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Buckling Restrained Braces andBuckling Restrained Braces andStructural Fuses

Michel Bruneau, Ph.D., P.Eng. Professor

Department of Civil, Structural, and Environmental EngineeringUniversity at Buffalo

Outline• Description of Structural Fuse Concept

(SFC)• Description of Buckling Restrained Braces

(BRB)• Applications of BRB and SFC to Bridges

Energy DissipationEnergy DissipationEarthquake-resistant design has long relied on hysteretic energy dissipation to provide life-safety level of protectionAdvantages of yielding steel

Stable material properties well known to practicing engineersengineersNot a mechanical device (no special maintenance)Reliable long term performance (resistance to aging)

For traditional structural systems, ductile behavior achieved by stable plastic deformation of structural members = damage to those membersIn conventional structural configurations, serves life-safety purposes, but translates into property loss, and need substantial repairs

Energy DissipationEnergy Dissipation

DuctilityDuctility

Brittle (↓) (Somewhat) Ductile (↓)Structural FusesStructural Fuses

From Energy Dissipation to Structural FuseResearchers have proposed that hysteretic energy dissipation should instead occur in “disposable” structural elements (i e structural fuses)structural elements (i.e., structural fuses)

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AnalogyAnalogy

Sacrificial element to protect the rest of the system.

Weak LinkWeak Link

Brittle (↓) (Somewhat) Ductile (↓)

Capacity DesignCapacity DesignRoeder and Popov (1977)Roeder and Popov (1977)

Ductile seismic behaviorConcentrating energy dissipation in special elements + capacity designLinks not literally disposable

“Ductile Fuse”“Ductile Fuse”

Eccentrically Braced FrameEccentrically Braced Frame

Other studies:Fintel and Ghosh (1981)Aristizabal-Ochoa (1986)Basha and Goel (1996)Carter and Iwankiw (1998)Sugiyama (1998)Rezai et al. (2000)

Eccentrically Braced FrameEccentrically Braced Frame

(((((Opening a parenthesis)

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Tubular Eccentrically Braced FrameTubular Eccentrically Braced Frame

EBFs with wide-flange (WF) links require lateral bracing of the link to prevent lateral torsional bucklingLateral bracing is difficult to provide in

b

dtf

tw

Fyw

Fyf

b

dtf

tw

Fyw

Fyf

b

dtf

tw

Fyw

Fyf

Lateral bracing is difficult to provide in bridge piersDevelopment of a laterallystable EBF link is warrantedConsider rectangular cross-section – No LTB

ProofProof--ofof--Concept TestingConcept Testing

ProofProof--ofof--Concept TestingConcept TestingFinite Element Modeling of Finite Element Modeling of ProofProof--ofof--Concept TestingConcept Testing

Hysteretic Results for Refined ABAQUS Model and Proof-of-Concept Experiment

))))(Closing a parenthesis)

Structural Fuse AnalogyStructural Fuse Analogy

EBF (Incomplete Fuse Analogy)Ductile linkMaybe not easily replaceabley y p

Need to configurations that decouple the energy dissipating system from the gravity carrying load systemBRB is one of many devices that could serve as a structural fuse

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What is a Buckling Restrained What is a Buckling Restrained Brace?Brace?

Explained by comparison with regular concentric brace not restrained

against buckling

PΔ

δ+

δ-

OAP

ΔD

E F

O

δ

AB

BC

CD

Δ

Δ= Real Hinge

Ductile Design of Steel Structures

B

CG

δ

A

DE

EFΔ

Δ= Plastic Hinge (Mpr)

Small residual deformationCrCr’

CBFsCBFs

KL/r – compression and tension strengths are unequal

Less energy dissipation in compression

Ductile Design of Steel Structures

Less energy dissipation in compressionUnbalanced force issuesLocal buckling and fracture

XMP

MPVV

VV

Ductile Design of Steel Structures

X

Buckling Restrained BracesBuckling Restrained Braces

The disadvantages of the CBF system can be overcome if the brace can yield during both tension and compression without buckling

