eccentrically braced frames

Eccentrically Braced FramesEccentrically Braced Frames

Presented by: Gustavo Cortes and JosePresented by: Gustavo Cortes and Jose MonarrezMonarrez

OutlineOutline

DesignDesign•• PhilosophyPhilosophy•• Preliminary design Preliminary design (ASCE (ASCE --7)7)

•• Force distribution Force distribution •• Design procedure Design procedure (AISC specifications and AISC Seismic provisions)(AISC specifications and AISC Seismic provisions)

•• Design summaryDesign summary

Performance EvaluationPerformance Evaluation•• Evaluation requirements Evaluation requirements (FEMA 356)(FEMA 356)

•• Findings and conclusionsFindings and conclusions

PhilosophyPhilosophy

To restrict the inelastic action to the To restrict the inelastic action to the links, andlinks, andto design the framing around the to design the framing around the links to sustain the maximum forces links to sustain the maximum forces that can be delivered by the links.that can be delivered by the links.

EBF AdvantagesEBF Advantages

If designed properly will achieve:If designed properly will achieve:•• High elastic stiffness,High elastic stiffness,•• Stable inelastic response under cyclic Stable inelastic response under cyclic

lateral loading,lateral loading,•• Excellent ductility, andExcellent ductility, and•• Energy dissipationEnergy dissipation

Preliminary designPreliminary designTENTEN--STORY OFFICE BUILDING DESIGNSTORY OFFICE BUILDING DESIGN

Building project is located in Los Angeles, Building project is located in Los Angeles, California, USA California, USA

(2/3) Maximum Credible Earthquake (MCE) (2/3) Maximum Credible Earthquake (MCE)

T=0.2 2 2 1.33T=1.0 0.75 1.1 0.733

SM (g) SD (g)Period (sec)

MCE with Soil factorsMCE Design Sa

Design DataDesign DataOccupancy Category: IOccupancy Category: ISeismic Design Category: ESeismic Design Category: EImportance Factor: 1Importance Factor: 1Seismic Weight W: 21,037 kipsSeismic Weight W: 21,037 kipsResponse Modification Coefficient: R=7Response Modification Coefficient: R=7System System OverstrengthOverstrength Factor: Factor: ΩΩo=2o=2Deflection Amplification Factor: Deflection Amplification Factor: CdCd=4=4Fundamental Period: Ta=1.1secFundamental Period: Ta=1.1secLoad combinations per ASCE 7Load combinations per ASCE 7--05:05:•• (1.2 + 0.2S(1.2 + 0.2SDSDS)D + )D + ΩΩoQoQEE + L + 0.2S+ L + 0.2S•• (0.9 (0.9 –– 0.2S0.2SDSDS)D + )D + ΩΩoQoQEE + 1.6H+ 1.6H

Base Shear and Force distributionBase Shear and Force distribution

151

322

513

734

965

1206

1467

1738

2009

19310

Fx (kip)Floor Level

∑ Base Shear = 1099 kip

Design ProcedureDesign Procedure

Link Link (yielding and strain hardening)(yielding and strain hardening)

Link length will control the stiffness and ductility of the systLink length will control the stiffness and ductility of the system em

Beam outside of the Link Beam outside of the Link (Elastic)(Elastic)

BeamBeam--column action will control size of frame memberscolumn action will control size of frame members

Brace Brace (Elastic)(Elastic)

Must be stronger than demands generated by the linkMust be stronger than demands generated by the link

Column Column (Elastic)(Elastic)

Forces from braces and beams.Forces from braces and beams.

The Eccentrically Braced Frame Scope:

To have significant inelastic deformation in the links “fuses”

Link Link DesignDesign

•• Factored Loads applied to the linkFactored Loads applied to the linkFrom Analysis:From Analysis:•• Ultimate Axial ForceUltimate Axial Force•• Ultimate Shear ForceUltimate Shear Force•• Ultimate Moment ForceUltimate Moment Force

•• PostPost--yielding behavior of link was controlled by shear yielding.yielding behavior of link was controlled by shear yielding.

•• Short links Short links (4ft)(4ft) are expected to yield in shear. are expected to yield in shear. (related to the shear span).(related to the shear span).

•• Link rotation angleLink rotation angle is the inelastic angle between the link and the BOL when is the inelastic angle between the link and the BOL when the the total story drift = design story drift.

