Buckling Restrained Braced Frames

Buckling-Restrained Braced Frames
by
Walterio A. López, SE
Rafael Sabelli, SE
Rutherford & Chekene
Walter P Moore
Buckling-Restrained Braced Frames
(BRBFs)
• Code Intent
• How BRBs work
• Brief History of BRBFs in US Codes
• Sample BRBF Construction
• Brief treatment on testing
• Building-Code Design
• Design Methodology
• Specification, Other Issues
• Gusset Connections
• Summary
Code Intent
Building-Code Philosophy
Objective:
Prevent collapse in the extreme
earthquake likely to occur at a
building site.
Objectives are not to:
limit damage
maintain function
provide for easy repair
To survive a strong earthquake
without collapse:
Design for Ductile Behavior
Material Ductility
Member Ductility
System Ductility
AISC Methodology
Designate fuses
Members that undergo inelastic strain
Provide ductility in fuse members
Prevent local buckling
Prevent member instability
Prevent connection failure
Design system to ensure ductility is concentrated
in fuses
How BRBs work
What is a Buckling-restrained Brace?
Two Definitions
Stress
resisted by
steel core
Buckling
resisted by
sleeve
De-Coupled Stress and Buckling
(Mechanics Definition)
Balanced Hysteresis
(Performance Definition)
BRB Definitions Explained:
Conventional Bracing
Brace behavior is
asymmetric with
respect to tension
and compression
and is subject to
strength and
stiffness
degradation
Tension
Ry Ag Fy
Pcr
Compression
BRB Definitions Explained:
Sleeved Column
2
λc ~ 0
kl/ ~ 0
r
Compression Strength
Steel core achieves Fy A
π EI
2
L
Fy A
Sleeve achieves π2EI/L2
Stress is zero
No material stress limit
0
1
2
Slenderness Parameter λc
3
Brief History of BRBFs
in US Codes
Historical Background
1st BRBF paper: 2000 SEAOC Convention
BRBF design presentations:
SEAOC: 2001-2006
NASCC: 2004, 2005
Steel TIPS Seminars: 2004
ASCE Structures Congress: 2005
AISC braced frame seminars: 2005, 2006
BRBFs in U.S. to date: >100 bldgs, >15,000
BRBs
Background (recent past/present)
SEAOC/AISC BRBF committee
Background (present)
Sample BRBF Construction
Sample Construction
Sample Construction
Sample Construction
Buckling-Restrained Brace Types
Buckling
Restrained
Brace
PowerCat
Brace
ACME
Bracing
Company
Unbonded Brace
Buckling-Restrained Brace Assembly
Core
Buckling-Restrained Brace Assembly
Sleeve
Buckling-Restrained Brace Mechanics
Unbonded
Brace Type
Encasing
mortar
Yielding steel
core
Decoupling
Debonding material
between steel core and
mortar
Steel tube
Buckling
Restraint
Brief treatment on testing
Selected Testing Data
Literature
Reference
Test Type
Number
of Tested
Braces
SIE, 1999
Uniaxial
3
2.1
SIE, 2001
Uniaxial
2
2.1
UC Berkeley, 2002
Frame
(Subassemblage)
3
1.8 - 2.1
Merritt et al., 2003a
Subassemblage
6
2.4 - 2.7
Merritt et al., 2003b
Subassemblage
8
1.8 - 2.6
Merritt et al., 2003c
Uniaxial
2
1.6, 1.7
SIE, 2003
Subassemblage
4
1.6 – 3.0
Brace
Strain
(%)
BRB Tests Short Summary
•
About 50+ different brace tests have been
performed in support of US projects
•
All tests results so far have met Appendix T’s
acceptance criteria
•
Tests have included Appendix T, moment frame,
near-field, and fatigue displacement protocols
•
Kinematic rotations of brace ends were not
detrimental to brace performance
Building-Code Design
ASCE 7 2005 (with Supplement 1)
R Values
7 for Basic BRBF System
8 for BRBF System with Rigid Beam-Column
Connections
8 for BRBF/SMF Dual System
ASCE 7 2005 (with Supplement 1)
Ωo Values
2 for Basic BRBF System
21/2 for BRBF System with Rigid Beam-Column
Connections
21/2 for BRBF/SMF Dual System
ASCE 7 2005 (with Supplement 1)
Cd Values
51/2 for Basic BRBF System
5 for BRBF System with Rigid Beam-Column
Connections
5 for BRBF/SMF Dual System
ASCE 7 2005 (with Supplement 1)
Height Limits
Separated by Seismic Design Category:
B&C D
E
F
NL
160* 160 100 for Basic BRBF System
NL
160* 160
100
for BRBF System with Rigid
Beam-Column Connections
NL
NL
NL
for BRBF/SMF Dual System
NL
(NL = Not Limited)
*Can be increased to 240 for regular buildings.
