level of performance. It is important to note that the parameters and the available scores are generally
predetermined through a decision-making body such as a committee of experts. However, depending on
the make-up of the committee, culture and their background, the index system developed may be
subjective.
Further detailed discussion on fire risk assessment methods can be found in generally-accepted fire
engineering literature in Canada and the US (SFPE, 2000; SFPE, 2008; NFPA, 2013).
5A.3 Comparative risk assessment for alternative solutions
In performing a fire risk assessment for tall wood buildings using the alternative solution provision of the
building code, it is important to recognize that the fundamental requirement of an alternative solution is
that it must perform “as well as” the acceptable solutions found in Division B for which a variance is
being sought. An assessment that simply shows an alternative solution is “good enough” based on
anecdotal evidence will not meet the alternative solution test. To demonstrate that an alternative solution
complies with the building code based on the provisions of the objective-based NBCC, a comparative risk
assessment must be carried out to assess the level of performance for each of the Division B and the
corresponding alternative solution with respect to the risk areas identified by the objectives and functional
statements.
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Building Enclosure Design
CHAPTER 6 Building Enclosure Design
Authors: Dave Ricketts and Graham Finch
Michael Lacasse, John Straube, Jieying Wang
Peer Reviewers: Robert Jonkman, Angela Lai, Mark Lawton, Paul Morris, Leslie Peer, Constance Thivierge
Abstract
The building enclosure system physically separates the exterior environment from the interior
environment(s). The focus on Chapter 6 of the guide is on the control of heat, air, and moisture transfer
through the building enclosure. Noise and fire control is also discussed. Together with the heating,
cooling and ventilation systems, the building enclosure maintains comfortable and healthy indoor spaces.
It is also a key passive design element for an energy efficient building and is one of the most important
systems in ensuring the durability of all other systems within a tall wood building.
Elements of the building enclosure include: roofs, above and below-grade walls, windows, doors,
skylights, exposed floors, the basement/slab on grade floor and all of the interfaces and details between.
As the focus of the Guide is specific to tall wood buildings, the unique considerations for the wood-based
above-grade wall and roof assemblies which are different than other high-rise structures are addressed in
this Chapter. The environmental loads (primarily wind, rain, temperature differentials) and structural
loads (primarily wind and seismic and related lateral movements) act on the building enclosure of tall
wood buildings. These loads will be the same as those acting on other tall buildings constructed of steel
or concrete. The structural loads will generally be greater for tall wood buildings than those experienced
by low-rise wood-frame buildings. Design considerations such as cumulative wood shrinkage of the
structure are potentially emphasized in taller wood-frame building and need to be accommodated by the
building enclosure.
The structural system utilized has a significant impact on the location of insulation, as well the details.
This Chapter therefore addresses five wood-based structural systems and associated wall assemblies:
platform (and balloon) framing, post-and-beam with wood-frame infill, mass timber (CLT), curtain-wall
with mass timber, and wood-frame infill within a poured-in-place concrete frame.
The high rain exposure conditions associated with tall wood buildings dictate that a rainscreen water
penetration control strategy be utilized for all wall assemblies. In addition, the energy efficiency
requirements combined with the structure types considered lead to a requirement for exterior insulated
wall assemblies. Various air barrier strategies can be considered, however an exterior liquid applied or
vapour permeable membrane air barrier is the easiest to implement with exterior insulated rainscreen wall
assemblies. The use of vapour permeable membranes and vapour permeable interior finishes will allow
for initial drying of the wood wall assemblies.
Roof assemblies considered include both conventional and protected membrane systems on mass timber
roof structures. In either case a durable membrane system such as a 2-ply modified bituminous membrane
is recommended because the wood substrate and structure is more susceptible to moisture damage than
other substrates.
The key to achieving durability is to prevent excessive moisture accumulation and to allow wood to dry
should it get wet during construction and in-service.
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6.1
Introduction
0 of this guide addresses the building enclosure (building envelope) and durability design considerations
for tall wood building in Canada.
Wood structures and specifically wood-frame building enclosures have a long history of successful
performance throughout Canada. The environmental loads (primarily wind and rain) and structural loads
(primarily wind, seismic and snow) acting on the building enclosure of a tall building constructed of
wood will be the same as those acting on a buildings constructed of steel or concrete, but will be greater
than those acting on a low-rise wood-frame building. Design parameters such as vertical differential
movement resulting from wood dimensional changes and loads are potentially emphasized in wood-frame
building enclosures. The focus of this Section is therefore to generally define the loads and effects of
loads that act on taller buildings and consider them in the context of tall wood buildings that utilize
traditional wood frame systems and newer wood systems such as cross-laminated timber (CLT), post-andbeam in filled with wood-frame or curtain walls, and concrete frame with infill wood-frame walls.
A number of publications on guidelines for building enclosure design, building science textbooks, and
other building science references, are available within Canada that provide general guidance regarding the
design and construction of durable and energy efficient wood-based building enclosures. They include:
Guide for Designing Energy-Efficient Building Enclosures (2013), Building Enclosure Design Guide
(2011), CLT Handbook (2011), High Performance Enclosures (2012), Building Science for Building
Enclosures (2005), and Builder’s Guide for North American Climates (various versions). The building
enclosure design fundamentals described within these publications may be applied to tall wood buildings,
but as most recommendations for wood building construction design are only applicable to buildings of
up to 6 stories (the current maximum building height depending on the jurisdiction) caution is urged when
considering the design of buildings exceeding such heights. Many assemblies, details, or materials
appropriate for use in low-rise wood-frame buildings but may not be suitable for taller buildings due to
the increased environmental loads, and nor will they accommodate the underlying structural systems in a
tall wood building.
This Section of the guide summarizes key building enclosure design considerations and particularly the
aspects of design where differences exist for tall wood buildings within the various climate zones of
Canada. The chapter is organized to take the reader from an understanding of the key building enclosure
loads, building and energy-related code requirements, followed by a summary of the fundamentals of
building enclosure design, building enclosure assemblies and detailing strategies, and concluded with a
discussion of wood protection and durability including on-site moisture management and use of wood for
exterior applications.
6.1.1
Building Enclosure Systems
The building enclosure (building envelope) is a system of materials, components, and assemblies that
physically separate the exterior environment from the interior environment(s). The building enclosure
primarily manages heat, air, and moisture transfer, and with the heating and ventilation systems, helps
maintain a comfortable and healthy indoor environment. It also forms a key passive design element of an
energy efficient building and is one of the most important systems in assuring the durability of all other
systems within a tall wood building.
The elements of the building enclosure include: roofs, above and below-grade walls, windows, doors,
skylights, exposed floors, the basement/slab on grade floor and all of the interfaces and details between.
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As the focus of this guide is specific to tall wood buildings, the unique considerations for the wood-based
above-grade wall and roof assemblies which are different than other high-rise structures are addressed
here. Detailing considerations for fenestration such as punched windows or curtain-wall assemblies, and
other components or penetrations through these wood assemblies are also discussed where appropriate.
Below grade concrete and other non-wood assemblies are not within the scope of this Section.
The structural system of a tall wood building has a significant impact on the selection and design of the
building enclosure assemblies. Four structural systems that could be utilized in a tall wood structure
(Figure 131) are considered:
1.
2.
3.
4.
Traditional platform framing (and balloon framing)
Post-and-beam framing
Wood-frame infill in concrete and steel structures
Mass Timber – such as cross-laminated timber (CLT)
The structural system will dictate the location and properties of thermal insulation that will be used within
the exterior wall or roof assemblies. The insulation then impacts the appropriate location and materials
used for the control of air and vapour transfer, and may also affect the fire safety and acoustic
performance. The structural system impacts the thickness of exterior walls, which may have implications
on usable floor space and building set-back restrictions. The structural movement between load bearing
and non-load bearing assemblies must be detailed so that non-load bearing components are not
accidentally loaded and that air and water seals between these interfaces are maintained over the life of
the building.
Platform Framing
Post-and-beam
Mass timber (CLT)
Wood-frame infill
Figure 131 Types of exterior wall enclosure systems utilizing wood components
Most modern low-rise wood-frame structures (up to 6 stories in height) are constructed utilizing stickbuilt platform framing (or balloon framing) where the wood roof trusses, floor joists and load bearing stud
walls, carry all structural loads of the building (i.e. gravity, wind, and seismic). The building enclosure is
constructed within this wood-frame structure, while protecting it from the exterior environment. The
design and construction of such wood-frame building enclosure assemblies are well understood across
Canada. The structural loads within a 10 to 20 storey tall wood building will dictate that larger and
heavier timber structures be used instead of stick-built framing for most parts of the building. This means
that traditional wood-frame enclosure assemblies may need to be modified to one of the alternate systems
discussed below.
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Post-and-beam framing (the oldest type of wood
structure) has been used in many wood buildings
where larger spans or the exposure heavy timber
framing is desired. The infill spaces between the large
wood beams and columns are typically filled with nonload bearing light wood-frame walls. The building
enclosure is constructed to both sides and within this
structure, while protecting the interior and the
structure from the exterior environment. The structural
interaction of the non-load bearing components with
the load-bearing components is taken into
consideration in the enclosure assembly design. Steel
framing and lateral bracing may be incorporated
within taller wood structures, particularly where noncombustible wall assemblies are deemed necessary
though they may be less thermally efficient though
compared with typical wood systems.
6 storey heavy timber office building (Bullitt Center in
Seattle, WA) utilizing timber columns and steel framing
for lateral loads. Building enclosure components consist
of non-load bearing infill walls and curtain wall
components placed outboard of the structure.
Wood-frame infill is another application of wood frame components used in a non-load bearing
application. An example of this enclosure framing system is wood frame walls within a steel or concrete
building where traditionally steel stud, masonry or concrete might have been used. This system may be
used within exterior walls to improve thermal performance, and for economic and other benefits.
Traditionally this type of structure has not been used widely in Canada due to building code fire safety
restrictions. The fire and building code requirements may be achieved by alternative solutions (see
Chapter 5). Infill walls do not carry the gravity load of the structure but will accommodate and transfer
wind loads to the primary structure. The movement and deflection of the primary structure of the building
relative to the infill wall needs to be considered in enclosure detailing. Similar to platform framing, the
building enclosure is constructed within or to the exterior of this structure, protecting the structure from
the exterior environment.
Mass-timber construction utilizes solid CLT or
other engineered timber products such as
parallel strand lumber (PSL), laminated strand
lumber (LSL) and laminated veneer lumber
(LVL). Mass timber construction consists of
solid wood shear walls, columns, roof, and
floors, part of which also forms part of the
exterior building enclosure. To save costs,
wood-frame infill walls, load bearing or nonload bearing, may also be utilized within masstimber
structures
where
mass-timber
components are not needed to save costs.
Mass-timber construction utilizing CLT panels (Ronald McDonald
Insulation and other components of building
House in Vancouver, BC). Insulation and critical control barriers
enclosure will typically be constructed to the
are placed outboard of the CLT panels which is ideal for the long
term durability of the wood structure.
exterior of the solid timber components.
Guidance on enclosure assemblies utilizing CLT walls and roofs are covered within the CLT Handbook
(2011) and the Guide for Designing Energy-Efficient Building Enclosures (2013).
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6.2
Building Enclosure Loads
6.2.1
Climate Considerations and Environmental Loads
Over the life span of a building the building enclosure will be subjected a wide range of exterior
environmental loads including variations in solar radiation, rain, snow, ice, hail, vapour condensation,
wind, temperature, and relative humidity (RH). Interior environmental factors include temperature, RH,
and vapour condensation as well as water arising from defects in appliances, sprinklers and interior
plumbing. The presence of rain on the exterior surface of the building, as well as differences in
temperature, air water vapour content, and air pressures between the exterior and interior environments
create the most critical loads acting on the enclosure. Fire, smoke, and noise separation must also be
considered.
The location or climate zone in which the building is constructed will dictate the magnitude and duration
of these environmental loads on the building enclosure. Climatic data within the National Building Code
of Canada (NBC) or provincial codes provides basic climatic design data (e.g. average temperatures,
heating and cooling degree days, design wind pressures, rainfall, and snow) for cities across Canada.
Every climate zone carries unique design, construction, and maintenance considerations. Colder climate
zones will have more stringent requirements for insulation as compared to warmer climate zones and the
need for condensation control (greater control of vapour and airflow) is likewise more significant. Rainy
marine climates also pose a challenge to wood buildings in terms of keeping the wood dry during
construction (on-site protection, Part 6.1) and in service (appropriate rainwater management, Parts 4 and
5). Construction within the Artic has unique challenges given that the buildings are exposed to extreme
cold and conditions of permafrost and the construction seasons is very short.
Climate zone maps, such as those shown in Figure 132 provide information on the delimitation of
standard climate zone classifications (left) and rainfall classifications (right) and are useful for
highlighting key design factors across Canada.
Figure 132 Climate Maps of Canada showing general climate zone classification (left) and annual rainfall levels
(right). Rainfall classifications - Extreme over 1500 mm/y, High between 1000 – 1500 mm/y, Moderate between
500-1000 mm/y and Low less than 500 mm/y. Maps from the “Guide for Designing Energy-Efficient Building
Enclosures” (2013) adapted from several industry references.
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The environmental loads acting on the building enclosure of a tall wood building will be the same as
those on other high-rise buildings. The fundamental consideration for the building enclosure of a tall
wood building, compared with a lower wood building, is to ensure that these larger loads, in particular
wind, rain, and air pressure differences, are addressed effectively in the design process.
6.2.2
Building Movement and Structural Considerations
The building enclosure must safely accommodate the structural loads acting on it (e.g. dead load, live
load, wind, and seismic) and transfer these loads to the primary structure of the building. This is
particularly relevant for the attachment of claddings, curtain wall systems, windows, and exterior doors,
as well as air-barrier materials subjected to the wind and seismic loads in taller buildings. Structural
attachments that penetrate thermal insulation are a source of thermal bridging and additional heat loss,
and optimization of structural connections for structural and thermal performance is necessary.
The movement of the primary structural system, resulting from
settlement, dimensional changes (e.g. wood shrinkage and swelling),
compression, deflection, creep, and lateral drift relative to the building
enclosure, is another factor that must be considered in the design of
building enclosure assemblies and details. Settlement mostly occurs
during construction, and load-induced movement such as compression
and deflection should be within a safe limit for a structurally sound
design. Movement caused by wood dimensional changes, however, is
usually the most variable and deserves careful consideration in design.
In tall wood buildings extra
attention should be made in the
enclosure
design
for
potentially high inter-storey
lateral drift and vertical
differential movement over the
height of the building resulting
from various loads and wood
moisture content changes over
time. The tolerances will
primarily depend on the
structural design and the
materials used, and will be
provided by the structural
engineer for incorporation into
the building enclosure design.
Tolerances of other materials
integrated, such as concrete
slabs, should also be taken into
consideration.
Wood dimensional changes, primarily resulting from changes in
moisture content (MC) from the construction stage into service become
more significant in taller wood buildings, traditional platform frame
buildings in particular, due to the cumulative effect over the building
height. The seasonal MC changes in service are much smaller. Wood
shrinks when losing moisture and swells when gaining moisture when
the MC is below the fiber saturation point (28-30%). The shrinkage or
swelling amount in construction depends on the shrinkage coefficient of
the material used and the percentage of MC change below the fiber
saturation point. The shrinkage or swelling of solid wood (such as dimensional lumber and solid wood
timbers) is much greater in the transverse grain orientation (i.e. the cross sections of horizontal wood
members). Most wood design books recommend using an average shrinkage coefficient of 0.20 or 0.25%
per 1% change in MC for cross sections of softwood lumber. On the other hand, wood is highly
dimensional stable in the longitudinal direction and its shrinkage or swelling can be largely neglected
compared with that in the transverse directions. Minimizing cross sections of wood in the gravity load
path, such as by using a balloon type structure or connecting columns without horizontal wood members
between them, can minimize the vertical movement.
