Introduction to the 2005 AISC Seismic Provisions

Transcription

AISC Live Webinar: Introduction to the 2005 AISC Seismic Provisions
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I-1
Today’s AISC Live Webinar
Introduction to
the 2005 AISC Seismic Provisions
Today’
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written and presented by
Thomas A. Sabol, Ph. D., S.E.
Principal, Englekirk & Sabol
Consulting Engineers, Inc, Los Angeles,
CA.
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American Institute of Steel Construction
1
Webinar
Version
Seminar Highlights
Introduction to
2005 AISC Seismic Provisions
Seminar addresses selected, key content from:
Seismic Provisions for Structural Steel Buildings
(ANSI/AISC 341-05)
Part I
Prequalified Connections for Special and
Intermediate Steel Moment Frames for Seismic
Applications (ANSI/AISC 358-05)
Seismic Design Manual (First Edition, 2006)
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Seminar Highlights
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Seminar Highlights
2005 Seismic Provisions (ANSI/AISC 341-05)
2005 Seismic Provisions (ANSI/AISC 341-05)
Presents seismic design and detailing requirements
for different structural steel systems
National, consensus standard referenced in 2006
model building codes
NEW: Combines Allowable Strength Design (ASD)
and Load and Resistance Factor Design (LRFD) into
a unified format
NEW: Introduces design provisions for Buckling
Restrained Braced Frames (BRBF) and Special Plate
Shear Walls (SPSW)
NEW: Introduces quality assurance and special
welding requirements for steel seismic systems
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2
Seminar Highlights
Seminar Highlights
Moment Frame Connection Prequalification
Standard (ANSI/AISC 358-05)
Moment Frame Connection Prequalification
Standard (ANSI/AISC 358-05)
First national consensus standard to replace FEMA
350 for design of moment frame connections
(FEMA 350 is a moment frame connection design
guideline developed after 1994 Northridge
Earthquake based on multi-year research program)
Provides design requirements, design limitations,
and design procedures for:
• Reduced Beam Section (RBS)
• Bolted End Plate (BEP) connections
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Seminar Highlights
Supplement 1 (2009) contains
liberalized requirements for BEP and
new provisions for Bolted Flange Plate
(BEP), Welded Unreinforced FlangeWelded Web (WUF-W), and Kaiser
Bolted Bracket (KBB)
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Seminar Highlights
Seismic Design Manual (Second Printing, 2006)
Seismic Design Manual (Second Printing, 2006)
Resource to help designers apply 2005 Seismic
Provisions and Prequalified Connection Standard
Provides practical examples to illustrate
• basic seismic concepts in structural steel
Contains a copy of 2005 Seismic Provisions and
Prequalified Connection Standard
• design examples for braced frames, moment
frames, and other system components
But, without Supplement 1
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3
Questions
Seismic Design Manual
Please ask (type-in) questions when
they occur to you – don’t wait until
the end of the seminar!
We may not be able to answer
every question, but all of them
help us understand what
content might not be
sufficiently clear.
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•
limit damage
•
maintain function
•
provide for easy repair
Conventional Building Code Philosophy
To prevent building collapse, design for
ductile behavior
Earthquake Load, V
Conventional Building Code Philosophy
Objective: Prevent collapse in the extreme
earthquake likely to occur at a building site.
Objectives are not to necessarily:
Ductility = Inelastic Deformation
V
Δ
Deformation, Δ
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4
As required elastic strength goes down (i.e. larger “R”factor) required inelastic deformation increases
As required elastic strength goes down (i.e. larger “R”factor) required inelastic deformation increases
Completely elastic
response
Completely elastic
response
Velastic
0.75Velastic
Δ
0. 5Velastic
V
0.25Velastic
Δyield
Δmax
Deformation, Δ
Earthquake Load, V
Earthquake Load, V
Velastic
As elastic design load
decreases, required
inelastic deformation
increases
0.75Velastic
0. 5Velastic
V
0.25Velastic
Δyield
Δmax
Deformation, Δ
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Seismic Provisions attempt to develop ductile
behavior in steel seismic systems
Ductility is provided by yielding
Earthquake Load, V
Fracture or instability reflect non-ductile behavior
V
Ductility = Inelastic Deformation
Δ
Δ
Choose frame elements ("fuses") that will yield in an
earthquake:
• Beams in moment resisting frames
• Braces in concentrically braced frames
• Links in eccentrically braced frames, etc.
Failure (fracture
or instability)
Deformation, Δ
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5
Detail "fuses" to sustain large inelastic deformations
prior to the onset of fracture or instability
Design all other frame elements to be stronger than
the fuses
• Detail fuses for ductility
• All other frame elements develop the plastic
capacity of the fuses
• Generally, this means other elements remain
elastic or nearly elastic
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Alternatively, in some areas of the country, you may
design to a higher force (i.e. use R = 3) and you do
not have to detail the seismic elements as required
You can’t use R > 3
by the Seismic Provisions.
and skip the
Thus, you must either
seismic detailing!
• Use R > 3 and seismic detailing from Seismic
Provisions
• Use R = 3 and you need not use seismic detailing
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Seismic Provisions
6
Organization of the Seismic Provisions Document
Major emphases of this Seminar