Ductile Design of Steel Structures

buckling. A braced frame that incorporates this type of brace is the buckling restrained brace (BRB) Frame (BRBF)BRBF is a special class of CBF that precludes brace buckling

BRB ConceptsBRB Concepts

Most of the BRBs developed to date are proprietary, but the concepts are similar

Ductile steel core, designed to yield during both tension and compressionSteel core placed inside a steel casing (usually a hollow

Ductile Design of Steel Structures

Steel core placed inside a steel casing (usually a hollow structure shape) Unbonding material wraps steel coreCasing is filled with mortar or concrete. Unbonding material minimizes / eliminates transfer of axial force from steel core to mortarNote: Poisson effect causes steel core to expand under compression

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BucklingBuckling--Restrained Brace Restrained Brace MechanicsMechanics

Encasing Encasing mortarmortar

Yielding steel Yielding steel corecore

Ductile Design of Steel Structures

Unbonded Brace TypeUnbonded Brace Type

DecouplingDecouplingBucklingBucklingRestraintRestraint

Steel tubeSteel tube

Unbonding material Unbonding material between steel core and between steel core and mortarmortar

WHAT IS A BUCKLING-RESTRAINED BRACE? Two Definitions

De-Coupled Stress and Buckling(Mechanics Definition)

Balanced Hysteresis(Performance Definition)

Ductile Design of Steel Structures Ductile Design of Steel Structures

TEST OBSERVATIONSTEST OBSERVATIONS

Ductile Design of Steel Structures

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Ductile Design of Steel Structures Ductile Design of Steel Structures

Ductile Design of Steel Structures Ductile Design of Steel Structures

Buckling Restrained Braces in Buckling Restrained Braces in Structural Fuse ApplicationStructural Fuse Application

Vargas, R., Bruneau, M., (2009). “Analytical Response of Buildings Designed with Metallic Structural Fuses”, ASCE Journal of Structural Engineering, Vol.135, No.4, pp.386-393.Vargas, R., Bruneau, M., (2009). “Experimental Response of Buildings D i d ith M t lli St t l F ” ASCE J l f St t l Designed with Metallic Structural Fuses”, ASCE Journal of Structural Engineering, Vol.135, No.4, pp.394-403.Vargas, R., Bruneau, M., “Experimental Investigation of the Structural Fuse Concept”, Technical Report MCEER-06-0005, Multidisciplinary Center for Earthquake Engineering Research, State University of New York at Buffalo, Buffalo, NY, 2006.Vargas, R., Bruneau, M., “Analytical Investigation of the Structural Fuse Concept”, Technical Report MCEER-06-0004, Multidisciplinary Center for Earthquake Engineering Research, State University of New York at Buffalo, Buffalo, NY, 2006.

Wada et al. (1992)Wada et al. (1992)DamageDamage--controlled or controlled or DamageDamage--tolerant Structurestolerant Structures

Ductile elements were used to reduce inelastic deformations of the main structureConcept applied to high

Other studies:Connor et al. (1997)Shimizu et al. (1998)Wada and Huang (1999)Wada et al. (2000)Huang et al. (2002)

Concept applied to high rise buildings (T > 4 s)

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mass, m

frame f

structural fuse, d

Ground Motion, üg(t)

frame, fbraces, b

Benefits of Structural Fuse Concept:Benefits of Structural Fuse Concept:

Seismically induced damage is concentrated on the fusesFollowing a damaging earthquake only the fuses

αK1 = Kf

V

Vp

VTotal

earthquake only the fuses would need to be replacedOnce the structural fuses are removed, the elastic structure returns to its original position (self-recentering capability)