•• Links are protected zonesLinks are protected zones

1)1) Check Local bucklingCheck Local buckling2)2) Determine the shear strength of the linkDetermine the shear strength of the link3)3) Check link rotation angleCheck link rotation angle4) Check lateral bracing requirements 5) Check stiffener requirements 6) Design of the welds connecting the stiffeners to the beam

Beam outsideBeam outside--ofof--the Link (BOL) the Link (BOL) DesignDesign

Determine the amplified LoadsDetermine the amplified Loads

•• The strength in the beam outsideThe strength in the beam outside--ofof--the link is based on the expected the link is based on the expected shear of the linkshear of the link

•• The resulting link end moment is based on the expected shear of The resulting link end moment is based on the expected shear of the the link.link.

•• The overThe over--strength factor is to be used in the proportioning of BOL strength factor is to be used in the proportioning of BOL elements that should remain in the linear range of response becaelements that should remain in the linear range of response because use nonnon--linear response is not acceptable.linear response is not acceptable.

1) Check Local Buckling

2) Determine unbraced length

3) Consider Second order Effects

BraceBraceDesignDesign

Determine the amplified loadsDetermine the amplified loads

Note: No part of brace connection shall Note: No part of brace connection shall extend over the link lengthextend over the link length

1) Check Local Buckling

2) Determine the unbraced length

3) Consider second order effects

4) Check combined loading

5) Check shear strength

ColumnColumnDesignDesign

Strength was determined based on Strength was determined based on the load combinations of ASCE 7the load combinations of ASCE 7--0505The columns met the seismically The columns met the seismically compact requirementscompact requirements

1) Calculate the effective length and slenderness ratio of the column

2) Check slenderness limits

Design SummaryDesign Summary

PlansPlans……

Performance EvaluationPerformance Evaluationfor the EBF Buildingfor the EBF Building

Modeling ParametersModeling Parameters

As per FEMA 356 for an EBF:As per FEMA 356 for an EBF:•• The rotation at the link:The rotation at the link:

eKQy

e

CE=θ

The Moment at yield depends on the length The Moment at yield depends on the length of the link:of the link:

CE

CE

VMe 6.1

≤ wyeCECE AFVQ 6.0==

Mathematical Representation Mathematical Representation Moment rotation relationMoment rotation relation--linklink

A

BC

D E

IO LS CP

Modeling Parameters and Modeling Parameters and Acceptance CriteriaAcceptance Criteria

FEMA 356, Table 5FEMA 356, Table 5--66

a b C IO LS CP

Mathematical Representation Mathematical Representation ForceForce--deformation relationshipdeformation relationship--bracebrace

CPLSIO

A

C

D E

B

Failure of 2nd Floor Link

BSE BSE –– 1 (Proportional to 1 (Proportional to CCvxvx))

19 in

Earthquake Hazard LevelEarthquake Hazard LevelBasic Safety Earthquake Basic Safety Earthquake –– 1 (BSE 1 (BSE –– 1): 10%/50 yrs. 1): 10%/50 yrs. Basic Safety Earthquake Basic Safety Earthquake –– 2 (BSE 2 (BSE –– 2) : 2%/50yrs. 2) : 2%/50yrs. (MCE)(MCE)

gTSCCCC eat 2

2

3210 4πδ =

For a very rigid building, the Te is small For a very rigid building, the Te is small (less than 1.0) (less than 1.0) –– results in a small target results in a small target displacementdisplacement

BSE BSE –– 1 (cont.)1 (cont.)

9.5 in

BSE BSE –– 2 (Proportional to 2 (Proportional to CCvxvx))

14.4 in

BSE BSE –– 1 (Uniform Load)1 (Uniform Load)

9.5 in

BSE BSE –– 2 (Uniform Load)2 (Uniform Load)

14.4 in

Acceptance CriteriaAcceptance Criteria

222222Uniform

000044CvxBSE – 2

000066Uniform

000022CvxBSE – 1CPLSIOLoad PatternHazard Level

Two links at the first floor.

NOTES:

The two links at floors 1 to 3.

The two links at floors 1 and 2.

First floor – 2 CP

Second floor – 2 LS

Third floor – 2 IO

Performance EvaluationPerformance Evaluation

Concluding RemarksConcluding Remarks

The building is very rigid The building is very rigid –– our our δδtt

requirements are smallrequirements are smallEven though our building satisfies Even though our building satisfies the Basic Safety Objectives we do the Basic Safety Objectives we do not feel very comfortable with its not feel very comfortable with its behaviorbehavior

RecommendationsRecommendations

It is highly recommended that a It is highly recommended that a performance evaluation be performance evaluation be conducted as part of the design conducted as part of the design Modify the beam sizes to increase Modify the beam sizes to increase ductility ductility –– hinges formed only at hinges formed only at floors 1 to 4floors 1 to 4

Questions?Questions?