Part I: LRFD and ASD Provisions
Part I of AISC Seismic Provisions

Part II: Composite Structural Steel and Reinforced
Concrete Buildings
Moment frames and braced frames

Commentary for Part I and Part II
R > 3 seismic system requirements
An unappreciated
resource in the AISC
Seismic Provisions
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Seismic Provisions
Seismic Provisions
Part I Contents

Symbols

Glossary

Part I Contents (continued)

4. Loads, Load Combinations, and Nominal
Strengths
1. Scope

2. Referenced Specifications, Codes and
Standards
5. Structural Design Drawings and Specifications,
Shop Drawings and Erection Drawings

6. Materials

3. General Seismic Design Requirements

7. Connections, Joints and Fasteners

8. Members
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Seismic Provisions
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Seismic Provisions
7
Moment
frame
systems
Provisions Specific to Steel Seismic Systems
Braced
systems
Provisions Specific to Steel Seismic Systems

9. Special Moment Frames (SMF)

13. Special Concentrically Braced Frames (SCBF)

10. Intermediate Moment Frames (IMF)

14. Ordinary Concentrically Braced Frames (OCBF)

11. Ordinary Moment Frames (OMF)

15. Eccentrically Braced Frames (EBF)

12. Special Truss Moment Frames (STMF)

16. Buckling-Restrained Braced Frames (BRBF)

17. Special Plate Shear Walls (SPSW)
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New shear
wall system
Seismic Provisions
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Seismic Provisions
Other Sections/Appendices
Other Sections/Appendices
Appendix S: Qualifying Cyclic Tests of Beams-toColumn and Link-to-Column Connections
Appendix T: Qualifying Cyclic Tests of BucklingRestrained Braces
Appendix W: Welding Provisions
Appendix X: Weld Metal/Welding Procedure
Specification Notch Toughness Verification Test