Wood exchanges moisture with surrounding air and the amount of moisture gain or loss depends on the
RH and temperature of the air, and the existing MC in the wood. Wood achieves equilibrium moisture
content (EMC) for certain environmental conditions; for example, its MC fluctuates slightly around 12%
at a RH around 65% at a temperature of 20°C. Engineered wood products, such as CLT and Glulam,
usually have much reduced shrinkage amount compared with “S-Dry” dimensional lumber (typically with
MC below 19%). One important reason is their MC after manufacturing (typically below 15% MC) is
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much closer to the EMC in service, therefore reducing the MC changes over time. Most engineered wood
products also have reduced shrinkage coefficient due to the use of adhesive and the mixed grain
orientation of wood. On the opposite, wood may swell when exposed to outdoor RH levels or if
significant wetting occurs during construction or in service, therefore the designs should accommodate
this as well.
For buildings it is the differential movement occurring between connected parts and components that
matters. Differential movement can occur between wood and other materials, such as steel, concrete and
masonry. For those materials, thermal expansion or contract may govern the dimensional changes instead
of MC changes. Differential movement can also occur between different wood components due to
different products, grain orientations, or exposure to different environment (exterior and interior). For
example, differential shrinkage may be noticeable across floors that are supported at one end on
conventional platform framed walls and at the other end on wood columns or CLT panels. Field
monitoring of vertical movement in mid-rise wood frame and post-and-beam buildings has been
conducted by FPInnovations and guidance on estimating differential movement can be found in relevant
wood design guides.
6.3
Building and Energy Code Considerations
In Canada two national model codes specify building enclosure and energy-efficiency design provisions
for buildings: the National Building Code of Canada (NBC, 2010), and the National Energy Code for
Buildings (NECB, 2011) (which was previously called the Model National Energy Code for Buildings
(MNECB)). These national codes have already been adopted or may soon be adopted either with or
without modifications by each of the provinces and territories. In addition to the provincial codes, some
local jurisdictions including the City of Vancouver have a modified version of the provincial building
code written into their municipal building bylaws. Buildings on all federal land (e.g., national parks,
Canadian forces bases, and First Nations reserves) are required to meet the current requirements of the
National Building Code and National Energy Code for Buildings regardless of the province. In addition to
these codes, some provinces and municipalities will reference one of the ASHRAE Standard 90.1
versions for building energy efficiency requirements.
6.3.1
Canadian Building Code Considerations
The design of the building enclosure of a tall wood building would fall under Part 5 “Environmental
Separation” of the NBC or the relevant provincial building codes. Part 5 is primarily concerned with the
control of precipitation and condensation on and within building enclosure components and assemblies,
and with the transfer of heat, air, moisture, and sound through building materials, components and
assemblies as well as the interfaces between them.
The application of Part 5 to a tall wood building would be similar to the application to other high-rise
buildings. Fundamentals of building enclosure design which meet the objectives of Part 5 of the building
code are addressed later in this Section.
6.3.2
Canadian Energy Code Considerations
There are two alternate reference energy standards in Canada which the design of a tall wood building
could follow: the National Energy Code for Buildings (NECB 2011, previously MNECB 1997) or
ASHRAE Standard 90.1 (2001, 2004, 2007 or 2010), depending on the location of the building. The
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adoption of either or both of these energy standards (and year of the
standard) as the minimum energy efficiency requirements varies by
province. For example, The Province of BC, formerly referencing
ASHRAE 90.1-2004, recently updated the code to adopt both ASHRAE
90.1-2010 and the NECB 2011 as alternate means of compliance to the
energy efficiency requirements of the BC code, effective December
2013. In addition to the provincial requirements, buildings undergoing
LEED certification are required to have completed an energy analysis
with reference to either the standards within MNECB 1997 or ASHRAE
90.1-2007 (which will be different than most provincial requirements).
Within all of these energy standards and codes, the building enclosure
along with the mechanical and ventilation systems, lighting, hot-water,
pumps, motors, equipment and other energy consuming devices all must
meet minimum criteria and work together to meet an overall minimum
energy efficiency target outlined by one of the various compliance
paths.
The adoption of Building and
referenced minimum Energy
Code Standards varies by
Province and jurisdiction
within Canada. In addition,
buildings undergoing LEED
certification will be subject to
additional requirements or
baseline energy modeling
criteria that may be different
that local energy code
minimums. As an example,
LEED references ASHRAE
90.1-2007 whereas 90.1-2010
may be the minimum energy
code requirement in the
jurisdiction. This means that
multiple energy models are
often required on building
projects.
Compliance with the building enclosure provisions of ASHRAE
Standard 90.1 or the NECB requires meeting some prescriptive and
mandatory requirements as well as one of the three alternate building enclosure compliance paths.
Without going into the complete details, the three alternate compliance paths include, in order of lowest to
highest complexity and level of work required to demonstrate building project compliance: Prescriptive
Building Enclosure Option, Building Enclosure Trade-off Option, and Whole Building
Modeling/Simulation Path (Energy Cost Budget Method). There are some slight differences between
ASHRAE 90.1 and the NECB, notably that ASHRAE 90.1 is energy cost based and NECB is energy
based within the whole building simulation path.
Pertinent to the selection and design of building enclosure assemblies and components, in both the
ASHRAE and NECB energy standards and all of the compliance paths, the effective R-value of each
building enclosure assembly needs to be determined (or some minimum nominal prescriptive insulation
level installed).
Nominal insulation R-values are the rated R-value of the insulation product being installed and do not
account for energy losses due to thermal bridging. Effective R-values are the true representation of
thermal resistance of an insulated assembly and account for thermal bridging. Thermal bridging mostly
occurs through structural elements, framing, gaps, fasteners, and any other penetrations through the
installed insulation. Within a tall wood building, this would primarily be through wood framing (studs,
columns, floor slabs, solid shear walls etc.) as well as through cladding attachments through exterior
insulation. While the degree of thermal bridging within a tall wood building might be less than a steel or
concrete frame building due to the much higher thermal resistance of wood than steel or concrete, it still
needs to be considered.
Both ASHRAE 90.1 (all versions) and the NECB 2011 require the use and analysis of effective R-values.
While nominal insulation R-values may be referenced within some prescriptive tables (ASHRAE 90.1),
there is some understanding that the most common assemblies will use batt insulation of a certain R-value
between wood or steel studs, and hence the need for integrating continuous insulation for some
assemblies to ensure a minimum effective insulation level (effective R-value).
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Continuous Insulation (ci) is a definition used in ASHRAE 90.1 and continuity of insulation is required in
the NECB, with both of the intended purposes of providing a minimum continuous layer of insulation that
has an effective R-value equal to or very close to its nominal R-value (i.e. minimal thermal bridging).
Continuous insulation is often specified in energy codes alone or in conjunction with thermally bridged
nominal insulation (i.e. between wood studs) to achieve higher effective R-values. This continuous
insulation requirement is commonly addressed with exterior rigid or semi-rigid insulation installed on the
exterior of a framed assembly, and for a tall wood building utilizing framing and mass timber
construction in particular. Continuous insulation could also be installed to the interior or within the
middle (sandwich construction) of some assemblies, although it would not meet the requirement for
continuity at floor levels in multi-storey buildings.
Figure 133 summarizes the minimum prescriptive thermal insulation requirements within the 2011 NECB
and ASHRAE 90.1-2010 for above-grade wood frame walls and roofs (flat roof) in climate zones across
Canada. Requirements for other building enclosure assemblies including windows, doors, skylights,
below grade assemblies and floors can be found within both standards. These minimum R-value targets
included within this table are good baseline targets for building enclosure assemblies within tall wood
buildings. However, as observed by the values within the table, most of these prescriptive R-value targets
are higher than the current standard practice for wood frame construction (i.e. batts within 2x6 studs) in
many jurisdictions of Canada and will require more highly insulated assemblies. Several options for
wood-based wall assemblies which can meet these higher targets are covered later in this Section.
Climate Zone –
By Zone and
HDD(°C)
Zone 4 <3000 HDD
Zone 5 3000 – 3999 HDD
Zone 6 4000 – 4999 HDD
Zone 7a 5000 – 5999 HDD
Zone 7b 6000 – 6999 HDD
Zone 8 >7000 HDD
NECB 2011 - Above
Grade Walls &
Roofs
(All Construction
Types)
Minimum Effective
Assembly R-values
[RSI]
Wall - 18.0 [3.17]
Roof - 25.0 [4.41]
Wall - 20.4 [3.59]
Roof - 31.0 [5.46]
Wall - 23.0 [4.05]
Roof - 31.0 [5.46]
Wall - 27.0 [4.76]
Roof - 35.0 [6.17]
Wall - 27.0 [4.76]
Roof - 35.0 [6.17]
Wall - 31.0 [5.46]
Roof - 40.0 [7.04]
ASHRAE 90.1-2010 –
Above Grade Walls &
Flat Roofs
- Residential Building
(Wood-Frame)
Minimum Effective
Assembly R-values
[RSI]
Wall - 15.6 [2.75]
Roof - 20.8 [3.66]
Wall - 19.6 [2.75]
Roof - 20.8 [3.66]
Wall - 19.6 [3.45]
Roof - 20.8 [3.66]
Wall - 19.6 [3.45]
Roof - 20.8 [3.66]
Wall - 19.6 [3.45]
Roof - 20.8 [3.66]
Wall - 27.8 [3.45]
Roof - 20.8 [3.66]
Figure 133 Minimum Effective R-Value Requirements for Above Grade Wall Assemblies within 2011 NECB
and ASHRAE 90.1-2010 (left) and NECB and ASHRAE 90.1 Climate Zones (right) (Note that ASHRAE 90.1
includes Climate Zone 4, Lower Mainland and Victoria, BC with Climate Zone 5 in Canada)
Where these minimum R-values cannot be prescriptively met, one of the two alternate trade-off paths
(Building Enclosure or Whole Building) must be followed. In these paths, addressing the largest source of
heat loss, typically windows, and choosing products with higher than code minimum R-values can be
used to offset the heat loss at walls or roofs which do not meet these criteria.
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6.4
Building Enclosure Design Fundamentals
6.4.1
Moisture Control
Appropriate moisture control is critically important in order to achieve long-term durable performance of
tall wood buildings. The major moisture sources for building enclosures include rain, snow and ice,
ground moisture, construction moisture, and vapour condensation. The frequency and intensity of winddriven rain is usually a determining factor in the amount of water likely available from outside. Snow and
ice are also potential sources of water and contributors to the risk of water penetration. Ground moisture
is the determining factor for the design of below-grade and adjacent assemblies, typically constructed of
concrete. Protection of the wood components near grade or in contact with grade is also important for a
tall wood building.
Occupants also generate a significant amount of indoor moisture, which should be taken into
consideration. In large wood buildings there could also be other important moisture sources including
common recreational areas with pools, hot tubs, kitchen areas and rooms requiring higher controlled
humidity which require special attention to moisture control in the enclosure. The relationship between
interior environment and building enclosure is influenced by many factors including the detailed buildingenclosure assembly; airtightness; design and operation of the heating, ventilation, and air conditioning
(HVAC) system; interior space layout; and type of usage and occupancy of the space. Because of the
nature of occupancy and the typical ventilation systems used, residential buildings tend to have higher
indoor moisture levels than commercial or institutional buildings, and as such some building enclosure
assemblies are more sensitive to wetting within residential buildings.
6.4.1.1
Wetting, Drying, and Safe Storage
The design and construction of a wood frame building enclosure for the purpose of moisture control is a
process of balancing moisture-entry mechanisms (wetting) and moisture-removal mechanisms (drying).
Wetting mechanisms include exterior moisture (rain, groundwater, snow, air vapour) and interior
moisture during building operation, as well as construction moisture. Drying mechanisms include
drainage, evaporation (through venting or ventilation in particular), and vapour diffusion. In addition to
wetting and drying, wood materials have an inherent capacity to safely store moisture or act as a moisture
buffer. As long as safe moisture levels are not exceeded, this moisture storage capacity will allow for
seasonal or short-term storage of moisture that accumulates within assemblies (either from seasonal
humidity or infrequent wetting) until drying occurs. Larger wood components, such as CLT panels or
glulam beams, usually have a higher moisture storage capability than wood studs or plywood sheathing,
but may dry more slowly once wetted.
An imbalance in wetting, drying and safe storage may result in the moisture accumulation and
deterioration of less moisture tolerant materials. Heat flow through building-enclosure assemblies plays
an important role in helping to maintain this balance. Thermally efficient building-enclosure assemblies
are particularly sensitive to moisture accumulation, and arguably have a smaller zone of moisture balance
than less insulated assemblies, due to the reduced heat flow through the assembly and the lower
temperatures of the exterior components. To improve this balance in favour of drying, this means utilizing
strategies like placing insulation on the exterior of the wood structure to keep it warmer and therefore
drier.
Wood and wood-based materials always contain some moisture; the amount varies over time with
exposure to humidity and water. Fortunately, the equilibrium moisture content of wood exposed to
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humidity alone is generally below levels conducive to the growth of decay fungi. As a general rule, liquid
water needs to be present to lead to decay conditions. Sustained high humidity conditions coupled with
warm temperatures is the primary exception to this rule; in these conditions, the presence of liquid water
on the surface of wood are sufficient to initiate fungal growth. The design of wood frame assemblies to
keep humidity levels low and away from liquid water is essential. See Part 6 of this Section about the
potential consequences of excessive wetting and additional wood protection and durability solutions.
6.4.1.2
Control Layers & Critical Barriers – Assembly Design and Detailing
A primary function of the building enclosure is to control environmental loads. Various materials are used
within enclosure assemblies and details to perform different control functions depending on their material
properties and placement. The notion of control functions and more specifically control layers can be
used to explain the function of different materials and components within building enclosures and to
select appropriate materials. The following control functions are typically considered: water
(precipitation, ground), water vapour, air, heat, sound, fire, light, and contaminants. The interior and
exterior finish can also be considered as part of the control layer function.
Applying the concept of control functions
and control layers, the term critical barrier
is used by many practitioners within the
building enclosure industry and used within
several reference documents to refer to
materials and components that together
perform a moisture-control function (as well
as air and heat control) within the building
enclosure. A critical barrier can be defined as
a control layer that must be essentially
continuous for the enclosure to perform as
designed. It has become common and
referenced within building codes to define
specific critical barriers within an enclosure
assembly,
such
as
a
vapour
retarder/barrier or air barrier. This
Section also refers to a water-shedding
surface (WSS) (first plane of protection, in
the terminology of the NBC and provincial Figure 134 List of Primary Building Enclosure Control
building codes) and a water-resistive Layers & Associated Critical Barrier Function
barrier (WRB) (second plane of protection) to facilitate discussion of water penetration control
strategies. In addition to the vapour retarder/barrier, air barrier and primary moisture-control layers, the
thermal barrier, and building form and features can be evaluated as discussed below. Noise and fire
control layers could also be evaluated in a similar manner.
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To make the link between the concept of control functions and
the associated critical barrier, Figure 134 shows the related
primary and secondary relationships. The use of building form
and features such as overhangs and canopies are discussed later
within this Section.
The use of critical barriers to evaluate assemblies and details is
consistent with industry best practices and are easier to assess
in the context of specific assemblies and details being
considered for a project. This is very important in the design of
tall wood buildings, as many assemblies and details will be new
to design teams and contractors. The application of this concept
will help all parties better understand the role and criticality of
certain materials and details. The following further discusses
each critical barrier and control function.
•
•
Water-Shedding Surface (WSS)
o In general, the water shedding surface (WSS) is
the outer surface of assemblies, interfaces, and
details that will deflect and/or drain the vast
majority of the exterior water from the
assembly. In simple terms, it is the exterior
surface of the building enclosure and part of
the water control function. The WSS reduces
the rain load on the underlying elements of the
assembly. For wall assemblies the WSS is the
cladding surface, conventional roofs is the
roofing membrane and protected membrane
roofs is the top surface of the insulation. Open
joint rainscreen claddings and porous claddings
such as brick veneer will allow a significant
amount of water past the WSS. Walls with
these claddings must take into account
measures to drain this moisture out of the wall,
typically with a more robust WRB. The WSS
concept can be used to encourage designers to
consider the differences between different
claddings like fiber-cement, brick, and open
jointed panels etc. The exterior surface of the
building enclosure is also exposed to solar
radiation and UV, and serves a solar control
function.