Δya Δyf u

KfKa

K1

Vyf

Vyd

Vy

Frame

Structural Fuses

αμmax

0.05

0.25

.0

.2

.4

.6

.8

1.0

.0 .2 .4 .6 .8 1.0.0

.2

.4

.6

.8

1.0

.0 .2 .4 .6 .8 1.0.0

.2

.4

.6

.8

1.0

.0 .2 .4 .6 .8 1.0

4

.6

.8

1.0

4

.6

.8

1.0

4

.6

.8

1.0

.0

.2

.4

.6

.8

1.0

.0 .2 .4 .6 .8 1.0

4

.6

.8

1.0

10 5 2.5 1.67

V/Vp

0.50

.0

.2

.4

.0 .2 .4 .6 .8 1.0.0.2

.4

.0 .2 .4 .6 .8 1.0.0

.2

.4

.0 .2 .4 .6 .8 1.0

.0

.2

.4

.6

.8

1.0

.0 .2 .4 .6 .8 1.0.0

.2

.4

.6

.8

1.0

.0 .2 .4 .6 .8 1.0.0

.2

.4

.6

.8

1.0

.0 .2 .4 .6 .8 1.0

.0

.2

.4

.0 .2 .4 .6 .8 1.0

.0

.2

.4

.6

.8

1.0

.0 .2 .4 .6 .8 1.0

Frame Damping System Total

p

u/Δyf

Structural Fuses

uu max

max

//ΔΔyfyf

ηη=0.2=0.2

αα= 0.05= 0.05

μμmaxmax = 10= 10Drift Limit (NL THA)Drift Limit (NL THA)Drift Limit (Suggested)Drift Limit (Suggested)

μμ ff==

T=T=

ηη

ηη=1.0=1.0

System PropertiesSystem Properties

IB, ZB

IC IC

L

H

Bare FrameBare Frame

IB, ZB

IC

L

HIC

θ θ

Ab Ab

BRBsBRBsbw

BRBsBRBsIB, ZB

IC IC

L

H

θ θ

Ab Ab

N plates

TT--ADASADAS

IB, ZB

IC IC

L

H

θ θ

Ab Ab

Shear Panel

Shear PanelShear Panel

h

t

ht

bftf

Model withModel withNippon Steel BRBsNippon Steel BRBs

Eccentric GussetEccentric Gusset--PlatePlate

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Test 1 Test 1 (PGA = 1g)(PGA = 1g)

Test 1Test 1First Story BRBFirst Story BRB

0

10

20

30

40

al F

orce

(kip

s)

-40

-30

-20

-10

0-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5

Axial Deformation (in)

1st S

tory

Axi

a

Test 1 (Nippon Steel BRB Frame)Test 1 (Nippon Steel BRB Frame)First Story Columns ShearFirst Story Columns Shear

25

50

75

100

ns S

hear

(kN

)

-100

-75

-50

-25

0-5 -4 -3 -2 -1 0 1 2 3 4 5

Inter-Story Drift (mm)

1st S

tory

Col

umn

Static Test Static Test -- Nippon Steel BRBsNippon Steel BRBsNote: Replacement is to reNote: Replacement is to re--center the building center the building

(not due to BRB fracture life)(not due to BRB fracture life)BRB and SFC in BridgesBRB and SFC in Bridges

Ductile DiaphragmsBRB SFC in end-diaphragms

R ki T (R ki B d F )Rocking Trusses (Rocking Braced Frames)SFC with BFB at base

ABC Piers BRB SFC between dual columns

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Ductile DiaphragmsDuctile Diaphragmswith Structural Fuseswith Structural Fuses

Zahrai, S.M., Bruneau, M. (1999). “Cyclic Testing of Ductile End-Diaphragms for Slab-on-Girder Steel Bridges”, ASCE Journal of Structural Engineering, Vol.125, No.9, pp.987-996.Zahrai, S.M., Bruneau, M. (1999). “Ductile End-Diaphragms for the Seismic Retrofit of Slab-on-Girder Steel Bridges”, ASCE Journal of Structural g ,Engineering, Vol.125, No.1, 1999, pp.71-80.Sarraf, M., Bruneau, M. (1998). “Ductile Seismic Retrofit of Steel Deck-Truss Bridges. I: Strategy and Modeling”, ASCE Journal of Structural Engineering, Vol.124, No.11, 1998, pp.1253-1262.Sarraf, M., Bruneau, M. (1998). “Ductile Seismic Retrofit of Steel Deck-Truss Bridges. II: Design Applications", ASCE Journal of Structural Engineering, Vol.124, No.11, 1998, pp. 1263-1271.Zahrai, S.M., Bruneau, M. (1998). “Impact of Diaphragms on Seismic Response of Straight Slab-on-girder Steel Bridges”, ASCE Journal of Structural Engineering, Vol.124, No.8, pp.938-947.