ASCE 7 2005 (with Supplement 1)
Coefficients for Determination of
Approximate Period
Ta = Cr (H/ft.)x
Cr
= 0.03
(ASCE to incorporate)
x
= 0.75
(Similar to EBF)
Combined effect of R and T
Design Base Shear
SCBF
BRBF
SCBF Building
BRBF Building
Period
Design Methodology
Design Procedure
Define appropriate BRB modeling
Determine required brace strength
Check drift
Determine brace displacements at 2.0 Δm
Compare required displacements to existing tests
Plan and conduct new tests?
Determine adjusted BRB strengths at 2.0 Δm
Requires test data or manufacturer’s summary
Calculate required strength of columns, beams, and
connections based on adjusted BRB strengths
BRBF Design Methodology
• BRB is energy dissipater
• Steel core material
specified as mild & ductile
• Design checks:
• BRB φPn
• Global drift
• BRB deformation, ΔbM
• Adjusted BRB strengths
•
•
•
Beam Ru/φRn
Column Ru/φRn
Connections Ru/φRn
Analysis
Gravity Load
Size frame to resist 100% of gravity
All load combinations
Do not model braces as resisting gravity load
Check that braces do not yield under Live Load
Seismic Load
Size braces for seismic load only
Do not model braces to resist gravity load
Size for 100% of seismic load?
Or consider shear in columns
Found by analysis
Size frame considering plastic mechanism
Design Summary
Load Combination
Gravity
Seismic
1.2D + 1.6L 1.2D + 0.5L + E
Frame
Braces
Design for maximum
Design for 100% of brace forces, plus 100% of
load
gravity
Check to make
sure live load does
not cause (cyclic)
yielding
Design for seismic force
from analysis; do not
include gravity
Brace Stiffness
Kbr = P/Δ
Δ ~ PLy/AyE
Ly = 0.5-0.8 L
(depending on
brace type and
configuration)
Kbr = 1.2 - 2.0 AyE /L
Ly
Flexibility E A
sc
L Ly
EA
nonyielding
BRB Modeling
Kbr
= 1.3 AscE /L ?
Kbr
= 1.6 AscE /L ?
BRB Modeling (Nonlinear)
Isotropic and kinematic strain hardening
Difference in tension/compression values
Modified DRAIN, PERFORM 3D
Steel Core Material
• Specifications
• ASTM A36 Grade 36/42
• JIS G3136 SN400B
• Wide range of yield strength not desired
• Solution: supplementary yield strength
requirements verified by coupon tests
• Current practice: material procured based on
MTRs, coupon tests performed prior to
fabrication
Preliminary BRB Design
F
Pu =
F
2 cosθ
Asc ≥
Pu
φFysc
Assume braces
resist 100% of
story shear
Design braces to
calculated capacity
(Pu = φPn = φFyscAsc)
θ
BRB Axial Deformation Check
Compute elastic story drift ΔX
Extract from analysis program ΔbX = Δbrace at ΔX
story drift
BRB Axial Deformation Check
ΔbX is computed at largest elastic story drift
(ρ = 1.0 for drift)
Compute ΔbM = Cd ΔbX = Δbrace at ΔM story drift
Compute max. brace strain εMAX= 2.0ΔbM / Lysc
εMAX cannot exceed maximum value tested
If εMAX exceeds tested values, resize BRB
BRB Axial Deformation Comparison
For a ASCE 7 earthquake (2/3 of MCE)
2.0 Δbm ~ 10 Δby (elastic methods, Ch. 16)
Mean = 9-11 Δby (Sabelli, Fahnestock)
For a 2%/50 year event
Not addressed in codes
Mean = 17-19 Δby (Sabelli, Fahnestock)
• Ductilities underestimated but not forces
• Solution: fabricate BRBs to Δby larger than
predicted by elastic methods
Plastic Mechanism
All braces yielding
Tension or compression
Strain Hardened
“Adjusted strength”
= Maximum force
Based on first mode
BRB Adjusted Strength
Compression: βωRy Fysc Asc
Tension:
ωRy Fysc Asc
Adjusted for Various Factors
ω Strain-Hardening
β Compression Overstrength
Ry Material Overstrength
If Fy is used as core yield strength Fysc, Ry is > 1.0
If Fysc is taken from material coupon test, Ry = 1.0.