Water-Resistive Barrier (WRB)
o The water-resistive barrier (WRB) also
sometimes called the moisture barrier is the
surface farthest from the exterior intended to
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Example of critical barrier analysis for
exterior insulated CLT wall detail from low
slope roof parapet to grade. Note the wall air
barrier and WRB are the same material (in
this case a self-adhered sheet applied
membrane) applied to the outside of the CLT
panel (as covered in Part 5.1.3)
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o
o
prevent liquid water from travelling further
to the interior. It has an integral water control function and is often relied upon for
assemblies to remain water tight. It would be considered a failure if water passed beyond
the WRB, but, depending on the design, may not result in an actual failure in the field.
In exterior-insulated wall assemblies, the WRB may be the surface of the insulation if it
is taped and sealed, or it may be a sheet, or fluid-applied sheathing membrane installed
behind the insulation on the wood sheathing/structure. To perform, it is critical that the
continuity of the WRB be maintained by membrane laps, along with quality tapes and
sealants, and occasionally projections such as flashings that act to shed water and provide
drainage. For many wall assemblies the WRB is the sheathing membrane in combination
with flashing and sealants at penetrations. Where the WRB is also part of the air-barrier
system, it will be made airtight by tapes, sealants, gaskets, and other airtight components.
The WRB may be vapour permeable or impermeable depending on the location and other
functions it may be performing within the wall. In most wood wall assemblies, the WRB
will be vapour permeable in most cases to facilitate drying towards exterior, but may be
vapour impermeable if the assembly can safely dry towards the interior under any
circumstances. In all cases, the materials selected for this function must be durable and
remain in service for the life of the assembly.
•
Air-Barrier System
o The air-barrier is a system of materials that controls
flow of air through the building enclosure, either
inward or outward. Air flow is significant with
respect to heat flow (space-conditioning), interstitial
vapour condensation (water vapour transported by
bulk air flow), and rain-penetration control. Air
flow control is particularly for thermally efficient
wood based building enclosure assemblies. Detailed
air-barrier strategies which are appropriate for taller
wood buildings are discussed later in this section.
•
Thermal Barrier (Insulation)
o While not typically considered a critical barrier, and
more of a control function – the thermal barrier, or
the line of insulation continuity is an important
factor within a thermally efficient building
enclosure. This barrier consists of insulation and
other low-conductivity elements and helps identify
thermal bridges or insulation discontinuities to be
addressed. Within a wood building, the primary
resistance to heat flow will be provided by thermal
insulation; however, wood components such as
CLT panels will provide some insulating value, and
by nature are often installed in a continuous
manner. In addition, wood may also be used within
details instead of metal to reduce thermal bridging
effects. As an example and shown in the figure
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Example of critical barrier analysis for
exterior insulated CLT wall to window
head and sill detail. The selection and
detailing of windows and curtainwall
assemblies would follow the same
critical barrier analysis as discussed
here.
295 of 374
above, wood blocking is used to support the metal cross cavity flashing outside of the
insulation and reduce the thermal bridging effect at this detail. The use of wood in these
situations also requires a review of the combustibility and fire separation requirements.
•
Vapour Diffusion Control Layer
o The vapour diffusion control layer consists of a material which retards or stops (barrier)
the flow of water vapour due to vapour pressure differences across enclosure assemblies.
Within Canada, a vapour retarder or barrier is typically placed inboard of the insulation
layer (warm or high vapour pressure side) to control vapour diffusion into and through
enclosure assemblies. Vapour diffusion is not typically considered a critical barrier as it
may not be needed within some climates or some assemblies and continuity of the vapour
retarder is not necessary to adequate control vapour diffusion in most cases.
o Vapour diffusion control is not to be confused with bulk air flow control (air barrier) and
continuity and sealing of air barrier details is very important. Therefore if the same
material is being used to control both vapour diffusion and air flow, then it is sealed as
such.
•
Building Form and Features
o Building form and features including as roof overhangs, balconies, canopies and other
protruding elements etc. also play a critical barrier function whereby these elements
deflect rain, provide shading from the sun, and buffering from wind. The protection
provided by these components may allow for alternate water control strategies to be
utilized at protected areas. For this reason, deflection elements can be considered as part
of the critical barrier analysis.
Foundations, walls, roofs, doors, windows, and other enclosure elements are combined in a building
project to form a complete and continuous enclosure that separates interior space from the exterior. The
critical barriers, i.e. air barriers, vapour retarders, water-resistive barriers, water-shedding surface, and
insulation must be present, not only in each assembly of the enclosure, but also at the interfaces and
through details between assemblies, as well as at penetrations through these assemblies. This is one of the
most common and challenging detailing tasks faced by designers and builders. Guidance on the selection
of critical barriers and detailing their connections for many wood-frame assemblies and details are
covered within the Guide for Designing Energy-Efficient Building Enclosures (2013) and the Building
Enclosure Design Guide (2011) and High Performance Building Enclosures (2012).
6.4.1.3
Control of Rainwater and Assembly and Detail Design
Rain is usually the largest moisture source for building enclosures, and the control of rainwater
penetration is essential across Canada, in the coastal climates in particular. While climate zone is
important as discussed in Part 2.1, building shape, surface-details, height and exposure are often just as
important: a five-storey building on an open site near Calgary can often be exposed to more rain than suburban house in Vancouver. Tall wood frame buildings can be expected to be exposed to much higher rain
deposition than the millions of low-rise wood frame buildings with which the industry has experience.
However, important steps can be taken in the design of a tall wood building to reduce the enclosure’s
exposure to wind-driven rain and to minimize potential rain moisture loads.
The basic principles of water-penetration control have been well understood for many years. Controlling
the exposure to rainwater is a two-step process:
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1. Limit, through the use of overhangs, canopies, balconies, and drip edges, etc. the amount of water
that is able to come in contact with the building enclosure. If it does come into contact with
assemblies limit the entry paths and control driving forces (kinetic energy, gravity, capillary and
pressure differentials).
2. Use appropriate assemblies to control the water that does reach the surface of the building
enclosure (WSS) such that it does not penetrate beyond the WRB and cause damage.
The classification of possible rain-control strategies for enclosures is covered by Straube and Burnett
(2005). The three strategies are perfect barrier (face-seal and concealed barrier), mass, and imperfect
barrier “rainscreen” type assemblies (drained and vented, drained and ventilated and pressure moderated).
A face seal strategy relies on stopping all water at a single surface located on the exterior face of the
assembly or detail. This strategy combines the WSS, WRB, and the air barrier into one layer at the
exterior of the assembly. This strategy relies solely on the elimination of all holes through the cladding. A
concealed barrier assembly is similar to a face seal assembly in that it utilizes a perfect barrier strategy.
The difference is that this barrier is located at the sheathing membrane where it is concealed and protected
by the cladding. Face seal and perfect barrier strategies have a poor history of successfully managing rain
penetrations in tall exposed buildings due to the difficulty of constructing a perfect barrier, and the likely
deterioration of fully-exposed materials. Therefore, the perfect barrier would not be recommended due to
the potential risk of damage to the wood framing.
A mass or storage approach is typically used in an assembly of materials with enough storage mass to
absorb and safely store moisture until it is eventually removed by evaporation when weather conditions
allow. Examples of mass assemblies include solid masonry, and would not
Compartmentalizing
the
be a strategy employed within a tall wood building.
A drained or screened approach (often termed a rainscreen) acknowledges
that some water will penetrate the outer surface and provide internal
drainage surfaces and a cavity (drain space) to help control rain
penetration. A rainscreen strategy for controlling water penetration of
wood-frame walls has, at a minimum, the following characteristics:
• a continuous WSS (the rainscreen),
• an airspace behind the cladding that is vented and drained to the
outside,
• a WRB (drainage plane), such as a sheathing membrane, inside of
the drainage space (WSS and WRB are separated),
• drain holes or gaps through the cladding so the water can leave the
cavity, with flashing at all penetrations and transitions (e.g. base of
wall, doors, and windows, etc. to direct draining water to the
outside.
In addition, many rainscreen wall assemblies will also have a continuous
air-barrier system at the WRB (i.e. one membrane that serves both
functions) to improve the control of rainwater penetration.
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cavity in rainscreen wall
assemblies can assist in
moderating
(partially
equalizing) the pressure
drops over the cladding or
stopping high speed wind
around corners. This is not
usually
critical
to
performance in low-rise
wood buildings. However,
for tall wood buildings
some compartmentalization
may be warranted. This can
be
accomplished
by
blocking
the
cavity
vertically
(resisting
horizontal flow) at building
corners and possibly at
some
intermediate
locations. Note that efforts
to compartmentalize should
never
compromise
the
capacity for drainage and
ventilation.
297 of 374
The drain space can (but need not) also create a capillary break to prevent water from migrating further
into the assembly, as well as an opportunity for air movement to facilitate drying. Common practice in
high-rise buildings and multi-storey wood-frame buildings is 10 to 50 mm utilizing wood strapping, metal
hat-tracks, metal clips, or masonry ties. An air cavity is also often included in assembly designs to
accommodate dimensional tolerances. A large gap allows for easier detailing at interfaces, and absorb
tolerances in structural elements (slabs, columns) often seen in taller
The use of open-rainscreen
buildings, without obstructing drainage or drying ability.
claddings (perforated water
shedding surface) is becoming
common in many building
designs. The open joints in
many of these cladding systems
allow more water to pass
through the WSS and hence,
increase the moisture load on
the drainage system and WRB.
These systems may also allow
UV radiation to reach the
WRB (which is often subject to
UV degradation). The use of
these
claddings
requires
careful consideration and
detailing on more exposed tall
wood buildings.
If the drain space behind the cladding allows for movement (i.e., more
than a millimeter or two) of air, a vented system can be created (i.e., a
drained and vented) that enables lateral diffusion and mixing of cavity
and outdoor air. If the drain space allows easy airflow, with vent holes
large enough and arranged to encourage air flow through the cavity,
then the assembly can be considered to be ventilated and has a muchimproved drying capability (i.e., drained and ventilated rainscreen). If
some attention is paid to the details of cavity size, the
compartmentalization and stiffness of the cladding, and the air barrier,
then some degree of pressure moderation (reduction of the windinduced pressure drop across the cladding) can be achieved, and thus
less water will penetrate past the cladding surface (i.e., a drained and
ventilated and pressure moderated rainscreen).
The presence of multiple lines of resistance (water-shedding surface
and water-resistive barrier) separated by a drained cavity provides
enormous benefit. Unplanned holes caused by construction errors or
aging in the inner and outer surface are generally not aligned so that
direct rain passage by momentum is eliminated. Water that passes
through the outer surface (the WSS or rainscreen) is driven either by
gravity, capillary action, or air pressure differences. It tends to run
down the back side of the cladding where it can be intercepted and
drained back to the outside at a cross-cavity flashing location. These
features mean that the amount of water reaching the inner surface of the
cavity and remaining in contact with potentially moisture-sensitive
materials is greatly reduced. This kind of detail incorporating multiple
lines of defence is the kind of constructed redundancy that is important for the durable performance of tall
wood buildings.
Details and materials exist to
provide the appearance of an
open rainscreen cladding
(black painted metal hattracks,
black
synthetic
membranes behind the joints
etc.) and would generally be
suggested in lieu of a true open
rainscreen cladding to reduce
water penetration past the
WSS.
For a tall building in Canada, particularly one constructed of wood, drained and ventilated assemblies are
recommended regardless of climate zone. This applies to the walls, windows, curtain walls, joints and
other interfaces exposed to rainwater. Drained assemblies, joints, and accompanying details are already
common in most high-rise construction in many parts of the country, and increasingly used for low-rise
wood-frame buildings. To address combustibility concerns of possible flame-spread through rainscreen
cavities in a fire, the air gap width must typically be less than 25 mm (50 mm in some scenarios) and
typically compartmentalized at every floor level by a non-combustible material (i.e. by use of a metal
cross cavity flashing).
It is important to note that the use of rainscreen wall assemblies that exhibit good resistance to water
penetration and are fundamentally less sensitive to moisture ingress does not eliminate the need for
implementing proper details and ensuring acceptable construction practice. Improper details are
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frequently sources of water entry and as such, will critically affect the moisture performance of the
assembly. A recommended design strategy is to understand the control functions and identify and label
the critical barriers of all building enclosure assemblies and details as covered in Part 4.1.2 such that
errors of omission are minimized prior to construction.
6.4.1.4
Accidental Sources of Moisture
Other sources of moisture within a high-rise building include the accidental leakage of water from
sprinklers, defects in plumbing, and from defects in water containing appliances (e.g. dishwashers,
washing machines, ice makers, refrigerators, water coolers etc.). While these are not the major concern
for designing building enclosures in this Section, such water leakage from one of these systems can cause
major damage to wood and other materials, and consideration should be given in design to facilitate
drying and ease of repair in case they occur. The design of building enclosure assemblies that have the
ability to dry out in the event of a small leak or large flood is recommended wherever possible. This
means that the interior surfaces of assemblies, particularly heavy timber elements should be vapour
permeable to allow for inward drying, or are easily removable or replaceable in the case of an indoor
flooding event.
6.4.1.5
Heat Flow Control & Thermal Bridging
Reducing space-heating energy use is a primary function of the building enclosure. While heat flow
through the building enclosure can never be stopped entirely, it can be controlled in order to reduce the
total energy consumption and improve comfort. This is achieved by constructing a thermally insulated
and airtight building enclosure, which is a fundamental strategy towards achieving an energy-efficient
building.
The control of heat flow for the building enclosures of wood buildings incorporates three primary
components:
• Use of the site, features of the building, and glazing properties to reduce solar gain in summer and
collect useful solar gains in the winter.
• Minimizing conductive heat flow through opaque enclosures by the use of insulating materials,
the avoidance of thermal bridging (such as at cladding attachments floor slabs, structural
columns) to form a continuous thermal control layer including the use of thermally efficient
window frames and glazing.
• Limiting unintentional air flow through the elements of the building enclosure by the construction
of airtight assemblies (a continuous airflow control layer).
Most energy conservation-related regulations target greater thermal insulation levels in opaque building
enclosures as being the key strategy for reducing energy use in buildings. In order to achieve effective
heat flow control, the continuity of thermal insulation should be maintained through assemblies, and
details should be provided to reduce thermal bridging. Because of the modest thermal conductivity of
wood (about 400 times less than steel and 20 times less than concrete) tall wood buildings have major
advantages when it comes to the control of thermal bridging. This is particularly relevant for CLT wall
and roof assemblies, and at approximately R-1.2/inch and often installed continuously will provide part of
the overall assembly thermal resistance. Effective R-value targets are outlined within Canadian energy
codes and were covered in Figure 133.
The use of thermal insulation must be considered together with the airtightness and vapour permeance of
various layers in the assemblies in order to ensure good durability performance.
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The placement on insulation plays a key role in the thermal efficiency and the hygrothermal performance
of a building enclosure assembly. Within a wood-frame assembly, insulation may be placed between the
structural elements, typically wood studs (interior or interstitially insulated), to the exterior of the
structure (exterior insulated), or divided between stud space and exterior of the structure (split insulated).
All of these options are possible in a tall wood building depending on the structure, though certain
assemblies would be recommended over others.
The higher the proportion of the total insulation located outside of the structure, the warmer and drier the
structure and, typically, the higher the effective R-value (less thermal bridging through the structural
framing). This means that in general exterior insulated assemblies will be more durable than internally
insulated ones (though both can achieve satisfactory performance with good material selection and
detailing). The split insulated approach is a good compromise in terms of cost (cavity insulation is
cheaper than exterior insulation) and wall thickness, though this assembly requires careful design. The
performance of a split insulated wall will depend on the thickness and type of exterior insulation, climate,
interior loads and other factors. The design of split insulated wood-frame walls is covered within many of
the referenced documents including the Guide for Designing Energy-Efficient Building Enclosures (2013)
and High-Performance Enclosures (2012).