18. Quality Assurance Plan

Appendix P: Prequalification of Beam-to-Column
and Link-to-Column Connections

Appendix Q: Quality Assurance Plan

Appendix R: Seismic Design Coefficients ad
Approximate Period Parameters
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Seismic Provisions
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Seismic Provisions
8
Glossary
1. Scope
Terms listed in glossary are generally
italicized where they first appear in a
subsection
Seismic Provisions intended for use in buildings
and “other structures”
“Other structures” have vertical and lateral systems
similar to buildings and are designed, fabricated and
erected in a manner similar to buildings
Seismic Provisions apply when R > 3 or when
otherwise required by the building code
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e.g. for cantilevered
column systems where
R = 2.2
Seismic Provisions
1. Scope
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Seismic Provisions
1. Scope
Seismic Provisions
not required in
“gray”areas
Seismic Provisions are used in conjunction with
AISC Specification for Structural Steel Buildings
(ANSI/AISC 360-05, March 9, 2005)
Seismic Provisions focus on seismic issues
Defers to the Specification for available and
nominal strength, etc. for most elements
Shows where
Seismic Provisions
are required based
on Soil Class
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Seismic Provisions
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Seismic Provisions
9
3. General Seismic Design Requirements
4. Loads, Load Combinations, and Nominal Strengths
Seismic Provisions defer to applicable building
code for
Required seismic strength (see slides on Section 4
for exception)
Determination of Seismic Design Categories and
Occupancy
Design story drift limits
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Seismic Provisions
4.1. Loads and Load Combinations
Seismic Provisions
Applicable Building Code
Determines loads and load combinations for required
strength of steel seismic systems using provisions in
ASCE 7 except Seismic Provisions may impose
additional requirements…
… except Seismic Provisions may impose
additional requirements:
When demand from one member can impose
higher loads on another member
0.9D + 1.0E (note that E is assumed to have both a
positive and negative sign in this combination)
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Seismic Provisions
Investigates
presence of
“net tension”
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Seismic Provisions
10
This is not the
same as Ω (ASD
Factor of
Safety)
Determines overstrength factor, Ωo, to multiply
horizontal earthquake load, E, when amplified
seismic loads are required by the Seismic
Provisions
Ωo is estimate of maximum load that can be
imposed on a member by another member
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Seismic Provisions
5. Structural Design Drawings and Specifications, Shop
Drawings, and Erection Drawings
Overstrength factor, Ωo, is estimate of maximum load
that can be imposed on a member by another member
Pseudo “mechanism load”
Tries to account for “unaccounted strength”
in seismic system
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Seismic Provisions
5.1 Structural Design Drawings and Specifications
The engineer, not the contractor or inspector, is in
the best position to know which components
are part of the seismic system and which
require special consideration
The engineer must communicate the design intent
to the contractor and inspector via the structural
design drawings
Significant change
in 2005 Seismic
Provisions
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Seismic Provisions
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Seismic Provisions
11
Structural design drawings need to indicate
Type of Seismic Load Resisting System (SLRS) (e.g.
SMF, EBF, etc.)
Member/connection material specifications
and sizes
Members and connections that are part of SLRS
Location of “demand critical welds”
Sections 5.2 and 5.3 contain similar requirements for
shop and erection drawings
Configuration of the connections
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Welds likely to
experience inelastic
demand – See
Section 7.3b
Seismic Provisions
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Seismic Provisions
6. Materials
Location and dimensions of protected zones
Welding requirements as specified in Appendix W,
Section W2.1
Locations in seismic
system with special
limitations related to
fabrication and
attachments – See
Section 7.4
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Seismic Provisions
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Seismic Provisions
12
6.2 Material Properties for Determination of Required
Strength of Members and Connections
6.1. Material Specifications
Specified minimum yield strength (Fy) for
members with anticipated inelastic behavior
shall not exceed 50 ksi (unless suitability is
proven by testing)
Limitation does not apply to columns where
inelastic behavior is assumed to be limited to
column base.
When specified in Seismic Provisions, required
strength shall be based on “Expected Yield
Strength,” RyFy, of an adjoining member
Underlying assumption is that actual yield strength
is greater than minimum specified strength
In seismic design, it is not appropriate (i.e. not
“conservative”) to underestimate demand on one
member created by another
e.g. Ru =
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Seismic Provisions
6.2 Material Properties for Determination of Required
Strength of Members and Connections
Table I-6-1(Abridged)
Ry and Rt Values for Different Member Types
Material Specification
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Seismic Provisions
7. Connections, Joints and Fasteners
New in 2005:
Used for
“tensile
strength”
Ry
Rt
ASTM A36 (shapes)
1.5
1.2
ASTM A572 Gr. 42
1.3
1.1
ASTM A500 HSS
ASTM A53 ( Pipe)
ASTM A36 (plate)
ASTM A992 (shapes)
1.4
1.6
1.3
1.1
1.3
1.2
1.2
1.1
