aisc seismic design-moduleug-brief overview
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Design of Seismic-
Resistant SteelBuilding Structures
Prepared by:
Michael D. Engelhardt
University of Texas at Austin
with the support of the
American Institute of Steel Construction.
Version 1 - March 2007
Brief Overview
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Design of Seismic-Resistant Steel Building
Structures: A Brief Overview
Earthquake Effects on Structures
Performance of Steel Buildings in Past Earthquakes
Importance of Ductility
Design Earthquake Forces: ASCE-7
Steel Seismic Load Resisting Systems
AISC Seismic Provisions
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Design of Seismic-Resistant Steel Building
Structures: A Brief Overview
Earthquake Effects on Structures
Performance of Steel Buildings in Past Earthquakes
Building Code Philosophy for Earthquake-Resistant Design
and Importance of Ductility
Design Earthquake Forces: ASCE-7
Steel Seismic Load Resisting Systems
AISC Seismic Provisions
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Ground
Acceleration
Building:
Mass = m
Building
Acceleration
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F = ma
Earthquake Forces
on Buildings:
Inertia Force Due to
Accelerating Mass
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Design of Seismic-Resistant Steel Building
Structures: A Brief Overview
Earthquake Effects on Structures
Performance of Steel Buildings in Past Earthquakes
Building Code Philosophy for Earthquake-Resistant Design
and Importance of Ductility
Design Earthquake Forces: ASCE-7
Steel Seismic Load Resisting Systems
AISC Seismic Provisions
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13/86Causes of Earthquake Fatalities: 1900 to 1990
Collapse of
Masonry Buildings
Fire
Collapse of Timber
Buildings
Other Causes
Landslides
Colla se of RC Buildin s
Collapse of
Masonry Buildings
Fire
Collapse of Timber
Buildings
Other Causes
Landslides Collapse of RC Buildings
Earthquake Fatalities: 1900 - 1949
(795,000 Fatalities)
Earthquake Fatalities: 1950 - 1990
(583,000 Fatalities)
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Design of Seismic-Resistant Steel Building
Structures: A Brief Overview
Earthquake Effects on Structures
Performance of Steel Buildings in Past Earthquakes
Building Code Philosophy for Earthquake-Resistant Design
and Importance of Ductility
Design Earthquake Forces: ASCE-7
Steel Seismic Load Resisting Systems
AISC Seismic Provisions
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Conventional Building Code Philosophy for
Earthquake-Resistant Design
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
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To Survive Strong Earthquake
without Collapse:
Design for Ductile Behavior
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H
HDuct i l i ty = Inelast ic Deform at ion
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HH
Strength
Reqd Ductility
MAX
Helastic
3/4 *Helastic
1/2 *Helastic
1/4 *Helastic
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HDuctility = Yielding
Failure =
Fracture
or
Instability
Ductility in Steel Structures: Yielding
Nonductile Failure Modes: Fractu re or Instabi l i ty
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Developing Ductile Behavior:
Choose frame elements ("fuses") that will yield in anearthquake.
Detail "fuses" to sustain large inelastic deformations prior tothe onset of fracture or instability (i.e. , detail fuses for
ductility).
Design all other frame elements to be stronger than the fuses,i.e., design all other frame elements to develop the plastic
capacity of the fuses.
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Key Elements of Seismic-Resistant Design
Required Lateral Strength
ASCE-7:Minimum Design Loads for Buildings and OtherStructures
Detailing for Ductility
AISC:Seismic Provisions for Structural Steel Buildings
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Design of Seismic-Resistant Steel Building
Structures: A Brief Overview
Earthquake Effects on Structures
Performance of Steel Buildings in Past Earthquakes
Building Code Philosophy for Earthquake-Resistant Design
and Importance of Ductility
Design Earthquake Forces: ASCE-7
Steel Seismic Load Resisting Systems
AISC Seismic Provisions
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Design EQ LoadsTotal Lateral Force per ASCE 7-05:
sV C W
V = total design lateralforce or shear at
base of structure
W = effective seismic
weight of building
CS= seismic response
coefficientV
Design EQ Loads Total Lateral Force per ASCE 7 05
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Design EQ LoadsTotal Lateral Force per ASCE 7-05:
SDS= design spectral
acceleration at
short periods
SD1= design spectral
acceleration at
1-second period
I = importance factor
R = response modification coefficient
T = fundamental period of building
WCVS
I
R
SC
DS
S
I
RT
S 1D
I
RT
TS
2
L1D
for T TL
for T > TL
TL= long period transition period
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R factors for Selected Steel Systems (ASCE 7):
SMF (Special Moment Resist ing Frames): R = 8
IMF (Intermediate Moment Resist ing Frames): R = 4.5
OMF (Ordinary Moment Resist in g Frames): R = 3.5
EBF (Eccentr ical ly Braced Frames): R = 8 or 7
SCBF (Special Concentr ical ly B raced Frames): R = 6
OCBF (Ordinary Conc entr ical ly B raced Frames): R = 3.25
BRBF (Buck l ing Restrained Braced Frame): R = 8 or 7
SPSW (Spec ial Plate Shear Walls): R = 7
Undetailed Steel Systems inSeismic Design Categories A, B or C R = 3
(AISC Seismic Provisions not needed)
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Design of Seismic-Resistant Steel Building
Structures: A Brief Overview
Earthquake Effects on Structures
Performance of Steel Buildings in Past Earthquakes
Building Code Philosophy for Earthquake-Resistant Design
and Importance of Ductility
Design Earthquake Forces: ASCE-7
Steel Seismic Load Resisting Systems
AISC Seismic Provisions
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Seismic Load Resisting Systems
for Steel Buildings
Moment Resisting Frames
Concentrically Braced Frames
Eccentrically Braced Frames
Buckling Restrained Braced Frames
Special Plate Shear Walls
MOMENT RESISTING FRAME (MRF)
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MOMENT RESISTING FRAME (MRF)
Advantages
Architectural Versatility
High Ductility and Safety
Disadvantages
Low Elastic Stiffness
Beams and columns with moment resisting connections; resist
lateral forces by flexure and shear in beams and columns - i.e. byframe action.