Figure 135 and Figure 136 demonstrate selected insulation strategies for wood-based assemblies which
would likely be within a tall wood building. Other assemblies which are more common in low-rise woodframe construction such as interior insulated vented or unvented roof and pitched roofs would not be
common in a tall wood building due to structural requirements for heavier timber components. Other
potential enclosure assemblies, like interior insulated CLT or mass timber walls or roofs would not be
recommended, so are not shown here.
Stud wall
CLT/mass timber
Stud wall
Interior-insulated wall assembly
Exterior-insulated wall assembly
Split-insulated wall assembly
Figure 135 Options for placement of thermal insulation within wood-frame wall assemblies.
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Low slope roof (tapered insulation)
Exterior insulated (conventional assembly)
Low slope roof (tapered structure)
Exterior insulated (inverted/protected membrane
assembly)
Figure 136 Options for placement of thermal insulation within low slope roof assemblies.
6.4.2
Condensation Control
Condensation of water vapour can lead to damage of finishes, structural components, and other materials
of the building enclosure, and it may affect indoor air quality and occupant comfort if not effectively
controlled. Condensation becomes more important for highly-insulated building-enclosure assemblies,
and critical for cold climate buildings with high interior relative humidity. In heating-dominated climates,
condensation control is generally achieved by controlling the indoor RH, controlling air flow (air barrier),
keeping potential condensing surfaces warm (insulation strategy), and controlling vapour diffusion
(vapour control layers).
Condensation occurs when the water vapour in the air changes phase to a liquid or solid form. The
variables that impact condensation potential include the temperature of surfaces, air temperature, and the
amount of vapour in the air. Warmer air can hold more moisture. The dew point temperature is a
measure of the temperature at which a given sample of air can hold no additional moisture. Condensation
occurs on surfaces that are colder than the dew point temperature of the air to which they are exposed.
Three conditions are required for the accumulation of condensation and the strategies for the control of
condensation therefore involve the management of these three variables:
•
•
•
A source of humidity
o Control indoor humidity levels by providing sufficient ventilation year round, or where
necessary mechanical dehumidification in the summer in more humid climates such as
many areas in Eastern Canada.
A sufficiently cold surface (at or below the dew point temperature of the air)
o Control by keeping surfaces warm. Strategies include insulating on the exterior of
moisture sensitive materials and the structure, minimizing/blunting thermal bridges;
improve air circulation at the exterior walls.
A mechanism(s)—vapour diffusion and/or air movement—to get the humid air to the cold surface
o Control of air movement within and through assemblies by continuous air barrier systems
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o
Control of vapour diffusion by use of low vapour permeance materials (vapour retarders
or barriers) where appropriate. Use of a climate appropriate vapour retarder at the warm
(high vapour pressure) side of the insulation is generally recommended, with
consideration for how this changes with season.
While these requirements seem simple, it can get complicated because materials used to keep water
vapour from moving into an assembly can also restrict water vapour from moving out. This is a problem in
situations where some drying is necessary to facilitate initial drying of wet building materials (such as wet
wood or concrete), or because of changes in the direction of the moisture drive (for example, inward drive
due to heated moisture in absorptive claddings). For example, exterior foam insulation (XPS, EPS, or
polyiso) in wood-frame assemblies should not be installed over wet sheathing, nor should water be
allowed to penetrate behind such insulation during construction or in service.
In addition, the low vapour permeability of solid wood such as that in CLT wall and roof assemblies is
low enough in most constructions that additional vapour diffusion control layers (such as polyethylene)
are not necessary and would in fact be risky to use in many assemblies. This may arise with the design of
some tall wood enclosure assemblies if prescriptive building code requirements were misinterpreted.
Vapour permeance of materials in assemblies must therefore be carefully selected in the context of the
permeability of the other layers within the assembly and the given climate zone. The control of vapour
diffusion is particularly important in highly thermally efficient assemblies.
6.4.3
Air Flow Control
The control of air flow by the use of air-barrier systems is important to minimize rain penetration,
interstitial vapour wetting/condensation, and space-heat loss from building enclosures.
Air-barrier systems are required for buildings within all Canadian climate zones. Air sealing measures are
prescriptively called out within the NBC, NECB, and ASHRAE 90.1, along with airtightness
requirements for materials and assemblies; however, no whole building airtightness target is required.
Overall building airtightness is very important for energy efficient and moisture management, and
included within a number of international building codes for large buildings. Typical airtightness targets
are below 2.0 L/s∙m2 (0.40 cfm/ft2) of enclosure area at 75 Pa which is relatively leaky (Washington State
and Seattle Building Codes). More stringent requirements below 1.27 L/s∙m2 (0.25 cfm/ft2) of enclosure
area at 75 Pa such as that set by the US Army Corps of Engineers are recommended for a large building
target and appropriate performance measure for a tall wood building. A recent report performed for
CMHC summarizes air leakage control strategies, testing measures and provides a database for large
buildings in Canada and the US (RDH 2013).
The air-barrier system must comply with five design requirements in order to function adequately.
1. All the elements (materials) of the air-barrier system must be adequately air-impermeable. They
must have air permeability less than 0.02 L/s∙m2 at 75 Pa based on definition within the NBC.
Assemblies of materials must have an air permeability of less than 0.2 L/s∙m2 at 75 Pa.
2. The air-barrier system must be continuous throughout the building enclosure. It must span across
dissimilar materials and joints. It must be sealed between assemblies and components (e.g. from
wall to roof, and wall to window) and around penetrations such as ducts and pipes.
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3. The air-barrier system must be structurally adequate or be supported to resist air pressure forces
due to peak wind loads, sustained stack effect, or fans.
4. The air-barrier system must be sufficiently rigid, or be supported, so that displacement under
pressure does not compromise its performance or that of other elements of the assembly.
5. The air-barrier system should have a service life as long as that of the wall and roof assembly
components; alternatively, it must be easily accessible for repair or replacement (be durable).
Air leakage in a building occurs through unintentional defects, joints, and interfaces in the building
enclosure, but also through open windows and mechanical penetrations. In tall buildings air leakage may
account for a significant portion of the space-heat loss, depending on the air-leakage rate, building height
and wind exposure, occupant behaviour, mechanical penetrations, and several other factors including the
effective enclosure thermal performance. Air leakage in a tall wood building will typically be higher than
in low-rise building due to the increased wind exposure, the stack effect, and the mechanical systems, all
of which contribute to higher and more sustained differential pressures across the building enclosure.
Air barriers are generally located on either the interior or the exterior side of the wall or roof assembly.
The most ideal location to install the air barrier may depend on ease of detailing, materials and other
factors related to the construction schedule and sequencing. In cold climates the air barrier is generally
installed on the interior side of the insulation to limit air exfiltration into the assembly and convective
looping within fibrous insulations, and to prevent moist indoor air from contacting cold exterior surfaces
(e.g. use of interior polyethylene sheet membranes or airtight drywall). In cold climates an air-barrier
material may also be installed on the exterior side of the wall to prevent wind washing (e.g. the use of an
airtight sheathing membrane or sealed rigid sheathing). Unlike vapour barriers, there is little to no
downside of redundancy in the air barrier provided that the materials used for the air barrier do not
negatively affect vapour flow.
The air-barrier system in building-enclosure assemblies, particularly for tall wood buildings must
accommodate the imposed wind load and transfer it to the building structure. In many cases, it is a
combination of materials that comprise the air-barrier system; however, there are usually one or two
materials that play a dominant role within any particular air-barrier strategy. For example, vapour
permeable sheathing membranes and tapes are often the key material in an exterior air-barrier strategy,
while the exterior sheathing or interior gypsum boards are the key materials in more rigid air-barrier
systems. Not only the material, but also their joints, tapes and sealant, must be capable of staying airtight
under applicable wind and air pressure loads. This can be a challenge in a tall wood building, and
attention must be paid in design and construction to ensure airtightness.
6.4.4
Noise Control
The building enclosure controls the transmission of undesirable outdoor noise pollution into indoor
spaces. Urban noise from vehicle traffic, rail, aircraft, industry, neighbours etc. is undesirable indoors and
interferes with many indoor activities including conversation, sleep and concentrated thinking. The
components that make up the building enclosure in terms of acoustic mass and dampening properties, as
well as air tightness affect outdoor noise transmission. The selection of appropriate windows, as well as
wall and roof assemblies all need to be considered in acoustic designs.
In North America, the most widely used rating for sound insulation of building enclosure components is
the sound transmission class (STC), which rates airborne sound reduction in the middle to high frequency
ranges. The STC rating does not properly rate against low frequency noise sources. The STC rating for
many common building enclosure components based on laboratory testing is provided within several
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industry sources. Window and glazing STC ratings are well documented within the industry as are some
common residential building enclosure assemblies (i.e. 2x4, 2x6 stud insulated walls etc.) (Bradley and
Birta 2000). Ratings for less common exterior insulated and heavy timber wall and roof assemblies
utilizing CLT panels and other engineered lumber products are not readily available at this time.
Currently there are no noise control/acoustic requirements for the exterior building enclosure within
Canadian building codes. The only requirements within codes are for addressing noise control within the
building and more specifically between units/spaces covered within Section 0 of this guide. This is not to
say that noise control of the building enclosure is taken into consideration by designers. In fact, many
municipal planning/building departments have noise control requirements written within municipal
bylaws which must be followed. These requirements are often based on NRC/CMHC industry guidelines
and will often contain minimum STC ratings or design requirements depending on the type of building,
site, and proximity to busy roads, airport, railway, or other industry.
As an example, planning departments may require certain glazing STC ratings or treatments (i.e. use of
laminated glass or dissimilar glass pane sizes) for certain building site (i.e. along busy streets). In some
instances, depending on the cladding and wall assembly make-up, additional layers of gypsum wall board
on resilient bars at the interior of stud framed walls may be required to achieve the desired performance
referenced by the municipal bylaw.
Tall wood buildings will sometimes be constructed in noisier urban areas. The assessment of more
traditional wood-frame wall and roof assemblies and windows should be relatively straightforward based
on existing performance data. However the lack of performance data for some of the heavy timber wall
and roof assemblies that are utilized in tall wood buildings will make acoustic design more difficult.
Fortunately mass timber panels, insulation materials, and gypsum drywall generally have good acoustic
properties (mass and dampening) and these assemblies will typically be able to meet municipal
requirements. Most often windows have the lowest STC rating compared to walls or roofs, and therefore
are often the limiting component.
Additional guidance on noise control for building enclosures can be found in the City of Vancouver Noise
Control Manual (COV 2005) as well as some older reference documents including: Controlling Sound
Transmission into Buildings (Quirt 1985), Road and Rail Noise: Effects on Housing (CMHC 1981),
Laboratory Measurements of the Sound Insulation of Building Façade Elements (Bradley and Birta
2000).
6.4.5
Fire Control
The building enclosure also controls the spread of fire and smoke in the unfortunate event of a fire. The
building enclosure must remain intact for a certain period of time to prevent the passage of flame outside
or into the building primarily to allow occupants to safely escape without structural collapse. Within a tall
wood building, which utilizes structural wood elements and building enclosure assemblies, this presents
some challenges for designers as discussed in Chapter 5. In general, the wood building enclosure must be
protected from fire, and the cladding and cladding system not contribute to the spread of the fire up the
exterior of the building. This generally means the use of non-combustible claddings, cladding supports
and non-combustible mineral fiber insulation, similar to other non-combustible high-rise buildings and
protection of wood elements. The use of foam plastic insulation within walls may be a challenge for
designers.
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To address combustibility concerns of possible flame-spread through rainscreen wall cavities in a fire, the
air gap width must typically be less than 25 mm (50 mm in some scenarios) and typically
compartmentalized at every floor level by a non-combustible material (i.e. by use of a metal cross cavity
flashing).
The basic premise of the approach to fire protection in tall wood buildings as addressed within Chapter 5
of this guide. There it discussed that all timber must be fully encapsulated to protect it from the effects of
fire except where it is specifically demonstrated that less or no protection is required.
For exterior cladding the fire code permits combustible wood cladding, and cladding systems containing
combustible components (i.e. foam plastic exterior insulation, membranes, and cladding supports),
provided that the system conforms to specific performance criteria found in NBC Article 3.1.5.5 when
tested to a full scale exterior fire plume as prescribed by CAN/ULC S134. This test is considered to
represent an appropriate design fire for exposure to both a window plume and exterior fire impingement.
It is essentially a test to confirm that a cladding system containing combustible components will not
support unacceptable flame propagation up the face of a building. Unfortunately these full-scale wall fire
tests can be quite expensive and time-consuming, therefore most cladding systems and combinations have
not been tested, nor will likely be over heavy timber wood structures, relying on analysis or modelling to
demonstrate conformance.
While CAN/ULC Standard S134 and the criteria of NBC Article 3.1.5.5 refer to the cladding system of a
non-combustible assembly in a non-combustible building, for a building containing combustible elements
within the exterior wall, it is appropriate to assess the entire wall system for conformance to NBC Article
3.1.5.5. Exposed wood which is not fire treated will not meet this performance criteria. Fire treatment of
the exposed wood would have to be a permanent treatment, not subject to degradation due to weathering
and at this time, availability of suitable treatment is limited and costly. Therefore any mass timber exterior
walls will need to be encapsulated on the interior face by a single layer of 5/8” Type X exterior gypsum
drywall as confirmed by testing by NRC/IRC in the 1990’s (Oleszkiewicz, 1990).
It is also probable that various forms of non-combustible exterior cladding systems and cladding systems
in combination with non-combustible exterior insulation can be shown to provide appropriate protection
to mass timber wall elements when installed over mass timber wall elements, however this will require
testing or analysis or modelling to demonstrate conformance. Similarly, use of minor combustible
components in these exterior walls can be demonstrated as acceptable based on testing or analysis to show
conformance with the code requirements.
6.5
Building Enclosure Assemblies and Details
6.5.1
Wall Assemblies
6.5.1.1
Structure and Insulation
Suitable opaque wall assemblies for a tall wood building will generally consist of rainscreen clad exterior
insulated or split-insulated to meet minimum requirements for thermal performance as given within the
NECB or ASHRAE 90.1 and those required to meet the durability objectives (to keep wood structure
warm and dry). Stud insulated wood frame walls with drained and ventilated cladding systems may be
utilized within some in-fill wall applications, though in climate zones 5 and higher, a 2x6 wall with glass
or mineral fibre insulation (up to effective R-17) will not meet the prescriptive R-value targets in either
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the 2011 NECB (R-20.4) or ASHRAE 90.1-2010 (R-19.6). Deeper 2x8 or 2x10 walls could be
considered, and may be required for high-rise buildings exposed to greater wind-loads. Split insulated and
exterior insulated assemblies will readily meet or exceed the minimum energy standard requirements.
Examples of split insulated stud and exterior insulated wall assemblies with alternate cladding support
strategies are shown within Figure 137.