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Seismic Provisions
RyFyAg
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Seismic Provisions
13
7.2. Bolted Joints
7.2. Bolted Joints
All bolts in SLRS shall be pretensioned highstrength bolts (i.e. no A307 bolts)
Bolts and welds shall not be designed to share force
in a joint or same force component in a connection
Faying surfaces shall be prepared as slipcritical with a Class A surface
Even though you prepare joint as if it were
“slip-critical,” you may use the higher bolt
“bearing” values (with some exceptions)
Faying surface
is where steel
plies come into
contact
Bolts
Line of action of
vertical force
Vertical force (and
possibly the horizontal
force) is resisted by
bolts and welds, but
designed so that either
welds or bolts take total
load
Welds
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Seismic Provisions
7.3. Welded Joints
Seismic Provisions
7.3a. General Requirements
Welding shall be performed in accordance with
All welds in members and connections within
SLRS shall use filler metal with minimum CVN
value of 20 ft-lbs at 0oF*
Appendix W
Welding Procedure Specification (WPS) per AWS
D1.1 and approved by the Engineer of Record
*See Section 7.3b
for additional CVN
requirements for
demand critical
welds
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Seismic Provisions
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Seismic Provisions
14
7.3b. Demand Critical Welds
Where frame is normally at 50oF or higher (i.e. most
conditioned structures), all welds designated as “demand
critical” shall use filler metal with minimum CVN value of
Although demand critical welds are identified in the
Seismic Provisions, there may be other welds that
warrant this designation by the designer.
20 ft-lbs at -20oF
40 ft-lbs at 70oF per Appendix X
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Seismic Provisions
7.4. Protected Zone
Examples of demand critical welds in SMF and IMF
include following CJP groove welds:
Welds of beam flanges to columns
Welds of single plate shear connections to columns
Welds of beam webs to columns
Columns splice welds, including column bases and
tapered transitions
Example
“demand
critical”
welds
Seismic Provisions
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Seismic Provisions
Certain areas of a seismic system are designated
as “protected zones”
Within the protected zone:
Welded, bolted, screwed or shot-in attachments
for perimeter edge angles, exterior facades,
partitions, duct work, piping, or other
construction are prohibited
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Seismic Provisions
15
7.4. Protected Zone
8. Members
Location of protected zones in a moment frame
Protected zones
in a moment
frame
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Seismic Provisions
Seismic Provisions
8.2. Classification of Sections for Local Buckling
8.2b. Seismically Compact
Seismic performance of members in the SLRS
may require yielding and high levels of inelastic
deformation
To facilitate this demand, Seismic Provisions
specify for selected members that they be
compact, λp (Specification Table B4.1) , or
seismically compact , λps, (Seismic Provision
Table I-8-1)
Seismically compact
limits, λps, for
“unstiffened elements”
(e.g. flanges of wide
flange sections)
b = bf /2
bf
More stringent than
Specification
requirements
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Seismic Provisions
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Seismic Provisions
16
8.2b. Seismically Compact
Tables 1-2 through 1-6 of Seismic Design Manual
list structural sections that satisfy local
buckling requirements (both “compact” and
“seismically compact”) for SMF, SCBF, and
EBF systems
h
Seismically compact
limits, λps, for
“stiffened elements”
(e.g. webs of wide
flange sections)
tw
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Seismic Provisions
8.3. Column Strength
λps limits for Wide Flange Sections
When axial loads on seismic columns are “large,” the
Seismic Provisions require that these columns
satisfy additional requirements.
This section satisfies local buckling
requirements for all listed applications
(shown by “•”)
This section does not satisfy local buckling
requirements for indicated application
(e.g. SMF beam and column)
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Seismic Provisions
17
Special requirements shall be met when
Pu/φcPn > 0.4 (LRFD) or
ΩcPa/Pn > 0.4 (ASD)
φc =
Pa =
Pn =
Pu =
Special requirements shall be met when
Pu/φcPn > 0.4 (LRFD) or ΩcPa/Pn > 0.4 (ASD)…
Without using Ωo to
calculate Pa or Pu for
checking the load to
strength ratio
0.9 (LRFD)
Ωc = 1.67 (ASD)
Required axial strength of a column using ASD load
combinations
Nominal axial strength of a column
Required axial strength of a column using LRFD load
combinations
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Seismic Provisions
If ratios are exceeded, axial compressive and
tensile strength, considered in absence of applied
moment, based on amplified seismic load
(i.e. if Pu/φcPn > 0.4 use Ωo if required by the
applicable building code load combinations)
If you “fail” the
test you then
have to use Ωo
to calculate Pa
or Pu
8.4. Column Splices
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Seismic Provisions
8.4a. General
Centerline of splice made with fillet welds or PJP welds
shall be located 4 ft. or more from beam-to-column
connections or at column mid-height, whichever is less
4 ft. or more
from connection
or at column
midheight
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Seismic Provisions
Column
splice
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Seismic Provisions
18
8.4b. Columns Not Part of the Seismic Load Resisting
System
9. Special Moment Frames (SMF)
Column splices in columns not in SLRS:
Splice required shear strength with respect to both
orthogonal axes shall be Mpc/H (LRFD) or Mpc/1.5H
(ASD), where Mpc is based on the appropriate
direction of applied load
Mpc
Mpc
Vu
H