BRB Adjusted Strength
Factors
Factors Taken from Test Results within 2.0
Δm.
Compression Strength Adjustment Factor
β = Cmax/Tmax
Strain-Hardening Adjustment Factor
ω = Tmax/FyA
Provided by brace manufacturers
BRB Uniaxial Test Results
Hysteresis courtesy of SIE, Inc.
BRB Adjusted Strength (example)
εMAX = 0.98 % at 2.0ΔbM
Go to graph from BRB manufacturer
and obtain:
ω = 1.22
ωβ = 1.25
β = ωβ/ω
= 1.25/1.22
= 1.03
BRB Adjusted Strength
Case at inverted-V beam
Frame Design: Model BRB Forces
Directly
ωRyFyscAsc
βωRyFyscAsc
βωRyFyscAsc
ωRyFyscAsc
ωRyFyscAsc
βωRyFyscAsc
βωRyFyscAsc
ωRyFyscAsc
ωRyFyscAsc
βωRyFyscAsc
Column flexural forces not calculated
Combine with 1.2D + 0.5 L + 0.2 Sds D
Frame Design: Model BRB forces Using
Temperature
E = 1 ksi
α = 1/oF
ΔT = ωRyFysc
Tension
ΔT = βωRyFysc
Compression
Axial force approximation
Column flexural forces not calculated
Combine with 1.2D + 0.5 L + 0.2 Sds D
Specification, Other Issues
Use of Proprietary BRBs
Engineer Specifies:
Brace Strength
Brace Core Area (or stiffness)
Maximum and Minimum Fy
Displacement range
Manufacturer Provides:
Braces that meet the specification
Test data that qualifies the braces
Typical Specification of BRB Size- ASC
Uncertainty in strength (example)
Calculations
φPn
Ry
= 0.9Aysc (38 ksi)
= 46 ksi/38 ksi = 1.21
Drawings
Asc
= 8.5 in.2
(for example)
Specifications
38 ksi
≤ Fysc ≤ 46 ksi
Manufacture
Asc
= 8.5 in.2
323 kips ≤ Pysc ≤ 391 kips
Proportioning of strength likely similar to design
Alternate Specification of BRB Size- Pysc
Uncertainty in stiffness (example)
Calculations
φPn
= 0.9Asc Fysc
Ry
Asc
= 1.0
= φPn /0.9 (44 ksi) [reasonably low stiffness for analysis]
Pysc
= 323 kips
Drawings
Specifications
where Fysc is measured during manufacture
and Asc is adjusted accordingly
(= Pu /φ)
38 ksi ≤ Fysc ≤ 46 ksi
Manufacture
Pysc
= 323 kips
7.0 in.2 ≤ Asc ≤ 8.5 in.2
Proportioning of stiffness likely similar to design
Construction Administration
General contractor
Steel fabricator
BRB
Detailer
Drawing
Exchange
Fabricator
Detailer
Coordinated submittals:
BRBs, gusset plates, frames
Code Issues
•
•
•
•
•
•
BRB is a better brace that doesn't buckle.
BRB is a performance-specification item.
Single diagonals in one direction and stacked
chevron allowed without penalty.
BRB and gussets often need not be fireproofed.
If manufactured in approved shop, inspections
may be waived.
Non-structural attachments to casing not
prohibited.
Gusset Connections
Sample Connections
Alternative Connections
Direct welding of core
Direct bolting of core
Courtesy of
CoreBrace
Courtesy of
STAR Seismic
Gusset Plate Design Issues
• Adjusted BRB strengths readily
determined from backbone
curve (first validation of
methodology)
• Frame fixity must be
acknowledged in analyses
• Recognize that cyclic testing of
gusset plates not fully
developed
• Avoid unnecessary connection
restraint
Potential Connection Issues
Beam (or column) yield at <1%
Rotation ductility not tested
These issues apply to all gussets at large
drift
SCBF and OCBF drift likely to be greater than
BRBF
EBF rotations may be much greater
Potential Connection Issues
Courtesy of K.C. Tsai
Pin Connection
Courtesy of
L. Fahnestock
Summary
BRBF Design Summary
• BRB is energy
dissipater
• Check BRB ductility
demands
• Check surrounding
elements for adjusted
BRB strengths
Overall Summary
BRBs harness steel ductility to provide
member ductility
BRBF provide a ductile system if
Connection failure is precluded
Braces are proportioned to earthquake demand
Frame is designed for plastic mechanism
Braces are properly specified.
Thank You