Intermittent metal clip supports
Screws through insulation with
continuous metal hat-track/furring
Intermittent fiberglass clip / screw
supports
EXTERIOR
EXTERIOR
EXTERIOR
• Cladding (WSS)
• Airspace (ventilated)
• Steel hat-track/furring and intermittent
metal clips
• Semi-rigid mineral-fibre insulation
(thickness to meet R-value requirement)
• Adhered or liquid applied Vapourpermeable sheathing membrane
(WRB/Air Barrier)
• Sheathing (plywood, OSB or gypsum)
• 2x6 wood framing with batt insulation
• Polyethylene film (vapour control where
needed)
• Gypsum board with paint
INTERIOR
• Cladding (WSS)
• Airspace (ventilated)
• Steel hat-track/furring and long support
screws through rigid insulation
• Rigid mineral-fibre insulation (thickness to
meet R-value requirement)
• Adhered or liquid applied Vapour-permeable
sheathing membrane (WRB/Air Barrier)
• Sheathing (plywood, OSB or gypsum)
• 2x6 wood framing with batt insulation
• Polyethylene film (vapour control where
needed)
• Gypsum board with paint
INTERIOR
•
•
•
Cladding (WSS)
Airspace (ventilated)
Steel hat-track/furring and long screws
through insulation
• Semi-rigid mineral-fibre insulation
(thickness to meet R-value requirement)
• Adhered sheet applied vapour-permeable
sheathing membrane (WRB/Air Barrier)
• CLT panel (vapour control layer)
• Furring and gypsum board with paint
INTERIOR
Figure 137 Split insulated wood stud frame (left and middle) and exterior insulated CLT wall (right)
The design and selection of appropriate wall assemblies requires control of exterior moisture (WSS and
WRB), appropriate air-barrier strategy, assessment of insulation placement and properties, cladding
attachment and support through exterior insulation, and vapour flow control. Effective R-values will be
governed by the amount and placement of insulation and thermal bridges within the assembly. The
reference document Guide for Designing Energy-Efficient Building Enclosures (2013) provides an indepth look at the appropriate wood-frame and CLT wall assemblies and provides calculated effective Rvalues and typical details. While that guide is generally intended for wood buildings up to six-stories, the
design analysis for a taller building is similar. The use of more robust and adhered WRB/AB membranes
and sealants, and more appropriate claddings would however be recommended for a taller wood building.
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Gypsum sheathing, mineral fiber insulation, and other fire protection measures may also be required for
some building types which would limit the design options for applicable wall assemblies.
6.5.1.2
Claddings and Cladding Attachment
Claddings used on tall buildings are typically noncombustible and are manufactured from durable and low
maintenance pre-finished materials such as: aluminum
core or galvanized steel panels or composites, fiber
cement, glass fiber composites, concrete, brick masonry,
terracotta, and glass etc. Curtain wall assemblies of glass
and aluminium and a range of spandrel panel claddings
are also common further covered in Part 5.1.4. Where
rigid claddings are used, appropriate joints must be
provided to accommodate potential vertical differential
movement and lateral drift of the tall wood structure,
which may be greater than a comparable concrete or steel
structure. Claddings are typically separated at floor levels
to accommodate such movement and improve moisture
management. Attachments for these cladding would be
similar to other non-combustible buildings, though would
take into account the potentially greater movement
tolerances.
While potentially desirable from an architectural
perspective, the use of fire-resistant wood claddings on a
tall wood building should be carefully considered in terms
of fire safety, durability, and life span. Very few fire
retardants on the market can meet the rigorous fire testing
requirements (e.g. weathering before fire testing). The use
of wood-based cladding, except at lower accessible floors,
may also create challenges for building maintenance as
covered in Chapter 9.
Wall assembly consisting of brick veneer over 6” of
semi-rigid mineral fiber over 3½” CLT panel wall.
Stainless steel brick ties and galvanized steel
galvanized shelf angle on bracket stand-off supports
are utilized to minimize thermal bridging and
maximize the effective R-value of the wall assembly
The cladding attachment can be a source of significant
thermal bridging in exterior and split-insulation woodframe wall assemblies. Optimizing structural cladding attachments is important for improving the thermal
efficiency of wall assemblies, while minimizing exterior insulation and overall wall thicknesses. It is also
critically important to adequately consider the gravity, wind, and seismic loads so that the claddings will
perform in service without excessively deflecting, cracking, or detaching from the structure. Many
strategies, products, and techniques, some of which have proved to work structurally and thermally, have
been developed over the years to meet this challenge. Several of these strategies are presented within the
Guide for Designing Energy-Efficient Building Enclosures (2013).
6.5.1.3
Appropriate Air Barrier Systems for Tall Wood Buildings
Efforts to achieve a satisfactory barrier to air movement in wood-frame construction in the early 1980’s
focused on the use of polyethylene sheet membranes. The polyethylene sheet is partially supported by the
wood framing, insulation, and interior sheathing, and also functioned as a primary vapour-retarder
material within the assembly. This approach is still commonly and successfully employed in low-rise
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wood-frame buildings though not recommended in tall wood buildings subject to higher air pressures and
difficulty of detailing in the field.
Alternative approaches to achieving control of air leakage are to seal the joints between the rigid sheet
materials that are used in construction with tapes, sealants, or gaskets. At the interior of the wall this
would be known as airtight drywall approach (ADA) and at the exterior sheathing known as a sealed
sheathing or sealed sheathing membrane approach. Most materials used in construction have a low
permeability to air leakage. Not only the material, but also their joints, tapes and sealant, must be capable
of staying airtight under applicable wind and air pressure loads.
Based on experience with high-rise concrete buildings and four to six-storey wood-frame construction
there are several possible approaches for achieving good airtightness in tall wood buildings as provided
below. All of these approaches utilize a rigid air barrier approach or use an adhered vapour permeable
membrane or liquid vapour permeable membrane to a rigid substrate (i.e. CLT panel or plywood
sheathing). Further details can be found within the Guide for Designing Energy-Efficient Building
Enclosures (2013).
•
•
•
•
Interior airtight drywall approach
o The interior gypsum board and framing members provide the air barrier in the rigid
airtight drywall approach. Continuity between different materials is created with sealant
or gaskets. Continuity between floors and at some interfaces can be a challenge and is a
limitation of an interior approach. This approach would be a challenge for designers and
contractors within a multi-storey CLT structure as it often is for stick-built wood frame.
Exterior sealed sheathing approach
o The rigid exterior sheathing (plywood, OSB, or gypsum) with sealed joints is the primary
air barrier element, particularly for wood-frame and post-and-beam construction. Joints
are sealed with a compatible sealant, tape, or membrane and details are interfaced to the
airtight sheathing substrate. These interfaces between the sheathing panels must consider
potential building movement. This approach would work well for sheathed and framed
wall assemblies, though with mass CLT assemblies would not typically be utilized due to
the number of ply joints within the CLT panels.
Exterior adhered or liquid applied vapour permeable sealed sheathing membrane
o A vapour permeable self-adhered or liquid applied WRB/sheathing membrane is applied
to a rigid substrate such as exterior sheathing or CLT panel. It must accommodate
potential building movement and substrate shrinkage and details are interfaced into the
rigid membrane/substrate. A vapour impermeable membrane (asphalt modified peel and
stick or torch-applied roofing membrane) may alternately use within some exterior
insulated assemblies, and in particular roofs, though not generally recommended for wall
assemblies. Where impermeable air barrier membranes are used, a hygrothermal analysis
should be performed and the assembly must be able to safely dry towards the interior.
Other approaches and systems
o Curtainwall assemblies are relatively airtight components. If the majority of the façade of
a building consists of curtain wall system then this will form the primary air barrier
system.
CLT building enclosure details can also pose unique and challenges to air-barrier detailing practices. CLT
panels themselves are airtight components (as tested in the laboratory); however, gaps between the
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lumber plies will open up in service after manufacture creating numerous air leakage pathways around the
panels themselves if not properly detailed. In addition CLT structural connections including angles and
clips can interfere and puncture air barrier membranes. For this reason, relying on sealing only the CLT
panels as an air-tight element is not recommended. The pre-installation of robust air-barrier membranes,
use of construction site mock-ups and air barrier commissioning will help improve air barrier installation
and performance with CLT construction. Figure 138 shows some example photographs of common
details requiring special consideration and Figure 139 shows two sketches showing possible air leakage
pathways for parapet and floor level/shear wall conditions.
Figure 138 Photographs of some of the unique air barrier detail considerations required for CLT panel
assemblies when utilized within tall wood buildings. Gaps between lumber plies and connections (left), structural
anchors interfering with installation of air barrier membrane (centre) and protruding structural elements (right).
Figure 139 Sketches showing potential air leakage paths and need for continuous adhered air barrier
membranes and transitions for CLT wall and roof details.
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6.5.1.4
Fenestration Selection and Installation Considerations
Punched windows, window wall, and curtain wall
utilizing thermally broken aluminum are all
commonly used within high-rise buildings. The use
of fiberglass, uPVC (vinyl) or wood-hybrid window
frames may also be considered within a tall wood
building to improve overall building thermal
efficiency. Exterior exposed wood window frames
are not recommended in a tall wood building due to
the need for frequent painting and maintenance
however interior exposed wood frames such as those
within wood curtain wall systems could function
adequately. The selection of window assemblies that
meet appropriate air, wind and water resistance
ratings is critical within a tall wood building to
ensure weather tightness of the assembly. As an
example, the minimum appropriate window for a 1020 storey tall wood building would have a NAFS
performance grade of PG 60 and higher, a water
penetration resistance of 510 to 730 Pa, and
performance class of CW or AW.
Sketch of curtainwall assembly integrated into a building
utilizing a heavy timber structural system using standard
aluminum curtain wall components and anchors bolted into
the CLT floor panels.
Integration of window systems into a tall wood
building is straightforward provided that best practices for installation details are followed. This means
the incorporation of rainscreen details in which the critical barriers of the wall (WSS, WRB, and air
barrier in particular) tie into the window assemblies. The key differences between a tall wood and a
concrete or steel-frame building will be structural connections (primarily slab anchors for curtainwall
systems), and potentially larger deflection tolerances made up by the window assemblies in a tall wood
building. Standard window anchor components designed for concrete and steel buildings can is most
cases be used as-is or with slight modifications to accommodate alternate wood fasteners.
6.5.2
Roof Assemblies
Tall buildings will typically utilized low-slope roof and roof-deck assemblies. Roof tops are often used
for placement of mechanical equipment, to provide access for maintenance of exterior walls, and for
additional outdoor space (roof decks or common amenity space). Pitched roofs may be incorporated as a
feature roof, but would be less common in a tall wood building. In many structural designs, the wood
roofing structure will behave as a lateral load resisting diaphragm element consisting of heavy timber
framing or CLT panels. This use of heavier framing rather than joists and plywood used in low-rise woodframe construction dictate certain approaches for a tall wood building.
Insulation is placed on the exterior of the wood structure and either a conventional or protected membrane
roofing assembly (also referred to as inverted) is recommended for roofs and roof-decks. Protected
membrane roofs provide greater protection of the roofing membrane and are recommended for roof decks
or roofs which expect high traffic. A protected membrane roof assembly will also form the basis of a
green roof, with modifications to the design to accommodate particular aspects to such roofs such as the
need for root barriers, a drainage layer, the incorporation of a water retention medium, and other aspects
related to soil and planting of vegetation.
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The selection of a green roof system, like any roof system on tall wood building should be carefully
considered in terms of life-span, maintenance, and the potential risk of leaks to damage the underlying
wood structure. The use of leak detection and moisture monitoring systems under roof membranes are
becoming common in Canada and provide the ability to detect and isolate leaks before damage can occur
to the underlying structure. The risk for water leaks are higher and less likely to be immediately observed
in a heavy timber CLT structure compared to a concrete slab.
Within a heavy timber roof deck assembly, slope to drains (minimum 2% recommended) can be achieved
by either a tapered insulation package (conventional or modified protected membrane assembly) or by
built-up tapered wood-framing and plywood above the wood deck (protected membrane or conventional
assembly). Alternately the roofing structure could be sloped to drain in some buildings.
The reference document Guide for Designing Energy-Efficient Building Enclosures (2013) provides an
in-depth review of appropriate roof assemblies and material recommended here and provides calculated
effective R-values and typical details. Figure 140 provides a sketch of both a conventional and a protected
membrane roof assembly over a mass timber CLT roof structure.
EXTERIOR
EXTERIOR
• Torch-on or mechanically fastened roofing membrane
• Protection board
• Roofing insulation (polyiso, mineral fiber, or EPS) ≥2 layers with
staggered joints to meet R-value target, tapered slope to drain
• Adhered or torch-on membrane (air-barrier/vapour barrier)
• Heavy timber CLT roof panels
• Ceiling finish
INTERIOR
• Ballast (concrete pavers or gravel)
• Filter fabric
• XPS insulation, ≥2 layers with staggered joints to meet
R-value target
• Adhered or torch-on roofing membrane (waterproofing airbarrier/vapour barrier)
• Built-up wood structure (sloped to drain), or tapered insulation
• Heavy timber CLT roof panels
• Ceiling finish
INTERIOR
Figure 140 Low slope conventional roof (left) and protected membrane roof (right)
Roofing with heavy timber framing can be a challenge in wet climates. Heavy timber roof components
such as glulam or LVL/LSL beams or CLT roof panels can absorb a considerable amount of water if
exposed to rain during transport or while under construction. Depending on the time of year, it could take
months for this wet wood to dry out, which may delay progress in roofing and overall building
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construction. Consideration for the pre-site protection by use of pre-applied roofing or adhered roof
vapour barrier membrane on CLT roof panels and other exposed heavy timber roof components is
recommended, with joints immediately addressed upon installation (i.e. protection of wood elements with
a temporary “construction roof” to allow for drying). As all low-slope roofing materials are vapour
impermeable, they should only be applied if the wood is dry and the ability for drying to the interior must
be facilitated in design.
Finally, where torch-on roofing membranes are used against wood a protection board (mechanically
attached asphalt underlay or gypsum protection board) is required to protect the wood from burning, in
addition to other measures used to ensure the prevention of fire when roofing a wood building.
6.6
Protection and Wood Durability
Wood has proven long-lasting performance in properly designed and constructed buildings across the
world. In Canada the major threat of durability comes from decay fungi and mould growth. The key to
achieving durability is to prevent excessive moisture accumulation and to allow wood to dry should it get
wet during construction and in-service. The risks will be greater within a tall wood building due to greater
exposure to rain and snow, and potentially longer construction schedules spanning over rainy seasons.
The main conditions for fungi to grow in wood are favourable moisture and temperature. Wood and
wood-based materials always contain some moisture; the amount varies over time depending on its
wetting and drying history, changes in RH, temperature, and liquid water in the environment. The
moisture content (MC) of wood exposed to humidity alone is generally below the levels conducive to the
growth of decay fungi. As a general rule, liquid water needs to be present to lead to wood decay. Research
has shown that at a temperature around 20°C, decay fungi can colonise kiln-dried wood products when
the MC rises to a threshold of 26% MC, which can be considered the low end of fiber saturation point.
Under such marginal moisture conditions, it takes at least several months or years for detectable structural
damage to occur when all other conditions are favourable for decay; but higher MC can speed up decay
dramatically. On the other hand, sustained high humidity conditions, coupled with warm temperatures,
may cause mould growth. Mould growth occurs on surfaces and does not reduce wood strength, but
affects appearance and raises other concerns. It takes at least months for mould to initiate on wood at a
minimum surface RH around 80% with a temperature at 20-25°C. Most incidents of mould growth in
buildings are associated with wetting caused by liquid water sources.
Wood species vary widely in natural durability. Sapwood of all wood species has low natural durability.
Heartwood is generally more durable than sapwood. The heartwood of SPF and hem-fir is not durable.
Douglas-fir and western larch heartwood are moderately durable. The heartwoods of species such as
western red cedar, and yellow cedar have high natural resistance to decay. Wood-based composites such
as plywood, OSB, PSL, LVL, LSL, glulam, and CLT have the same level of durability as the wood from
which they were made, unless treatment such as using a preservative has been provided during the
manufacturing process.
6.6.1
On-site Moisture Management
It is important to limit the amount of wetting that wood-frame assemblies receive through the shipping,
on-site storage, and construction stages, to the stage that the building is closed in or at least protected
from exposure to rain. This becomes even more critical where engineered wood products such as parallel
strand lumber, laminated strand lumber (LSL), laminated veneer lumber (LVL), CLT, and glulam are
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used because these massive wood elements may absorb and store more moisture than dimensional lumber
and swell when exposed to liquid water sources, and they tend to dry slowly. Moreover, it can be difficult
for insulated wood based assemblies to dry out after the insulation is installed. Ideally it should be
targeted to keep the wood MC below 20% at most during storage and construction to provide a margin of
safety and reduce long-term differential shrinkage after installation. While the manufacturing of most
engineered lumber such as CLT occurs within the 10 to 14% range, the wood when installed will often
reach higher moisture contents depending on the equilibrium outdoor RH levels and wetting during
storage and transportation to site.