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Seismic Provisions
Seismic Provisions
9.1. Scope
9.1. Scope
SMF are expected to withstand significant
inelastic deformations (R = 8) when subjected to
design an earthquake
Basic Design Procedure
Calculate demands based on building code
Analyze frame
Size “fuses” (i.e. frame girders)
Size other members so fuses will govern
Confirm that frame satisfies drift criteria
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Seismic Provisions
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Seismic Provisions
19
9.2. Beam-to-Column Connections
9.2a. Requirements
All beam-to-column connections in SLRS shall
satisfy:

An interstory drift angle at least 0.04 radian
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Seismic Provisions
Seismic Provisions
9.2a. Requirements
9.2a. Requirements
All beam-to-column connections in SLRS shall satisfy:
Deformed shape
of test specimen

θ
Δ
Measured flexural resistance of connection, at face of
column, is at least 80% of Mp of connected frame
beam at interstory drift angle of 0.04 radian
Interstory Drift Angle
θ =
Beam Moment at Face of Column (in-kips)
40000
Δ
Lbeam
Lbeam
M 0.04 ≥0.8 M p
30000
0.8 Mp
20000
10000
0
-10000
-20000
- 0.8 Mp
-30000
-40000
-0.08
II-79
Seismic Provisions
M 0.04 ≥0.8 M p
-0.06
-0.04
-0.02
0
0.02
Interstory Drift Angle (rad)
0.04
0.06
0.08
II-80
Seismic Provisions
20
9.3. Panel Zone of Beam-to-Column Connections (beam
web parallel to column web)
9.2b. Conformance Demonstration
Requirements of 9.2a shall be satisfied by one of
the following:
SMF connection recognized by Prequalified
Connection Standard (ANSI/AISC 358)
Qualifying tests per Appendix S of Seismic
ProjectProvisions
Relevant tests reported in the literature
specific
Relevant project specific tests
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Seismic Provisions
Seismic Provisions
9.3a. Shear Strength
Panel zone must be strong enough to resist
demand from connecting beam without
excessive deformation
Yielding of panel zone recognized as an efficient
method of providing ductility
Required strength (shear) based on demands
generated by beams framing into column
Beam 1
Mf1
db -tf
Panel
zone
Mf1
db -tf
Beam 2
Vc
Mf1
Mf2
Vc
Panel Zone Required Shear Strength = R u =
II-83
Seismic Provisions
Mf2
db -tf
Mf2
db -tf
∑M
f
(d b − t f )
− Vc
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Seismic Provisions
21
9.4a. Width-Thickness Limitations
When Pu ≤ 0.75 Py in column, shear strength of
panel zone:
Where:
dc
=
column depth
db
=
beam depth
bcf =
(AISC Spec EQ J10-11)
tp
db
Use φ =
1.0
⎡ 3b t 2 ⎤
Rv = 0.6Fy d ct p ⎢1 + cf cf ⎥
⎣⎢ d b d ct p ⎦⎥
Beam and column members shall meet requirements
of Section 8.2b (i.e. seismically compact per Table
I-8-1), unless otherwise qualified by tests
column flange width
dc
tcf
=
column flange thickness
Fy
=
minimum specified yield stress of column web
tp
=
thickness of column web including doubler plate
tcf
bcf
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Seismic Provisions
Seismic Provisions
9.6. Column-Beam Moment Ratio
Strong Column – Weak Beam provision is intended to
prevent global frame instability rather than prevent
yielding of individual columns
Delaying column yielding helps force beam yielding
at multiple levels and provides greater overall
frame stability
Use Fy
for
column
M*pc-1
Use
1.1RyFy
for
beam
M*pb-1
M*pc-2
∑M
∑M
II-87
Seismic Provisions
M*pb-2
*
pc
*
pb
> 1.0
Note:
M*pc is based on minimum specified
yield stress of column
M*pb is based on expected yield stress
of beam and includes allowance for
strain hardening
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Seismic Provisions
22
9.7. Lateral Bracing at Beam-to-Column Connections
Exception: Eq. 9-3 need not apply if either (a) or (b) is
true:
(a) Columns aren’t too heavily loaded and (i) they are
located at the roof or (ii) there aren’t too many
columns that don’t satisfy Eq. 9-3
(b) Columns are sufficiently strong compared to the
columns on the floor above
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II-90
Seismic Provisions
9.8 Lateral Bracing of Beams
Both flanges of beams shall be laterally braced.
Unbraced length between lateral braces shall not
exceed Lb = 0.086ryE/Fy
Braces need to possess sufficient strength and
stiffness (Appendix 6 of Specification)
Lateral
torsional
buckling
These photographs show lateral
torsional buckling in frame girders.
This behavior can twist the column
out-of-plane unless the column is
adequately braced (see Section
9.7a.). Required frame girder bracing
is discussed in Section 9.7b.
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Seismic Provisions
Seismic Provisions
Lb ≤ 0.086ryE/Fy
Lateral bracing
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Seismic Provisions
23
Both flanges of beams shall be laterally braced.
