Keith Palmer

Seismic Behavior of Steel Concentrically Braced Frame SystemsConcentrically Braced Frame Systems

St t Of Th P ti CBF D iState‐Of‐The‐Practice CBF Design

• AISC Seismic ProvisionsAISC St l C t M l• AISC Steel Const. Manual

d h• Based on component research• Not based on system behavior

Points C & D – brace fracturePoints G & H – column fracture

(Uriz and Mahin 2008)

NEES CBF R h PNEES CBF Research ProgramNEESR‐SG – International Hybrid Simulation of y

Tomorrow’s Braced Frame Systems

• C. Roeder, D. Lehman, University of Washington

• S. Mahin, University of California Berkeley

• K.C. Tsai, National Taiwan University

• C. Shield, University of Minnesota

• T. Okazaki, Hokkaido University

NEES CBF R h O tNEES CBF Research Outcome

(a) 2t linear offset - Current practice (b) 8t elliptical offset - Proposed

Elliptical offset – thinner, more compact gusset plate design

C t & F t W kCurrent & Future WorkMotivation

• 3D effects not completely understood

• Loading and deformation perpendicular to CBF

• Effect of concrete floor systemffect of concrete floor system

• Limited studies of center connection for Single‐story X configurationstory X‐configuration• System behavior of pin‐ended, collar‐type BRB with typical detailing not clearwith typical detailing not clear

3D CBF System Tests

1st Test Frame:• SCBF• HSS buckling braces• Single-story X-configSingle story X config.

3D CBF System Tests

2nd Test Frame:• BRBF• Star Seismic BRBs• Single diagonalSingle diagonal config.• Pinned ends

MAST Laboratory

3D CBF System Tests

Loading protocolLoading protocol

• Bi-directional• Cyclic• Displacement controlled

3D CBF System Tests

Cyclic Loading ProtocolCyclic Loading Protocol) Drift at initial brace b ckling (SCBF) 0 3%

Rat

io (r

ads) y – Drift at initial brace buckling (SCBF) ~ 0.3%

y – Drift at initial BRB yielding (BRBF) ~ 0.33%

ory

Drif

t RSt

o

6@0.5y 6@1.0y

2@0.75y 2@1.25y

2@1.5y

2@1.75y

2@2.0y

2@2.5y

2@3.0y

2@3.0y

Note: complete protocol not shown (continued in increments of y after 3.0y

3D CBF System Tests

InstrumentationInstrumentation

• Strain gauges (~220)

LVDT ( 74)• LVDTs (~74)

• String potentiometers (~41)• String potentiometers ( 41)

•Metris Kryptonyp

SCBF Test Highlights

First brace to buckle (~0.3% ISDR)

SCBF Test Highlights

Mode 2 brace UnsymmetricalMode 2 brace buckling

1.3% ISDR

Unsymmetrical brace buckling

2.1% ISDR

SCBF Test Highlights

3D CBF System TestsGusset yield lines (+2% ISDR)

3D CBF System Tests

C t W kCurrent Work

Brace Fracture2.0% ISDR

Second StoryY di St Sh (kN) SDR ( d 100)

SCBF Test HighlightsY-dir Story Shear (kN) vs. SDR (rads x 100)

YY

X

First StoryY-dir Story Shear (kN) vs. SDR (rads x 100)

2nd story brace yielding (~0.3% ISDR)

3D CBF System Tests

BRBF Test Highlights

Gusset plateweld tear Columnweld tear

3.1% ISDRColumn flange

Beam flange

3D CBF System Tests

Column flange

BRBF Test Highlightsg

local buckling3.2% ISDR

3D CBF System Tests

Column web and

BRBF Test Highlights

flange tearing4% ISDR

Beam web yielding and tearing, flange

tearing3 6% ISDR3.6% ISDR

Gusset plate

Beam web yielding and tearing, flange

tearing3 6% ISDR3.6% ISDR

Second Story

BRBF Test Highlights

Y-dir Story Shear (kN) vs. SDR (rads x 100)

X

Y

Second StoryX-dir Story Shear (kN) vs. SDR (rads x 100)

S (SCBF)Summary (SCBF)• Out-of-plane frame deformation appears to have had little to no effect on SCBF frame deformation capacity and strength

• XBF deformation capacities the same as those achieved in l t t t UW ith i il fplanar tests at UW with similar frame

• 8t elliptical offset method accommodated large inelastic rotations after brace bucklingafter brace buckling

• Gusset edge deformation caused by frame action – no apparent effect on global performanceeffect on global performance

S (BRBF)Summary (BRBF)• Out-of-plane frame deformation appears to have had little to no p ppeffect on frame deformation capacity and strength

• Frame action had dominating effect on gusset interface weld demands

• Considerable damage to frame occurred before and after brace f tfracture

• The BRB cores fractured at 3.5 and 4% story drift

• No instabilities occurred in the BRB prior to fracture

• Different behavior than BRBF tests at UW and UCB

• UW, UCB drifts prior to BRB instability ~ 2 to 2.5%

A k l d tAcknowledgements

• National Science Foundation (NSF)• Network for Earthquake

Engineering Simulation (NEES)

• American Institute of Steel Construction (AISC)

A k l d tAcknowledgements

MAST LaboratoryMAST Laboratory

• Professor Carol Shield, Director• Paul Bergson, Operations Manager

• Drew J Daugherty IT• Drew J. Daugherty, IT• Rachel Gaulke, Instrumentation Engineer

• Angela Kingsley, Floor Manager

•Mitch Reierson ITMitch Reierson, IT

Questions?

B kli R t i d B d FBuckling‐Restrained Braced Frames

B kli R t i d BBuckling‐Restrained Braces

AA

A

Steel CoreCasing Steel Core

Gap and

CasingSteel jacket

Mortar

Section A-A

Gap and Debonding material

H t ti C iHysteretic Comparison

Conventional Buckling Brace Buckling Restrained BraceConventional Buckling Brace Buckling‐Restrained BraceBlack et. al (2002)Black et. al (1980)

NEES CBF R h O tNEES CBF Research OutcomeBalanced Design ProcedureBalanced Design Procedure

Brace Brace Connection Beam/Column BraceBuckling Yielding Yielding Yielding Fracture

• Ensure that adequate ductility capacity is provided• Unwanted failure limit states are suppressed

SCBF Test Highlights

3D CBF System TestsGusset plate tear1.7% ISDR

3D CBF System Tests

BRBF Test Highlights

Initial Gusset Yielding

~ 1.6% ISDR 1.6% ISDR

SCBF Test Highlights

3D CBF System TestsBase gusset yield pattern