The NBC and provincial building codes
state that the "Moisture content of
lumber shall be not more than 19% at
the time of installation" for Part 9
buildings. While this requirement is not
explicitly stated in Part 5, it is generally
accepted as best practice and would be
prudent when constructing a tall wood
building. This threshold may be difficult
to achieve if wood is left exposed to rain
in a roof or wall – so protection of
wood during construction is paramount.
The control of construction moisture
can also be more difficult in a taller
under
more
exposed
building
construction and the interior walls and
floor slabs will also be subject to
extensive wetting from unfinished floors
above. This cascading effect of water
running down the interior of a wood
building under construction should be
prevented.
Temporary construction roofs and
heated
wall
enclosures
are
recommended for the construction of
tall wood buildings within many areas
of Canada, particularly during the
wintertime. While temporary protection
and heating can be quite expensive, it
will allow for construction under
inclement weather and reduce the time
required to dry out wetted wood prior to
closing in wall and roof assemblies.
In addition to outdoor relative humidity, sources of liquid water
which wood will be exposed to include rain, snow, ground
moisture, and vapour condensation. Wood absorbs water most
rapidly through end grain, which can be exposed by end cuts,
knots, drilled holes and cracks. Checking on upper surfaces
exposed to rain or condensation can trap moisture and increase
water uptake. Many engineered wood products have much
exposed end grains, as well as small pores for capillary
absorption, depending on the manufacturing compared with
solid wood products.
In most cases people handle dried wood such as various
engineered wood products and “S-Dry” lumber. These products
are usually covered with wraps when they arrive on
construction site. Plans should be made in advance to minimize
on-site storage time and wetting. The products should be kept
under shelter, or in well drained and ventilated area, and should
be off the ground. The wraps should be kept on the wood
products until they are ready to use. They should be re-covered
with waterproof tarps if the original wrapping is damaged. Also
be aware that these plastic wraps or tarps may also trap
moisture and slow down drying if water is allowed to get into
the packages.
If used, wet wood such as “S-Green” lumber and solid timbers
should be stored in dry and well ventilated areas to promote
drying. In most cases chemical formulations will have been
applied at the sawmill to provide temporary protection against
stain and mould fungi before the wood dries.
Wood-based panels (e.g. plywood and OSB) and various engineered products (PSL, LSL, LVL, Glulam
beams or CLT) usually require more attention during storage and handling. Most of them are
manufactured at lower MC levels (10-14%) and may be more susceptible to moisture uptake and
dimensional change than lumber. Massive wood elements may absorb and store more moisture than
dimensional lumber when exposed to liquid water sources; moreover, they tend to dry slowly. The wood
should be allowed to dry effectively once it gets wet, such as through space heating and ventilation.
Factory finishing with special coatings and sealers can provide temporary rainwater protection for wood
products such as glulam. But these products may also trap moisture and slow down drying if water gets
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into the wood. All engineered and prefabricated products such as Glulam require special care during
storage and transportation to prevent any structural damage.
Good construction sequencing can be used to protect wood and minimizing wetting, such as applying
wall and roof membranes as soon as the construction allows. In cases of roofs or even floors this would
mean potentially applying temporary or permanent waterproof membranes prior to erection to provide a
temporary construction roof and protect the underlying wood. Experience has also found that many
adhered and liquid WRB membranes as recommended within this Section do not adhere well to damp
wood or surface water run-off if exposed to rainwater while curing, additional reasons to pre-protect
wood elements in a tall wood building.
6.6.2
Exterior Wood and Preservative Treatment
Exterior exposed wood is subjected to a high risk of decay and other potential damages, in the mild and
rainy costal climates in particular, and therefore the materials must be carefully selected in design.
Wood pressure treated with preservatives must be used for all ground contact applications. Any aboveground wood framing or cladding should be separated from ground, with a minimal clearance of 150 to
200 mm. The bottom of the wood must be allowed to drain and dry if wetted. When wood is used in an
exterior application, whether it is preservative-treated or untreated, coated or uncoated, it is always most
effective to protect the wood from the exterior climate using design features such as overhangs and
canopies. For exposed above-ground applications, pressure preservative treated wood or naturally durable
wood species such as cedar and others previously mentioned should be used. This is required by building
codes for coastal areas of Canada. The building enclosure assemblies and details should always be
carefully designed to prevent moisture entrapment and encourage drainage and drying as covered within
this Section.
Like other materials, wood used exterior without any coating weathers naturally, a slow surface
deterioration process but fast in terms of appearance change resulting from the exposure to UV, water,
oxygen, visible light, heat, windblown particulate matter, atmospheric pollutants, and microorganisms. Light woods typically darken slightly and dark woods lighten, but all woods eventually end up
a silvery-grey colour. The surface will also roughen, check and erode, and have a “rustic” look. Coatings
are the most common way to reduce such deterioration and improve the aesthetic appearance. However,
all coatings have a limited service life and coating maintenance is critical to maintain its aesthetic
appearance and functions. Typically an opaque coating with a high pigment amount is better in protecting
the base material from damage caused by sunlight and moisture and therefore tends to last long, up to
several years or longer. On the other hand, a transparent or semi-transparent finish can better expose the
grain and texture of the base material, but tends to fail quickly and requires more frequent recoating. The
selection of coatings affects the maintenance needs and costs, particularly for less accessible locations,
and must be taken into consideration in the design and the maintenance plan (covered within Section 9.4).
When the wood is not naturally durable enough to prevent attack by decay fungi or insects in building
service, it can be treated with preservatives to improve long-term durability. Even for some massive
timber products, preservative treatment can be very beneficial to the durability of the buildings, for
locations with higher moisture risk in particular. For massive timbers such as CLT, manufacturers should
ensure that the water-based preservatives used before laminating do not adversely affect glue bonds or
that resin modifiers are added as needed. Non swelling oil-based treatments used for industrial glulam
post manufacture are not a preferred approach for buildings due to VOC emissions; most are not
registered for interior use. Non penetrating surface treatments are not likely to be effective against decay
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but may be effective against surface mold. Where moisture ingress will be highly localized and
predictable, boron or boron/copper rods can be used for local protection. In most cases boron rods should
be used in combination with a borate/glycol surface treatment and a film-forming coating to prevent
leaching if it could occur. The testing for the compatibility of wood treatments with building enclosure
membranes, adhesives, and sealants will need to be performed where treated wood will be in contact.
More information related to wood durability can be found at www.durable-wood.com and the
maintenance of tall wood structures is covered within Chapter 9.
6.7
Concluding Remarks
This Section provides guidance to assist practitioners in designing building enclosures for tall wood
buildings in Canada. It emphasizes moisture, heat, air control strategies and details to achieve the
durability and energy performance of the building enclosures and accommodate the increased
environmental and structural loads given the height of the building. Guidance on on-site moisture
management and the use of exterior wood is also provided in context of taller wood buildings and longer
construction schedules.
6.8
References
ASHRAE Standard 90.1 – Energy Standard for Buildings except Low-Rise Residential Buildings. 2010.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers. Available at:
www.ashrae.org.
APEGBC Technical Practice Bulletin: Structural, Fire Protection and Building Envelope Professional
Engineering Services for 5 and 6 Storey Wood Frame Residential Building Projects, Association of
Professional Engineers and Geoscientists of British Columbia, 2009.
Bradley, J.S., and Birta, J.A. 2000. Laboratory Measurements of the Sound Insulation of Building Façade
Elements.
IRC
Internal
Report,
IRC
IR-818.
Available
at
archive.nrccnrc.gc.ca/obj/irc/doc/pubs/ir/ir818/ir818.pdf
Building Enclosure Design Guide – Wood-frame Multi-Unit Residential Buildings. 2011. Homeowner
Protection Office, Branch of BC Housing. Available at: www.hpo.bc.ca.
The Case for Tall Wood Buildings – How Mass Timber Offers a Safe, Economical, and Environmentally
Friendly Alternative for Tall Building Structures, 2012. Michael Green. Available at www.cwc.ca.
City of Vancouver. 2005. City of Vancouver Noise Control Manual. Prepared by Wakefield Acoustics
Ltd. Available online: vancouver.ca/files/cov/noise-control-manual.pdf.
CMHC. 1981. Road and Rail Noise: Effects on Housing. Canada Mortgage and Housing Corporation.
Available at www.cmhc-schl.gc.ca.
Cross-Laminated Timber (CLT) Handbook (Canadian Edition). 2011. FPInnovations.
www.woodworks.org.
Available at
Cross-Laminated Timber (CLT) Handbook (US Edition). 2013. FPInnovations, Forest Products
Laboratory, BSLC, American Wood Council, APA, WoodWorks. Available at www.woodworks.org.
Finch, G., Wang, J., and Ricketts, D. 2013. Guide for Designing Energy-Efficient Building Enclosures –
for Wood-Frame Multi-Unit Residential Buildings in Marine to Cold Climate Zones in North
America. FP Innovations, Special Publication-53. Available online: www.fpinnovations.ca.
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Lstiburek, Joseph. Various. Builders Guide Climate Series. Building Science Press. Available at
www.buildingsciencepress.com
Oleszkiewicz, I. 1990. Fire Exposure to Exterior Walls and Flame Spread on Combustible Cladding, Fire
Technology, Volume 26, Issue 4, pp 357-375, November 1990.
Quirt, J.D. 1985. Controlling Sound Transmission into Buildings. Division of Building Research,
National Research Council Canada. Available online: archive.nrc-cnrc.gc.ca
RDH Building Engineering. 2013. Air Leakage Control in Multi-Unit Residential Buildings –
Development of Testing and Measurement Strategies to Quantify Air Leakage in MURBS. Report for
CMHC. Available online: www.rdhbe.com.
Straube, J. 2013. High Performance Enclosures: Design Guide for Institutional, Commercial and
Industrial Buildings in Cold Climates. Building Science Press. Available at
www.buildingsciencepress.com.
Straube, J., Burnett, E. 2005. Building Science for Building Enclosures. Building Science Press.
Available at: www.buildingsciencepress.com.
National
Building
Code
of
Canada
(NBC).
www.nationalcodes.nrc.gc.ca/eng/nbc/index.shtml.
National Energy Code for Buildings (NECB).
www.nationalcodes.nrc.gc.ca/eng/necb/index.shtml.
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2010.
2011.
NRC-CRNC.
Available
at:
NRC-CRNC.
Available
at:
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Prefabrication and Inspection of Assemblies
CHAPTER 7 Prefabrication and Inspection of Assemblies
Lead Author:
Co-Authors:
Peer Reviewers:
Gerald Epp
Richard Aarestad, John Boys, Bernhard Gafner, Ciprian Pirvu
John Bowser, Robert Drew, Sylvain Gagnon, Jens Hackethal, Ken Koo
Angela Lai, Thomas Leung, Robert Malcyck, Ghassan Marjaba
Abstract
Building multi-storey buildings with wood in the modern context is pioneering work – as importantly in
the construction aspects as in the design. It is believed that such construction will not be possible without
a large degree of prefabrication and the attendant planning, similar to what is common with structural
steel and precast concrete buildings.
Currently, there are few facilities in North America prepared to take on this responsibility. Further, with
wood, there are many possible means of building, including various forms of solid wood and engineered
wood panels, as well as beams and columns. Standard practices for construction, without knowing the
details of what may emerge, should thus be broad and performance-based.
This Chapter seeks to establish good practices and standards which can give confidence that what is
designed in accordance with the intent of relevant building codes, can in fact be built with confidence in
quality and with excellence. This is especially important for the projects which pioneer this field.
This Chapter is limited to structural aspects only, and is organized as quality and performance goals to be
met, and not detailed requirements, which will rather be the responsibility of the fabricator as part of their
planning submissions for each project.
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7.1
Preamble
Building multi-storey buildings with wood in the modern context is pioneering work – as importantly in
the construction aspects as in the design. It is believed that such construction will not be possible without
a large degree of prefabrication and the attendant planning, similar to what is common with structural
steel and precast concrete buildings.
Currently, there are few facilities in North America prepared to take on this responsibility. Further, with
wood, there are many possible means of building, including various forms of solid wood and engineered
wood panels, as well as beams and columns. Standard practices for construction, without knowing the
details of what may emerge, should thus be broad and performance-based.
This Chapter seeks to establish good practices and standards which can give confidence that what is
designed in accordance with the intent of relevant building codes, can in fact be built with confidence in
quality and with excellence. This is especially important for the projects which pioneer this field.
This Chapter is limited to structural aspects only, and is organized as quality and performance goals to be
met, and not detailed requirements, which will rather be the responsibility of the fabricator as part of their
planning submissions for each project.
7.2
General
Personnel of various professions and trades need to coordinate their work in all phases of construction.
The roles and responsibilities of the personnel vary depending on the type and complexity of the project,
experience and expertise of personnel, and available resources. Typical roles of the personnel involved in
the prefabrication and inspection of assemblies are defined below for information purposes, to help the
reader understand the concepts presented in this Chapter; however, the information should be taken with
care because sometimes the roles vary and responsibilities are shared within the team.
•
Registered Architect of Record (“AOR” henceforth) – A suitably qualified professional architect
registered in the jurisdiction where the project is being constructed. The AOR is engaged to be
the overall designer of the building and provides architectural specifications and drawings with
details sufficient for the fabricator and erector to price, plan, fabricate, and erect the structure;
reviews the shop drawings to confirm general compliance with the original design intent.
•
Registered Professional Structural Engineer of Record (“EOR” henceforth) – A suitably
qualified professional structural engineer registered in the jurisdiction where the project is being
constructed. The EOR is engaged to be the overall structural engineer for the building. He
provides structural engineering specifications and drawings with details sufficient for the
fabricator and erector to price, plan, fabricate, and erect the structure; reviews the shop drawings
to confirm general compliance with the original design intent.
•
Supporting Registered Professional Structural Engineer (“SRP” henceforth) – SRP engaged by
the fabricator and/or erector to develop the assemblies and connections as per structural
requirements outlined by the EOR. Provides detailed specifications and oversees production of
detailed drawings sufficient to fabricate and erect the structure. The SRP is responsible for
certifying that the fabrication of the assemblies and connectors is in compliance to his designs
when engaged by the fabricator. The SRP is responsible for the erection of the assemblies for the
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structure is executed in compliance of his erection sequencing and procedures when engaged by
the erector.
•
Fabricator – Develops the shop drawings with information needed by the manufacturing crew to
fabricate an assembly, and with information about erection of the assembly for the erector. The
fabricator employs a skilled manufacturing crew working in a manufacturing facility, which
prefabricates the assembly in accordance with reviewed and approved shop drawings.
•
Erector – Employs a skilled construction crew that assembles on-site the prefabricated structure
composed of prefabricated assemblies produced by the fabricator.
•
Quality Control Officer – In some cases, the owner may require that the Fabricator retains a third
party agent to provide audited quality control. The agent reviews and assists the Fabricator in
meeting the minimum quality requirements.
•
Building Official – Performs audits for fire safety, life safety, and structural sufficiency of a
building at different times during the construction process.
•
Specifier – Provides specifications including specifications for the fabricator about prefabrication
of an assembly. Specifications may also be for owner review, for cost estimation, and for
approval by code authorities. The specifiers may be the AOR, EOR, SRP and others as suitable.
•
Approver – AOR, EOR, SRP, and/or others, as may be required.
7.2.1
Qualification of Personnel
Both the fabricator and erector should prove to have extensive experience with fabrication and/or erection
of timber buildings, as applicable, of past successful building projects built to standards of excellence to
meet the requirements of Canadian codes. Such projects should show experience which is either directly
applicable to the project proposed, or can give strong indication of competence in performing same. The
SRP should be duly registered in the jurisdiction where the project is located, and should have extensive
experience in the type of work proposed, similar to the requirements for fabricator and erector given
above.
7.2.2
Quality Assurance Programs
Both the fabricator and erector should submit, in addition to drawing submission requirements given
below, evidence of companywide and project-specific quality assurance programs. Such programs should
indicate experience of key personnel which should be responsible to address quality and safety concerns
in both methods and outcomes, in order to show that the required standards are achievable. Such
programs are typically audited by third parties. Acceptable third parties should be defined in the
specifications.