Required strength of lateral braces provided
adjacent to plastic hinges:
Plastic hinge
Strength of bracing >
0.06Mu/ho
Lateral bracing provided by fullheight perpendicular framing
Bracing adjacent to
plastic hinge
Lateral bracing provided by
shallow perpendicular framing
and stiffener
II-94
II-93
Seismic Provisions
Seismic Provisions
9.9. Column Splices
Required strength of lateral braces provided
adjacent to plastic hinges:
When splices are not made with CJP welds
required flexural strength based on smaller column
RyFyZx
(LRFD)
Splice not made with
CJP (e.g. fillet welds
or bolts)
Lateral bracing at RBS provided
by structural slab
(ASD)
Mu = RyFyZx
Lateral bracing provided angles
– check stiffness of bracing)
(Note: deck not in place)
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Seismic Provisions
RyFyZx/1.5
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Seismic Provisions
24
9.9. Column Splices
When splices are not made with CJP welds
required shear strength based on
ΣMpc/H
ΣMpc/(1.5H) (ASD)
(LRFD)
Mpc1
Vu
Mpc2
H
where ΣMpc is sum of nominal plastic flexural
strengths of columns above and below the splice
II-97
II-98
Seismic Provisions
Seismic Provisions
Seismic Design Manual Table 4-1
Comparison of Requirements
for SMF and IMF Systems
Special Moment Frame
(SMF)
Special Moment Frames
(SMF)
Intermediate Moment Frame
(IMF)
Interstory Drift
0.04 radian
0.02 radian
Connection Flexural Strength
80% of nominal plastic moment of
the connection at interstory drift
angle of 0.04 radian
80% of nominal plastic moment of
the connection at interstory drift
angle of 0.02 radian
Vu for load combination 1.2D +
0.5L + 0.2S plus shear from
application of moment of
2[1.1RyFyZ/distance between
plastic hinge locations]
Vu for load combination 1.2D +
0.5L + 0.2S plus shear from
application of moment of
2[1.1RyFyZ/distance between
plastic hinge locations]
Connection Shear Strength
― or ―
― or ―
Lesser Vu permitted if justified by
analysis
Lesser Vu permitted if justified by
analysis. See also the exception
provided in Seismic Provisions
Section 10.2a. II-99
Seismic Provisions
Intermediate Moment Frames
(IMF)
For Pr < 0.75Pc
No additional requirements
beyond AISC Specification
Panel Zone Shear Strength
with φv = 1.00
Rn = Per Specification Eqn. J1012, with φv = 1.00
Panel Zone Thickness
t > (dz + wz)/90
Continuity Plates
To match tested condition
To match tested condition
Beam-Column Proportion
II-100
Seismic Provisions
25
11. Ordinary Moment Frames (OMF)
11.1. Scope
OMF are expected to withstand minimal inelastic
deformations (R = 3.5) in their members and
connections when subjected to design
earthquake.
Model codes place significant limits on where
OMF may be used
II-101
II-102
Seismic Provisions
11.1. Scope
Seismic Provisions
11.2a. Requirements for FR Moment Connections
Maximum Building Height per Seismic Design Category per
2006 International Building Code
Seismic
Design
Category
A or B
C
D
E
F
Maximum
Height
No limit
No Limit
Not
permitted1,2
Not
permitted1,2
Not
permitted1,2
Notes
1.
2.
OMF may be used in a single story building ≤ 60 ft. tall with bolted end
plates and roof dead load ≤ 15 psf and any dead load of any wall > 35 ft. is
≤ 15 psf
OMF may be used in a building ≤ 35 ft. tall with roof, floor and wall dead
load ≤ 15 psf
II-103
Seismic Provisions
Special weld
access hole
Figure 11-1 Special Weld
Access Hole Geometry
II-104
Seismic Provisions
26
11.3. Panel Zone of Beam-to-Column Connections (beam
web parallel to column web)
No additional requirements beyond those in the
Specification
11.4. Beam and Column Limitations
No additional requirements beyond those in
Section 8.1 of Seismic Provisions
II-105
II-106
Seismic Provisions
11.5. Continuity Plates
When FR connections use welds of column flanges
to beam flanges or beam-flange connection
plates, continuity plates shall be provided
Continuity plates also required when
t cf < 0.54 bf t bf Fyb / Fyc
t cf < bbf / 6
tcf
No requirements.
tbf
bbf
or when
Seismic Provisions
II-107
Seismic Provisions
II-108
Seismic Provisions
27
Specification
11.8. Lateral Bracing of Beams
Specification
II-109
II-110
Seismic Provisions
1.1 Scope
ANSI/AISC 358
Prequalified Connections for Special and Intermediate
Moment Frames for Seismic Applications
Supplement 1 issued
June 2009
www.aisc.org/freepubs
Seismic Provisions
Provide design, detailing, fabrication, and quality
criteria for special and intermediate moment
frames
To be used as prequalified connections with Seismic
Provisions
Not intended to preclude use of other connections
tested per Seismic Provisions Appendix S
III-111
III-112
28
5. Reduced Beam Section (RBS) Moment Connection
5.1 General
In reduced beam section (RBS), portions of beam
flanges are selectively trimmed in a region
adjacent to beam-to-column connection
Yielding and hinge formation are intended to