Vulnerable Vulnerable Bridge Bridge Bridge Bridge SubstructureSubstructure

Inelastic Behavior of Proposed andInelastic Behavior of Proposed andExisting EndExisting End-- DiaphragmDiaphragm

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Implementation of ConceptImplementation of ConceptMinato Bridge (Hanshin Expressway Corporation)Minato Bridge (Hanshin Expressway Corporation)

Ductile Cross-Frames implemented as part of a comprehensive seismic rehabilitation processKANAJI, H., KITAZAWA, M., SUZUKI, N., “Seismic Retrofit Strategy using Damage Control Design Concept and the Response Reduction Effect for a Long-span Truss Bridge”, US-Japan Bridge Workshop, 2005

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BRB and SFC BRB and SFC in Rocking Truss Piers in Rocking Truss Piers

Pollino, M., Bruneau, M., (2010). “Bi-Directional Behavior and Design of Controlled Rocking 4-Legged Bridge Steel Truss Piers,” ASCE J. of Struct. Eng. (in press).Pollino, M., Bruneau, M., (2010). “Seismic Testing of a Bridge Truss Pier Designed for Controlled Rocking,” ASCE J. of Struct. Eng. (in press).Pollino, M., Bruneau, M., (2007). “Seismic Retrofit of Bridge Steel Truss Piers Using a Controlled Rocking Approach” ASCE J of Struct Eng Vol 12 No 5 Using a Controlled Rocking Approach , ASCE J. of Struct. Eng. , Vol.12, No.5, pp.600-610.Pollino, M., Bruneau, M., (2008). “Analytical and Experimental Investigation of a Controlled Rocking Approach for Seismic Protection of Bridge Steel Truss Piers”, Technical Report MCEER-08-0003, Multidisciplinary Center for Earthquake Engineering Research, State University of New York at Buffalo, Buffalo, NY, 2008.Pollino, M., Bruneau, M., “Seismic Retrofit of Bridge Steel Truss Piers using a Controlled Rocking Approach”, Technical Report MCEER-04-0011, Multidisciplinary Center for Earthquake Engineering Research, State University of New York at Buffalo, Buffalo, NY, 2004.

Controlled Rocking/Energy Controlled Rocking/Energy Dissipation SystemDissipation SystemAbsence of base of leg connection creates a rocking bridge pier system partially isolating the structure

Retrofitted Tower

structure

Installation of steel yielding devices (buckling-restrained braces) at the steel/concrete interface controls the rocking response while providing energy dissipation

Existing Rocking BridgesExisting Rocking BridgesSouth Rangitikei Rail Bridge Lions Gate Bridge North Approach

Static, Hysteretic Behavior of Controlled Static, Hysteretic Behavior of Controlled Rocking PierRocking Pier

Device Response

FPED=0FPED=w/2

General Design Constraints for General Design Constraints for Controlled Rocking SystemControlled Rocking System

(1) Deck-level displacement limits need to be established on a case-by-case basis

Maintain pier stabilityBridge serviceability requirements

(2) Strains on buckling-restrained brace (uplifting (2) Strains on buckling restrained brace (uplifting displacements) need to be limited such that it behaves in a stable, reliable manner(3) Capacity Protection of existing, vulnerable resisting elements considering 3-components of excitation and dynamic forces developed during impact and uplift(4) Allow for self-centering of pier

Velocity

Limit forces through vulnerable members using structural “fuses”

Acceleration⇒

Design ProcedureDesign Procedure

Design ConstraintsDesign Chart:

6

8

10h/d=4

(in2) ubVelocity

Control impact energy to foundation and impulsive loading on tower legs by limiting velocity

⇒

Displacement DuctilityLimit μL of specially detailed, ductile “fuses”

⇒

β<1⇒ Inherent re-centering (Optional)

0 100 200 300 4000

2

4

constraint1constraint2constraint3constraint4constraint5

Lub (in.)