7.2.3
Design Criteria for Prefabricated Assemblies
To allow the fabricator flexibility in achieving innovative and economical solutions, the specifiers should
supply a set of design performance criteria that must be met (in addition to the overall design where
applicable). These criteria should include architectural requirements as well as all climate and
environmental criteria, loading and performance expectations, with reference to applicable codes. Such
criteria should be indicated in the submission documents, and the SRP should be responsible for ensuring
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structural requirements are met. When an innovative and alternative solution1 is proposed in lieu of the
design by the EOR, it is the responsibility of the fabricator to provide creditable technical background
information with adequate test data to demonstrate that the proposed alternative solutions meets the
structural design performance criteria. The economic impacts for the EOR to evaluate these alternative
solutions should be the responsibility of the fabricator. Critical elements and connections pertaining to
the overall design of the building including load paths that cannot be altered should be address by the
EOR in the design documents.
7.2.4
Coordination and Fabrication Drawings / 3D Modelling
Well planned and error-free fabrication and erection drawings are key to control the production, logistics
and installation of a project. The complexity of modern timber buildings, including tall ones, places
excessive demands on 2D drawing methods of the past. Even current 3D tools like Autodesk® REVIT®
and other Building Information Modeling (BIM) software packages are not able to provide the data and
accuracy required for the production of prefabricated elements. A 3D manufacturing model is required for
final design and fabrication (“shop”) drawings. The BIM package used can and should however provide
the base data used for the manufacturing model. The design team should create a BIM execution plan
(including the level of detailing required for all consultants) that best addresses the accuracy required by
the fabricator. This includes the main architectural, structural, mechanical and electrical elements and
their interfaces. Ownership of the overall BIM model and its transfer rights have to be clearly addresses in
the specifications.
The manufacturing model also has to address the interfaces to other structural framing such as concrete,
masonry and steel. These interfaces and required / achievable tolerances should be communicated to these
trades via the general contractor or construction manager. It should be noted that default construction
tolerances such as CSA A23.1 governing concrete construction are incompatible with the demands of a
CNC fabricated structure.
Given the increasing complexity of building services, it is essential that structural aspects be coordinated
with these in an integrated fashion, again best achieved by use of the manufacturing model.
Early coordination with the erector is important to confirm the construction/erection sequence and details
of the prefabricated structure. The general contractor or construction manager should be included in that
process to ensure the interfaces to other trades and their construction sequencing are coordinated.
7.2.5
Testing for Design
Materials and systems testing prior to concealment or completion, where required, include not only the
structural aspects of prefabricated assemblies but also the fire, acoustic, mechanical/electrical, and
envelope details where applicable. Details about testing of systems for design are provided in Section 0.
Fire safety and building enclosure considerations are made in Chapter 5 and 0, respectively.
7.2.6
Submittals
Approval drawings are to be generated from the 3D model for submission. Specifiers should indicate in
tender documents what is required on such drawings. Fabricator submits the drawings sealed by the SRP
in charge of the project.
1
This differs from the alternative solution as discussed in Chapters 2, 4 and 5, which are “alternatives” to a code
acceptable solution. Here, the proposal is an alternative to a solution developed by the EOR.
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The fabricator should submit fully detailed and dimensioned layout drawings indicating all aspects of
construction of the prefabricated elements, including assurances that the design meets the required
structural performance criteria to the AOR, EOR and others as suitable for review.
Erection sequence, temporary supports, bracing requirements against all conditions during erection should
be clearly indicated on the erection drawings. Lifting connections and locations should be detailed on the
erection drawings along with indications of the lifting equipment to be used.
All structural component and connection detail drawings as well as erection drawings and information
should be under the seal of the SRP in charge of the project.
7.3
Fabrication
7.3.1
General
Fabrication and prefabrication are always recommended in controlled factory environment, under
continuous manufacturing processes, and following strict standard qualification and quality control (QC)
requirements. However, sometimes, large and complex structural assemblies are specified for a project.
Given the size, nature, location of the project and (most likely) limited number of required assemblies, the
fabricator may find it more feasible to prefabricate some assemblies on site. Regardless of whether an
assembly is factory-built or site-built, it is the responsibility of the fabricator to make sure the minimum
qualification and QC requirements that have been established to ensure acceptable performance are met.
Additional considerations need to be given to developing appropriate qualification and QC procedures for
assemblies prefabricated on site.
Aside from wood, other types of building materials will be used in the manufacture of prefabricated
assemblies for multi-storey buildings. Sometimes, premature material degradation or even failure may
occur when incompatible materials are placed adjacent or are connected to each other. Such
incompatibilities should be addressed by the specifier, fabricator, and builder early in the process.
Different materials have different thermal expansion coefficients; other materials may swell/shrink more
than others due to the seasonal changes in ambient humidity and temperature. Appropriate tolerances
should be specified and strictly followed in the manufacture of an assembly, and in the erection of a
prefabricated structure.
Development of appropriate qualification and QC procedures is the responsibility of the fabricator.
Qualification and QC data should be documented and approved by the AOR, EOR and others as suitable
in charge of the project. All details identified in the procedures shall be clearly understood by the design
team.
Qualification and QC procedures for prefabricated assemblies are presented generically in the following
two sections. Qualification and QC procedures tailored to structural glued wood assemblies that cannot
follow qualification requirements in standards or evaluation reports are shown in Appendix 7A.
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7.3.2
Qualification Procedures
Qualification procedures (Quality Assurance Procedures) for generic products, manufactured in
controlled factory environment and under continuous manufacturing processes, should follow the
applicable standard, such as CSA O177-06/O122-06 for structural glued-laminated timber or similar
standards for qualification of other products. Where standards do not exist, these standards are a good
source of information for establishing an appropriate performance criterion for qualification testing.
Qualification tests are intended to establish if the performance targets can be met with the components
and details selected. The SRP should understand the objectives of the tests and how best representative
samples should be generated for testing, especially when applying tests from standards that are outside
their scope. The SRP should oversee the evaluation and interpret the results.
Following are some principles for qualification of unique assemblies, manufactured in limited number
and tailored to specific projects, which do not follow the qualification requirements in standards or
evaluation reports.
•
•
•
•
7.3.3
Materials used in prefabricated assemblies should be of the exact specifications given by the
specifier. Replacements with lower quality or grade products should not be allowed as they may
affect performance.
Fabricator of elements or components of mechanically fastened assemblies should follow the
exact specifications for tolerances and product limits given by the specifier. Use of dry lumber
components (i.e. MC of 15%) will minimize shrinkage/swelling and consequent retightening or
premature repair of mechanically fastened connections. The allowance for minor checks and
splits of timber can follow, for example the allowed specifications outlined in the NLGA
Standard Grading Rules for Canadian Lumber (NLGA, 2010).
The manufacture process of prefabricated glued assemblies is sensitive to several adhesiverelated parameters, substrate-related parameters, and environmental-related parameters that need
to be controlled during the manufacture to ensure high quality bonds. A qualification procedure
for structural glued assemblies should define acceptable limits for the critical parameters, based
on the product performance criteria supplied by the specifier and the adhesive performance
criteria supplied by the adhesive manufacturer. The acceptable range for quality can then be
defined through preliminary testing by using combinations of the minimum and maximum
acceptable limits of the critical parameters. This step can be done in a third party testing facility
prior to the manufacture of the assembly.
A summary of the qualifications should be submitted to the AOR, EOR and others as suitable for
review prior to fabrication.
Quality Control Procedures
QC procedures for generic products, which are manufactured in controlled factory environment and under
continuous manufacturing processes, generally follow recognized standard requirements. The SRP shall
oversee QC procedures which are not covered by recognized standards.
Following are some principles for QC of unique assemblies, manufactured in limited number and tailored
to specific projects, which do not follow the QC requirements in standards or evaluation reports.
•
Check grade stamps, species, dimensions, engineering values (if applicable), etc. to verify that
elements or components for prefabricated assemblies meet the specifications.
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•
•
•
•
7.3.4
Ensure that the orientation and arrangement of elements or components in prefabricated
assemblies meet the specifications.
Inspect mechanical fasteners of prefabricated assemblies to ensure they are of the specified sizes
and grades, there are no missing fasteners, and they are correctly installed and to the
tightness/torque in the specifications.
A QC procedure checks whether the bond quality of a structural glued assembly is within the
acceptable quality range defined during qualification. The same manufacturing process for
prefabricated glued assemblies used in the qualification phase should be followed. QC involves
testing of small coupons taken from inconspicuous parts of the actual prefabricated assembly.
Testing of the small coupons can be done in a third party testing facility after the manufacture of
the actual assembly. Results should be sent to the SRP in a timely manner.
A summary of the quality control procedure should be submitted to the AOR, EOR and others as
suitable for review prior to fabrication.
Storage
As prefabricated, precision made structural elements require a certain degree of protection from the
elements, special provisions need to be made to store and transport such elements prior and during
erection. Fabricator should indicate how such provisions are met for the given project circumstances,
including storage of wood products on level supports and under protection from moisture, sunlight,
humidity, temperature extremes, etc. Information about durability and protection of wood materials is also
provided in 0.
7.4
Execution
7.4.1
Coordination
It is important to convene a meeting between all relevant parties prior to the fabrication phase to review
connections for constructability, logical scope breaks, confirm the construction schedule and sequencing,
and confirm everyone’s responsibility in the project. Access to the 3D model for all parties as outlined
under 7.2.4 is recommended.
The effort put into coordinating on-site activities can also help with reducing hazards on the construction
site and increase safety. Some information about fire safety on construction site is given in Chapter 5.
7.4.2
Transportation
In general, “just in time” delivery of all prefabricated parts and pieces to site is a good strategy. By
minimizing material storage on site, it can help to reduce site logistic problems and it decreases the risk of
site accidents. Further it decreases the potential for damage to the prefabricated assemblies and materials
due to weather and handling.
Just in time delivery is an optimal strategy, but truck loading, safety and efficient manufacturing will most
likely make it very challenging to avoid on site storage. Working “off the truck” presents challenges
regarding fall protection requirements as per the Occupational Health and Safety Act.
Fabricator /erector should clearly indicate the strategy proposed for transportation and storage, to suit the
requirements of the project. The EOR in charge of the project should approve the proposed strategy.
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7.4.3
Site Modifications
Site modification should be avoided where ever possible. Reality is that site modifications are usually
required. They should be pre-planned where possible (trim and infill pieces), identified as such in the
planning procedure / fabrication drawings and be approved by the AOR, EOR and SRP in charge of the
project.
Unforeseen site modifications should be approved by the he AOR, EOR and SRP in charge of the project
before executed.
7.4.4
Erection
Erection of the structure should be carefully planned for quality, durability and safety, and all erection
methods should be designed by the SRP and strictly followed by the erector.
A clear strategy is required to protect wood elements after they are installed and should be part of the
specifications issued by the AOR and EOR. They should then be identified on the submittal drawings
issued by the fabricator/erector. Potential risk are but not limited to:
•
•
•
•
Weather - water damage and excessive UV exposure.
Over rapid moisture change - relative humidity during construction needs to be carefully
managed.
Contamination of wood with other construction materials – concrete run offs, steel welding, etc.
Wood damages due to other trades – handling and moving of materials.
Erection drawings sealed by the SRP should be submitted prior to erection. Details about building
enclosure are provided in 0.
7.5
Inspection and Records
The AOR, EOR and SRP or his designate should do the site inspection of the prefabricated assembly so
as to take ownership of the inspection.
The erector should keep a log of items such as but not limited to:
•
•
•
Site deliveries including verified load manifests with notes of damaged or missing elements.
Element install log with sign off for QC on hardware/fastener installation
Log of any changes or modifications.
Where building officials require inspections during construction stage or post-construction stage, special
provisions should be made to accommodate this.
To ensure long term performance of the prefabricated assembly, regular inspections should be part of the
ongoing monitoring schedule. Such inspections should include an overall examination of the
prefabricated assembly and a detailed examination of the elements/components and connections of the
assembly. Details about monitoring and maintenance are provided in Chapter 9.
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7.6
References
NLGA Standard Grading Rules for Canadian Lumber. 2010. National Lumber Grades Authority. Surrey
BC.
CSA A23.1-09/A23.2-09 - Concrete materials and methods of concrete construction/Test methods and
standard practices for concrete. 5060 Spectrum Way, Suite 100, Mississauga, Ontario, Canada L4W
5N6.
CSA O122-06 - Structural Glued-Laminated Timber. 5060 Spectrum Way, Suite 100, Mississauga,
Ontario, Canada L4W 5N6.
CSA O177-06 - Qualification Code for Manufacturers of Structural Glued-Laminated Timber. 5060
Spectrum Way, Suite 100, Mississauga, Ontario, Canada L4W 5N6
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Appendix 7A
Qualification and Quality Control Principles for On-Site Prefabrication
of Structural Glued Wood Assemblies
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7A.1 General
Engineered wood products for structural purposes are typically mass-produced in controlled factory
environment, under continuous manufacturing processes, and following strict standard qualification and
quality control (QC) requirements. A product passing the qualification and QC requirements is expected
to meet the minimum performance requirements for such products in service.
Of critical importance for structural glued wood products is maintaining the integrity of the bond during
their service life. Structural/mechanical properties calculated by engineers are based on the assumption
that a product is well-manufactured and has high quality bonds, because strength of a glued wood product
is only as good as the quality of its bond. Bond quality assessment is part of every QC procedure for
structural glued wood products.
Often times, large and complex structural glued wood assemblies are specified for a project. It is always
recommended to prefabricate in controlled factory environment, under continuous manufacturing
processes, and following strict standard qualification and QC requirements. However, given the size,
nature, or limited number of required assemblies, the fabricator may find it more feasible to carry out the
final prefabrication steps on construction site. If the assembly is not mass-produced in a continuous and
controlled process, there needs to be an accepted procedure in place for qualification and QC assessment
to ensure the assembly has the quality needed to meet the performance requirements in service.
7A.2 Qualification Procedure
In a manufacturing process of a glued wood product, several adhesive-related parameters, substraterelated parameters, and environmental-related parameters need to be controlled to ensure high quality
bonds. For example, adhesive spread rate and uniformity, assembly time, pressure level (i.e. adhesiverelated parameters), wood quality, moisture content, surface temperature (i.e. substrate-related
parameters), ambient temperature and relative humidity (i.e. environmental-related parameters) are all
critical in the manufacturing process. While such parameters are controlled for products mass-produced in
continuous and controlled processes, it is challenging to control all these parameters when the assembly is
manufactured on site.
A qualification procedure for structural glued wood products manufactured on site should follow the
principles outlined in standards such as CAN/CSA O122 - Structural Glued-Laminated Timber. The
qualification procedure has to define acceptable limits for the critical parameters, based on the assembly
performance criteria supplied by the specifier and the adhesive performance criteria supplied by the
adhesive manufacturer. The acceptable range for the structural glued wood assembly quality can then be
defined through preliminary testing by using combinations of the minimum and maximum acceptable
limits of the critical parameters. This step can be done in a third party testing facility prior to the final
fabrication steps of the assembly on site.
7A.3 Quality Control Procedure
A QC procedure should check whether the bond quality of a structural glued wood assembly whose final
fabrication steps occur on construction site is within the acceptable quality range defined during
qualification. QC involves testing of small coupons taken from inconspicuous parts of the assembly
prefabricated on site. Testing of the small coupons taken from the prefabricated assembly can be done in a
third party testing facility after the fabrication of the assembly on site.
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Project and Construction Costing
CHAPTER 8 Project and Construction Costing
Lead Author:
Co-Authors:
Joe Rekab
Richard Aarestad, Olivier Barjolle, Gerry Epp, Sylvain Gagnon, Angela Lai, Ashley Perry
Peer Reviewers:
Kevin Below, John Davidson, Vince Tersigni
Abstract
This Chapter of the Guide advises on preparing capital budgets for tall wood buildings, providing
guidelines for developing realistic cost estimates, identifying challenges and constraints, as well as
proposing procedures and techniques to manage risks. Although the focus of the Guide is on tall wood
buildings, the information is applicable to non-traditional wood construction, particularly those using
Mass Timber. The Canadian Institute of Quantity Surveyors (CIQS) Elemental Cost Analysis Format 4th
Edition is used to describe the impact, if any, on design and construction costs.