occur primarily within the RBS
Trimmed
(reduced)
flange
5.3.1 Beam Limitations
Beams shall satisfy the following limitations
Beams shall be rolled wide-flange or built-up Ishaped members conforming to Section 2.3
Beam depth is limited to W36 (and equivalent for
built-up shapes)
Beam weight is limited to 300 lbs/ft
Depth: W36 x max or equivalent for built-up member
Weight: 300 plf max
Reduced beam
III-115
section
Reduced Beam
Section
Yielding in RBS
III-114
5.3.1 Beam Limitations
Beams shall satisfy the following limitations
Beam flange thickness is limited to 1.75 in.
Clear span-to-depth ratio is limited to
7 or greater for SMF and 5 or greater for IMF
For same drift angle,
greater beam depth
requires larger extreme
fiber strain
Depth
III-113
Clear span
III-116
29
5.5 Beam Flange to Column Flange Weld Limitations
Weld access hole geometry shall conform to
requirements of Section J1.6 of AISC Specification
(i.e. not the special weld access hole)
5.6 Beam Web to Column Connection Limitations
For SMF:
Beam web shall be connected to column flange with a
CJP weld extending between weld access holes
Single plate shear connection, with minimum thickness
of 3/8 in., may be used as backing
III-117
III-118
5.6 Beam Web to Column Connection Limitations
5.8 Design Procedures
For IMF:
Procedures outline steps to design RBS connection
Note that currently there is no HSS or weak-axis
wide flange RBS connection that has been
prequalified
c
Beam web shall be connected to column
flange per requirements for SMF
Exception:
Bolted web connection using single
shear plate is permitted
Bolts shall be designed as slip-critical
Nominal bearing strength at bolt holes
per Section J3.8 of AISC Specification
b
a
RBS Dimensions
III-119
III-120
30
IV-121
IV-122
Seismic Provisions
13.1. Scope
13.1. Scope
SCBF are expected to withstand significant
inelastic deformations (R = 6) when subjected to
design earthquake.
SCBF are expected to have increased ductility
compared to OCBF because negative
consequences caused by strength degradation
in buckled OCBF compression braces is
minimized
IV-123
Seismic Provisions
Seismic Provisions
Preferred mode of behavior: tension brace yielding
F
RyFyAg
Δ
Consider maximum
effects due to brace
force (RyFyAg)
IV-124
Seismic Provisions
31
13.1. Scope
13.1. Scope
Preferred mode of behavior: compression brace buckling
F
Unfavorable modes of behavior
Connection fracture
RyPn,
Column buckling
0.3Pn
Beam failure
Δ
Consider maximum effects due
to brace force (sometimes P =
RyPn, sometimes P = 0.3Pn)
IV-125
IV-126
Seismic Provisions
13.1. Scope
Seismic Provisions
13.2b. Required Strength
Basic Design Procedure
Calculate demands based on building code
Analyze frame
Size “fuses” (i.e. braces)
Typical
example:
slotted HSS
Where effective net area of bracing is less than
gross area, required tensile strength of brace
based on limit state of fracture in the net section
shall be greater than the lesser of:
Expected yield strength, in tension, of bracing
RyFyAg/1.5 (ASD)
member: RyFyAg (LRFD)
Size other members so fuses will govern
Maximum load effect indicated by analysis that can
be transferred to brace by the system
IV-127
Seismic Provisions
ΩoQE does not
satisfy this
requirement
IV-128
Seismic Provisions
32
132b. Required Strength
Typical
example:
slotted HSS
132b. Required Strength
gross area…
Objective is to yield gross section of brace prior to
fracture of net section
gross area…often requires local strengthening
of the brace
Slot needs to be
neatly radiused to
avoid brittle fracture
Plate added to each
side to compensate
for slot
IV-129
IV-130
Seismic Provisions
Seismic Provisions
13.2c. Lateral Force Distribution
13.2c. Lateral Force Distribution
Along any line of braces, braces shall be deployed
in alternate directions such that, for either
direction of force parallel to bracing, at least
30% but no more than 70% of total horizontal
force is resisted by tension braces unless…
F
F
Δ
Braces oriented in alternate
directions
F
F
Δ
Braces oriented in same
direction
IV-131
Seismic Provisions
IV-132
Seismic Provisions
33
13.2d. Width-Thickness Limitations
Column and brace members shall meet
requirements of Section 8.2b (i.e. seismically
compact per Table I-8-1)
For rectangular HSS (A500 Gr B steel) there are many
sections that will not satisfy Table I-8-1:
b
E
29000 ksi
≤ 0.64
= 0.64
= 16.1
t
Fy
46 ksi
Examples of brace buckling shows local
buckling (and fracture) at the mid-length
of the brace
IV-133
Seismic Provisions
IV-134
Seismic Provisions
13.3. Required Strength of Bracing Connections
b
≤ 16.1
t
(There aren’t a lot
of them)
IV-135
Seismic Provisions
IV-136
Seismic Provisions
34
13.3a. Required Tensile Strength
13.3b. Required Flexural Strength
Required tensile strength of bracing connections
(including beam-to column connections if part
of bracing system) shall be lesser of:

Expected yield strength of bracing member,
RyFyAg (LRFD)
RyFyAg/1.5 (ASD)

Maximum load effect, indicated by analysis, that
can be transferred to brace by the system
ΩoQE does not
satisfy this
requirement
In direction brace will buckle, required flexural
strength of connection shall be equal to
1.1RyMp (LRFD) or 1.1RyMp/1.5 (ASD) of brace
about critical axis.
Exception: Brace connections are permitted that:

Brace
Satisfy Section 13.3a,
tensile
Can accommodate inelastic rotations capacity
associated with post-buckling deformations
IV-137
IV-138
Seismic Provisions
Seismic Provisions
Plastic
Hinges
Fixed-End
Braces
P
P
M
P
1.1RyMp-brace = 1.1RyFyZbrace
P
Plastic Hinge
M
1.1 Ry Mp-brace
Plastic hinges form at
ends and mid-length
of brace. Brace
imposes moments on
connections and
adjacent members
IV-139
Seismic Provisions
P
Pin-ended Braces
For "pinned" end braces: flexural plastic hinge will
form at mid-length only. Brace will impose no
bending moment on connections and adjoining
members.
Must design brace connection to behave like a "pin"
IV-140
Seismic Provisions
35
2t
2t
Fold line
Fold line
>2
t
Fold line is free to form: OK
Fold line is NOT free to
form: NG
IV-141
Seismic Provisions
13.4. Special Bracing Configuration Requirements
IV-142
Seismic Provisions
13.4a. V-Type and Inverted V-Type Bracing
V-Type and Inverted-V-Type braced frames
Undesirable behavior
IV-143
Seismic Provisions
M
em
be
r
Strong beam member
mobilizes tension brace
once compression brace
buckles
Te
ns
io
n
Te
ns
io
n
M
em
be
r
Weak beam member
neutralizes tension
brace once compression
brace buckles
Desired behavior
IV-144
Seismic Provisions
36
V-Type and Inverted-V-Type braced frames
Two-story braces
eliminate the need to
design this beam to
support the unbalanced
vertical brace load
V-Type and Inverted-V-Type braced frames shall
meet following requirements:
For load combinations that include earthquake
effect on beam, E shall be determined as follows

Forces in tension braces shall be assumed to
equal RyFyAg

Forces in all adjoining braces in compression
shall be assumed equal to 0.3Pn
Two-story braces
IV-145
IV-146
Seismic Provisions
Seismic Provisions
13.4b. K-Type Bracing
V-Type and Inverted-V-Type braced frames shall meet
following requirements:
K-Type braced frames are not permitted for SCBF.
Wgravtity = 1.2D + 0.5L
( Ry Fy Ag + 0.3 Pn ) cos θ
0.3 Pn
θ
Ry Fy Ag
( Ry Fy Ag - 0.3 Pn ) sin θ
Beam is designed to
support gravity
load, horizontal
axial load, and
unbalanced vertical
load without relying
on braces
IV-147
Seismic Provisions
K-type braced frame
(not permitted)
IV-148
Seismic Provisions
37
13.6. Protected Zone
where ΣMpc is sum of nominal plastic flexural
strengths of columns above and below the
splice
Protected zone at
gussets
Protected zone
on braces at
expected hinges
L

50% of available flexural strength of smaller
connected section.
Required shear strength shall be ΣMpc/H (LRFD)
or ΣMpc/(1.5H) (ASD)
d

L/
4
In addition to meeting requirements of Section 8.4,
column splices in SCBF shall:
d
13.5. Column Splices
Miscellaneous attachments (cladding, plumbing,
etc.) not permitted in the Protected Zone
IV-149
IV-150
Seismic Provisions
14. Ordinary Concentrically Braced Frames (OCBF)
Seismic Provisions
14.1. Scope
OCBF are expected to withstand limited inelastic
deformation (R = 3.25) in their members when
subjected to the forces from the design
earthquake.
IV-151
Seismic Provisions
IV-152
Seismic Provisions
38
14.2. Bracing Members
Basically
the same
as SCBF
Bracing shall meet the requirements of Section 8.2b
(i.e. seismically compact)
Exception: braces filled with concrete need not comply with this
provision
Braces with Kl/r greater than 4√(Es/Fy) shall not be
used in V-type or inverted-V-type configurations.
V-Type, Inverted-V-Type and K-type braced frames
shall meet following requirements:
Beam that is intersected by braces shall be continuous
between columns (V-Type, Inverted-V-Type)
Column that is intersected by braces shall be
continuous between beams (K-Type)
Basically
the same
as SCBF
Unique to
OCBF
IV-153
IV-154
Seismic Provisions
Required strength of beam intersected by braces,
their connections and supporting members shall be
determined based on load combinations of building
code assuming braces support no dead and live
loads.
Basically
the same
as SCBF
V-Type, Inverted-V-Type and K-type braced frames shall meet
following requirements:
For load combinations that include earthquake effect on
beam, E shall be determined as follows
Forces in tension braces shall be assumed to equal
RyFyAg
For V-type and Inverted V-type, brace tension forces
need not exceed maximum force developed by system
Forces in compression braces shall be assumed equal to
0.3Pn
Basically
IV-155
Seismic Provisions
Seismic Provisions
the same
as SCBF
IV-156
Seismic Provisions
39
QUESTIONS?
Both flanges of beam shall be laterally braced with
maximum spacing of Lb = Lpd per Equation A-1-7 and A1-8 of Appendix 1 of the Specification.
Braces need to possess sufficient strength and
stiffness (See notes on Section 9.8 of Seismic
Provisions and Appendix 6 of Specification for example
requirements)
Basically
the same
as SCBF
IV-157
Seismic Provisions
IV-158
AISC eLearning
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CEUs/PDHs are available
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40
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41

Introduction to the 2005 AISC Seismic Provisions

Transcription

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