Aub

(

Lub

A u

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Experimental TestingExperimental TestingArtificial Mass Simulation Scaling Procedure

λL>5 (Crane Clearance)λA=1.0 (1-g Field)Wm=70kN (We=76kN)T 0 34 (T 0 40 )

Δ

λL=5h/d=4.1

6.1mTom=0.34sec (Toe=0.40sec)Loading System

Phase I5DOF Shake Table

Phase II6DOF Shake Table

λt=2.2

1.5mSynthetic EQ 150% of DesignFree Rocking

Synthetic EQ 150% of DesignTADAS Case ηL=1.0

Synthetic EQ 150% of Design – Free Rocking Synthetic EQ 175% of Design - Viscous Dampers

ABC Bridge Pier with ABC Bridge Pier with Structural FusesStructural Fuses

El-Bahey, S., Bruneau, M., (2010). “Structural Fuse y ( )Concept For Bridges”, Transportation Research Record (a Journal of the Transportation Research Board), (in press).El-Bahey, S., Bruneau, M., (2010). “Structural Fuse Concept For Bridges”, MCEER Report (in press).

Simulate ABC ConstructionSimulate ABC ConstructionSimulate ABC ConstructionSimulate ABC Construction

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Simulate ABC ConstructionSimulate ABC ConstructionSimulate ABC ConstructionSimulate ABC ConstructionNew “Short Length” BRB New “Short Length” BRB Developed by Star Seismic Developed by Star Seismic

Specimen S2Specimen S2--11 Experimental versus Analytical Experimental versus Analytical Results for Specimen S2Results for Specimen S2--11

Onset of BRB yielding

Onset of Column yielding

Specimen with BRB FusesSpecimen with BRB Fuses Specimen with BRB FusesSpecimen with BRB Fuses

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Pushover Comparison of Frame Pushover Comparison of Frame with Different Structural Fuseswith Different Structural Fuses

1000

1100

1200

1300

1400

BRB60%

SPSL (with Restraints)

0

100

200

300

400

500

600

700

800

900

0 20 40 60 80 100 120 140 160 180 200 220Top Displacement (mm)

Tot

al F

orce

(kN

)

μmax=3.3

30%

Bare Frame

SPSL (no Restraints)

ConclusionsConclusionsRecently developed options for seismic design and retrofit illustrated (BRB with Fuse, TEBF, Rocking)Instances for which replacement of sacrificial structural members (considered to be structural fuses dissipating hysteric energy) was accomplished in some cases repeatedly hysteric energy) was accomplished, in some cases repeatedly. Article/Clauses for the design of some of these systems are being considered by:

CSA-S16 committee for 2009 Edition of S16AISC TC9 Subcommittee for the 2010 AISC Seismic Provisions

Emerging field: opportunities to develop structural fuse concepts still exist

AcknowledgmentsAcknowledgmentsFormer Ph.D. Students:

Michael Pollino (Case Western University) – Rocking Steel Framed SystemsJeffrey Berman (University of Washington) – Seismic Retrofit of Large Bridges Braced BentRamiro Vargas (University of Panama) – Enhancing Resilience using Passive Energy Dissipation SystemsResilience using Passive Energy Dissipation SystemsSamer El-Bahey (Stevenson and Associates, Phoenix) –Structural Fuses for BridgesMajid Sarraf (Parsons) – Ductile Cross-Frames in TrussesMehdi Zahrai (University of Tehran) – Ductile Diaphragms

Funding from:National Science Foundation (to MCEER)Federal Highway Administration (to MCEER)NSERC (Canada)

Thank you!Thank you!yy

Questions?Questions?