Major challenges and constraints include the lack of specific costing data (particularly when using Cross
Laminated Timber, or structural composite lumber products as Mass Timber elements), , a limited
number of suppliers with the capacity to undertake such a project, and a substantial initial cost premium
associated with innovation and risk on a first-of-its-kind structure. Other issues include transportation
and distance to site, site construction challenges, storage, and weather protection of Mass Timber
elements.
The cost advantages of prefabrication, standardization and modular systems are identified, reporting
potential cost savings in high-level percentage terms as well as schedule savings. It also reports on a
qualitative review of constructability, and notes the benefits of off-site prefabrication (consistent quality
and rapid construction), ease of deconstruction, and potential for re-use of Mass Timber and its
advantages in scoring LEED points.
Given the novelty of the undertaking, the use of Agency Construction Management forms of contract or
an Integrated Design Process, followed by a Design-Build form of contract are recommended for their
promise in yielding the optimum competitive pricing. It is also advisable to seek contractors with
experience in Mass-Timber buildings or large pre-cast concrete structures and have the ability to deliver
large projects using an Integrated Design Process (IDP) and Building Information Modelling (BIM).
A Basic Framework for Developing an Integrated Cost Estimating and Cost Control System is presented.
This includes a process for developing a cost estimate for tall wood buildings, with the recommendation
that the design team (architect, structural engineer, code consultant and mechanical engineer) should
invest a significant amount of time discussing early stage design documents. This will help ensure
everyone has a thorough understanding of the proposed building. Once all the documentation and project
information is assembled, a list of items should be entered into an elemental standard format for the
preparation of estimates.
The Guide further provides guidelines for estimating soft (or non-construction) costs such as professional
fees, permits, municipal charges, insurance and financing charges.
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8.1
Introduction
Developing reliable budgets for construction projects incorporating innovative technology presents a
challenge to developers, government agencies and their consultants. Risks associated with new
technology may hinder their introduction, even when there are compelling reasons that make them
desirable. Allaying the risk of cost overruns through careful budgeting and cost monitoring is, therefore,
a crucial step in realising innovative projects and, through them, advancing the options available to the
construction industry.
The purpose of this Chapter of the Guide is to provide those preparing capital budgets for tall wood
buildings with guidelines for developing realistic cost estimates. The challenges and constraints of cost
planning for innovative technology are outlined and procedures and techniques to manage risks are
proposed.
This Chapter’s primary purpose is to provide guidelines for consultants tasked with estimating costs of
tall wood buildings higher than six storeys. The Subsections establish a landscape of known and
unknown factors that may have an impact on design and construction costs.
Our findings in gathering and generating cost information reflect key components that should be
considered when assembling estimates for mass timber buildings. We have used the Canadian Institute of
Quantity Surveyors (CIQS) Elemental Cost Analysis Format 4th Edition to describe the impact on costs, if
any, that the use of mass timber would have on each specific element.
8.2
Knowledge Gaps in Costing Tall Wood Buildings
To date, no tall wood building higher than six storeys has been built in Canada that uses the latest in glued
mass timber products. This lack of real-world experience leaves knowledge gaps in estimating the costs of
mass timber structures, particularly those contemplated to be built with Cross-Laminated Timber (CLT)
panels or large Structural Composite Lumber (SCL) elements.
Much of the information now available as well as the gaps were identified in The Case for Tall-Wood
Buildings: How Mass-Timber Offers a Safe, Economical, and Environmentally Friendly Alternative for
Tall Building Structures. This landmark study, released in February 2012, provided a comprehensive
overview of project costs based on preliminary design drawings.
The study’s cost analysis covered 12-storey and 20-storey mass timber buildings, providing high-level,
elemental cost analyses in the CIQS Elemental Cost Analysis Format, 4th Edition for both structures. The
study concluded that constructing a tall wood building using mass timber could be cost competitive when
benchmarked against cast-in-place concrete structures. The study identified further savings when schedule
and carbon credits, if accessible, were considered.
In the report, the key cost drivers identified included:
•
•
•
•
Substructure – a mass timber building’s foundations are likely to be lighter;
Finishes – additional wallboard is required on ceilings to achieve an acceptable fire rating;
General Requirements – less on-site management is required than with cast-in-place concrete;
Schedule – construction is faster for lighter, pre-fabricated panels.
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Some key issues, however, are not addressed in detail but should be noted by project teams, including:
•
•
•
•
•
•
•
•
•
•
•
•
•
8.2.1
A shortage of manufacturers in the market capable of supplying such products;
A lack of historical data for cost estimating;
Connection mechanism suppliers are few;
Higher insurance costs due to innovative technology;
Comparison with steel frame construction;
Cost differentials due to location factors;
Issues related to crane installation in high-rise construction;
Intumescent paint for steel components providing structural support of wood members;
Temporary weatherproofing requirements of mass-timber components;
Core drilling and cutting of wood panels (as sleeves cannot be placed as in cast in place concrete
slabs.);
Temporary bracing requirements; and
Installation of mechanical and electrical services in ceilings as compared to laying into concrete
slabs;
Flexibility of design to accommodate functional use changes once structure has been erected.
Availability of Data
The use of mass timber, in particular CLT, for a tall wood building is relatively uncharted territory. Since
a project of the scope for which these guidelines are intended has not yet been attempted, specific costing
data is limited. While suppliers of mass timber products are key sources of unit rates for material,
installation and implementation, costs are unlikely to be reliably defined until a project taller than six
storeys is delivered.
Despite this limitation, manufacturers of CLTs are the best source of cost information in Canada,
especially supply-only costs. Costs can be obtained for standard-sized panels of varying thicknesses but
will usually be exclusive of transportation costs. Some manufacturers also have experience installing the
panels and can be a valuable source of information regarding assembly methods and durations and the
costs that they entail. Given the lack of unit–rate data assembled over a series of projects, a detailed
understanding of all the components and other resources required can provide a valuable basis for
preparation of an estimate.
The best available Canadian data are from projects under six storeys that have been completed in BC and
Quebec. Despite interest from developers, the material has yet to be used in the Prairie Provinces, the
Maritimes and Central Canada.
8.2.2
Cost of Innovation
Field research and market surveys conducted with key contractors and sub-contractors have focused on
three areas:
•
•
•
Market appetite for taking on an innovative project;
Premium for research and innovation; and
Commercial opinion on profit levels.
From the responses, it has been concluded that:
•
•
A champion will be required to construct the first tall wood building in excess of 10 storeys.
Contractors will include a high risk premium to cover unknowns.
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•
•
Public education will be required to alleviate fire and structural safety concerns.
Building life-cycle and longevity will need to be addressed.
As construction of a tall wood building is a relatively new undertaking, there would be a substantial initial
cost premium associated with innovation and risk. Contractors will be pricing less competitively than
with traditional structural frames of similar height. Cost drivers of this innovation premium include:
•
•
•
•
•
•
•
8.2.3
Risk associated with building with a combustible product;
Risk associated with market acceptance;
Lack of competition;
Lack of historical data;
Lack of designer experience with wood assembly details, connections, and tie-in with non-wood
components;
Construction methodology; and
Prefabrication and limitations with panels cut to size.
Market Premium for Learning Curve
A survey of major construction firms identified a lack of skilled labour familiar with this type of
construction as a key factor in raising costs and, potentially, causing delays (see the appendix to this
Chapter). The consensus from responding firms in British Columbia, Alberta, Saskatchewan and Quebec
was that with increased training and use of mass timber, these costs would continue to decline.
Concerns affecting the market premium identified in the surveys included:
•
•
•
•
•
•
•
•
•
•
•
8.3
Typical exterior and interior finishes to be used with CLT;
Market appetite;
Contractors’ appetite for adopting a new technology;
Heating and hoarding required;
Premium on CLT due to lack of availability;
Panel size limitations;
Lack of familiarity with CLT installation;
Expectations from building operators/managers from performance, maintenance, and adaptability
perspective;
Ability to meet equivalent steel/concrete durability, longevity, and performance;
Separation of supply and installation contracting; and
Storage of the wood products
Procurement
Contractors and manufacturers in North America and overseas have been surveyed exploring both options
and challenges for procurement of contractors and material. In many cases, given the specialized nature
and early stage of mass timber building, the manufacturer is also the installer.
8.3.1
Procurement of Contractors
The authors of the surveys focused on establishing guidelines for contractor selection and the best
methods of procuring a contractor to get competitive pricing for a project of this scale. Selecting a
competent contractor will largely depend on previous experience with successful mass timber installation
or similar structures. There are several construction companies in Canada that have experience in
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constructing buildings with mass timber products. There are also numerous contractors with the capability
to undertake construction of tall wood buildings, which should assist in establishing a more competitive
process.
Guidelines for prequalifying contractors may include as a minimum:
•
•
•
•
•
Experience with currently completed mass timber buildings (e.g. CLT and Glulam);
Completion of large pre-cast concrete structures;
Access to and availability of a skilled labour pool;
Ability to act as an experienced construction manager to deliver large projects through the
Integrated Design Process (IDP) and Building Information Modelling (BIM); and
Demonstrated experience and interest in construction using innovative methods.
Given the novelty of this technology to the construction industry, the use of Agency Construction
Management forms of contract or the use of an Integrated Design Process, followed by a Design-Build
form of contract, would likely yield the best competitive pricing from the industry. Analysis of these
procurement options should be reviewed as more projects are completed.
8.3.2
Procurement of Material
The investigation of the current landscape for sourcing and pricing material for Tall-Wood Buildings
extended across North America and included European and Asian sources of supply. While expanding,
the number of suppliers with the capacity for a project of the scope under consideration in this guide is
still limited.
8.3.2.1
Availability of Material
In the Canadian marketplace there are currently three large suppliers of mass timber products (include
Glulam, etc.); two are based in British Columbia and one in Quebec. This does not include suppliers of
SCL, which can be manufactured in mass timber sizes. There are also several smaller suppliers entering
the Canadian market, but these plants have low production capacity. Outside Canada, there are several
manufacturers supplying mass timber. These manufacturers are based in the U.S., Germany, Austria and
Finland.
Confirmed availability of supply is essential to successful delivery of a project of the scope contemplated
in this guideline. Over time, manufacturers in the U.S., Europe and Asia will likely drive down prices and
increase the volume of production, but such adjustments would depend on market interest.
8.3.2.2
Transportation of Material
Distance and transportability of prefabricated large assemblies that may be composed of CLT, SCL or
other wood components are two chief cost-related concerns for material procurement. Because of the
potential size and weight of the elements, there are two main factors regarding transportation that must be
considered when planning the project: highway regulations and construction site limitations. The
distance from the point of manufacture to the building site naturally affects costs. There would also be
significant scheduling challenges on a project in a rural location if and when just-in-time delivery was
used.
Transporting such large elements can be costly, and depending on the size of the elements, may require
specialized transportation services. Prefabricated assemblies are typically transported by flatbed truck.
Typical crane and hoisting techniques should be utilized to ensure timely and safe construction similar to
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pre-cast systems. Designers should consider the size of the assemblies, as this will affect the ability to
deliver to sites and will have an impact on cost. The capacity of flatbed trucks should also be considered
when designing panel size in order to prevent logistical problems with transportation.
8.3.2.3
Construction Site Limitations and Considerations
Transporting Mass Timber elements to the construction site is only part of the challenge. The construction
site itself may have restrictions that are more limiting than weights and dimension regulations. First, the
design team should make sure that the route from the plant to the construction site will allow movement
of the truck, including its load, without any obstacles. This is especially critical for oversize loads.
A common problem at construction sites occurs when a long trailer arrives and the width of the driving
space (which was fine for a short dump truck) does not allow enough clearance for the off-tracking of the
rear trailer wheels when a short radius turn is needed. Moving a fence, a shed, piles of materials, for
example, to make driveway changes can disrupt and delay deliveries and increase costs.
This can be a challenge when working in tight urban areas where the space for piling building materials
and the allowance for turns is very limited. The off-tracking is a function of the sum of the squares of the
vehicle combination wheelbases so an extra-long trailer will intrude inward on a tight turn much more
than shorter wheelbase trailers. A data chart and other methods to estimate off-tracking (SAE J 695) are
available from the Society of Automotive Engineers.
Awareness of local city regulations and pre-planning to match construction site challenges are advisable
to ensure a smooth, efficient delivery without delays and cost overruns.
8.3.2.4
Storage of Materials
Weather protection is one of the more important issues when dealing with CLT. Compared to glulam,
CLT has more exposed end-grain and is generally manufactured with the narrow faces of the laminations
unglued. These features provide more points for moisture to penetrate the panel. This is discussed
further in Chapter 9. Manufacturers advise that the product must be protected from the elements during
transportation, storage and prior to and during installation.
Given the size of the panels, any laydown yard or storage facility would require cover, an unusual
requirement in the industry. Depending on the location of the project and climatic conditions,
consideration should be given to applying a factory installed peel-and-stick protective membrane to roof
panels to protect the product. The balance of the non-roof panels is normally shrink-wrapped at the
manufacturer’s plant. This additional step in the prefabrication process would mitigate weather delays
and reduce construction costs.
8.3.2.5
Temporary Protection during Construction
When necessary, the wood components should be protected as much as possible against the elements
during frame set-up operations. The wood elements are primarily intended for use in dry conditions with
limited exposure to water, so they should be protected from direct rain, snow and ice, especially from
long exposure to these elements. Otherwise, the wood may tarnish or become dirty during construction.
In addition, due to the hygroscopic nature of wood, prefabricated wood elements, such as CLT, may
change slightly in size as a result of moisture content changes during construction and problems can occur
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at joints. For example, connections can be difficult to perform on the construction site, if allowance is not
made in the detailing of the connections.
As wood is a natural breathing material, moisture content control and acclimatization will require careful
consideration, such as specifically manufacturing in a moderately humid climate and then transporting to
a low humidity climate for installation.
8.4
Quantifying Schedule Benefits of Prefabrication
This section investigates the cost advantages of prefabrication, standardization and modular systems.
Potential cost savings (in high-level percentage terms), as well as schedule savings, were identified and a
qualitative review of constructability was also carried out. Off-site prefabrication can generally be
expected to provide consistent quality and contribute to rapid construction. But other considerations can
affect construction time.
8.5
•
Availability of adequate site storage. This is a key factor in the use of prefabricated large
wooden elements including CLT panels. Sites with limited storage space for mass timber
products may affect the schedule. Site availability will have an impact on how fast or how much
product can be manufactured in advance of use. Panelizing off-site will have the additional cost
of transport. (See Sections 8.3.2.2 and 8.3.2.3).
•
Detailing of wood to steel connections. This can also have a significant impact on the shop
drawing process as well as an impact on construction costs. This type of connection is usually
bulky and requires a significant amount of labour to install. It also engages another supplier with
its own logistics of supply, manufacture and delivery.
•
Utilization. The higher the repetition of the building systems, the lower the cost for supply as
well as installation. Increased repetition also reduces material handling and storage requirements
and panels can be delivered to site “just in time”. When properly executed, installation of prefabricated elements can yield significant reductions in total construction time. The process is also
far less likely to be interrupted by inclement weather as compared with concrete or steel
structures.
•
Architectural requirements and/or engineering detailing. Both can slow the rate of production
and contribute to higher costs
•
Construction detailing. Challenges and delays can arise when sealing joints for fire-rating and
acoustic separation between walls and floors.
Disposal Costs and Opportunity for Re-Use/Recycling
Mass timber can potentially be as simple to deconstruct as to erect. Large members can be inspected and
evaluated for re-use in another application (Section 0) provides some guidance on evaluation). If needed,
members can easily be cut and machined.
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In factory fabrication, wood waste material may be used for energy generation. The opportunities for reuse of a wood product can score LEED Points and potentially earn additional credits for Inno

90% draft - GHL Consultants Ltd.

Transcription

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