Develop ductility primarily by flexural yielding
of the beams:
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Moment Resisting Frame
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Inelastic Response
of a Steel Moment
Resisting Frame
C t i ll B d F (CBF )
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Concentrically Braced Frames (CBFs)
Beams, columns and braces arranged to form a vertical truss.
Resist lateral earthquake forces by truss action.
Develop ductility through inelastic action in braces.
- braces yield in tension
- braces buckle in compression
Advantages
- high elastic stiffness
Disadvantages
- less ductile than other systems (SMFs, EBFs, BRBFs)
- reduced architectural versatility
T pes of CBFs
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Types of CBFs
Single Diagonal Inverted V- Bracing V- Bracing
X- Bracing Two Story X- Bracing
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Inelastic Response of CBFs under Earthquake Loading
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Inelastic Response of CBFs under Earthquake Loading
Inelastic Response of CBFs under Earthquake Loading
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Inelastic Response of CBFs under Earthquake Loading
Tension Brace: Yields(ductile)
Compression Brace: Buckles(nonductile)
Columns and beams: remain essentially elastic
Inelastic Response of CBFs under Earthquake Loading
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Inelastic Response of CBFs under Earthquake Loading
Compression Brace(previously in tension):
Buckles
(nonductile)
Tension Brace (previously incompression): Yields
(ductile)
Columns and beams: remain essentially elastic
Eccentrically Braced Frames (EBFs)
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Eccentrically Braced Frames (EBFs)
Framing system with beam, columns and braces. At least one end ofevery brace is connected to isolate a segment of the beam called a
link.
Resist lateral load through a combination of frame action and truss
action. EBFs can be viewed as a hybrid system between momentframes and concentrically braced frames.
Develop ductility through inelastic action in thelinks.
EBFs can supply high levels of ductility (similar to MRFs), but can
also provide high levels of elastic stiffness (similar to CBFs)
e Link
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e
Link
Link
e Link
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e Link
Some possible bracing arrangement for EBFS
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Some possible bracing arrangement for EBFS
e e e e
e
e
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Inelastic Response of EBFs
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Inelastic Response of EBFs
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Buckling-Restrained Braced Frames (BRBFs)
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g ( )
Type of concentrically braced frame.
Beams, columns and braces arranged to form a vertical truss.
Resist lateral earthquake forces by truss action.
Special type of brace members used: Buckling-Restrained
Braces(BRBs). BRBS yield both in tension and compression
- no buckling !!
Develop ductility through inelastic action (cyclic tension and
compression yielding) in BRBs.
System combines high stiffness with high ductility.
Buckling-Restrained Brace
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Buckling-
Restrained Brace:
Steel Core+
Casing
Casing
Steel Core
Buckling-Restrained Brace
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Buckling-
Restrained Brace:
Steel Core+
CasingA
A
Section A-A
Steel Core
Debonding material
Casing
Steel jacket
Mortar
Buckling-Restrained Brace
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P P
Steel coreresists entire axial force P
Casingis debonded from steel core
- casing does not resist axial force P
- flexural stiffness of casing restrains buckling of core
Buckling-Restrained Brace
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Buckling-
Restrained Brace:
Steel Core+
Casing
Steel Core
Yielding Segment
Core projection and
brace connectionsegment
Bracing Configurations for BRBFs
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g g
Single Diagonal Inverted V- Bracing V- Bracing
X- Bracing Two Story X- Bracing
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Inelastic Response of BRBFs under Earthquake Loading
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Compression Brace: Yields Tension Brace: Yields
Columns and beams: remain essentially elastic
Special Plate Shear Walls (SPSW)
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Assemblage of consisting of rigid frame, infilled with thin
steel plates.
Under lateral load, system behaves similar to a plate girder.
Wall plate buckles under diagonal compression and forms
tension field.
Develop ductility through tension yielding of wall plate along
diagonal tension field.
System combines high stiffness with high ductility.
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Plate-Girder Analogy
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Design of Seismic-Resistant Steel Building
St t A B i f O i
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Structures: A Brief Overview
Earthquake Effects on Structures
Performance of Steel Buildings in Past Earthquakes
Building Code Philosophy for Earthquake-Resistant Designand Importance of Ductility
Design Earthquake Forces: ASCE-7
Steel Seismic Load Resisting Systems
AISC Seismic Provisions
2005 AISC Seismic Provisions
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2005 AISC Seismic Provisions