AISC Seismic Design-ModuleUG-Brief Overview

5/28/2018 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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Building Code Philosophy for Earthquake-Resistant Design

and Importance of Ductility





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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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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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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 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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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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Tension Brace: Yields(ductile)

Compression Brace: Buckles(nonductile)

Columns and beams: remain essentially elastic



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Compression Brace(previously in tension):

Buckles

(nonductile)

Tension Brace (previously incompression): Yields

(ductile)


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



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Buckling-

Restrained Brace:

Steel Core+

CasingA

A

Section A-A

Steel Core

Debonding material

Casing

Steel jacket

Mortar



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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



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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


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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St t A B i f O i


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Building Code Philosophy for Earthquake-Resistant Designand Importance of Ductility




2005 AISC Seismic Provisions


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2005 AISC Seismic Provisions

Documents

AISC Seismic Design-ModuleUG-Brief Overview