buckling-restrained braced frames - seaot · buckling-restrained braced frames (brbfs) • code...

Buckling-Restrained Braced Frames

by Walterio A. López, SE Rutherford & ChekeneRafael Sabelli, SE Walter P Moore

Buckling-Restrained Braced Frames (BRBFs)• Code Intent• How BRBs work• Brief History of BRBFs in US Codes• Sample BRBF Construction• Brief treatment on testing• Building-Code Design• Design Methodology• Specification, Other Issues• Gusset Connections• Summary

Code Intent

Building-Code Philosophy

Objective: Prevent collapse in the extremeearthquake likely to occur at a building site.

Objectives are not to:

limit damagemaintain functionprovide for easy repair

Presenter

Presentation Notes

Before going further, it is instructive to review the basic philosophy of building codes for seismic design of ordinary structures. The basic intent is to prevent collapse of a structure in the event of an extreme earthquake, in order to prevent loss of life. More specifically, the performance goal of current codes for most ordinary structures is to prevent collapse in the maximum considered earthquake (MCE). In the western US, the MCE is based on the largest earthquake that can be generated by known faults. In the rest of the US, the MCE is defined as an earthquake with a 2-percent probability of exceedance in 50 years (which translates to a recurrence interval of about 2500 years). In the event of the MCE, the objective is to prevent collapse in order to prevent loss of life. The objectives are not to limit damage to the building, maintain the function of the building, or to provide for easy repair of the building. Thus, the primary goal is to protect lives, not property. If a code-compliant building sees a very strong earthquake, one would expect substantial and costly damage to the structure. If a higher level of performance is desired, then it is necessary to go beyond a basic code design. A useful analogy is as follows...... We design buildings for extreme (but rare) events like strong earthquakes like a car manufactures designs a vehicle for an extreme event like a head-on collision. In the event of a major collision, the design goal is to protect the occupants of the car; not to protect the car itself. In fact, for a major collision, the car is used in a sacrificial manner to absorb the energy of the impact, thereby protecting the occupants. In a similar way, in the event of a major earthquake, a building is used in a sacrificial manner to absorb the energy of the earthquake, in order to prevent collapse and protect the occupants. If we designed cars to withstand a head-on collision without damage to the car, probably most of us could not afford to buy a car. Similarly, if we designed buildings to withstand an extreme earthquake without damage, we couldn't afford to construct most buildings.

To survive a strong earthquake without collapse:

Design for Ductile BehaviorMaterial Ductility

Member Ductility

System Ductility

Presenter

Presentation Notes

In order to economically design a building to withstand a very strong earthquake, the key is not provide to a building with high strength, but rather to provide a building with high ductility. Ductility is the key to building survival is strong earthquakes. The key objective of the AISC Seismic Provisions is to provide steel building structures with high ductility.

AISC Methodology

Designate fuses Members that undergo inelastic strain

Provide ductility in fuse membersPrevent local buckling

Prevent member instability

Prevent connection failure

Design system to ensure ductility is concentrated in fuses

Presenter

Presentation Notes

These are the major topic areas to be covered in the first module.

How BRBs work

What is a Buckling-restrained Brace? Two Definitions

De-Coupled Stress and Buckling(Mechanics Definition)

Balanced Hysteresis(Performance Definition)

Stress resisted by steel core

Buckling resisted by sleeve

BRB Definitions Explained: Conventional Bracing

Brace behavior is asymmetric with respect to tension and compression and is subject to strength and stiffness degradation Pcr

Ry Ag Fy

Compression

Tension

0 31 2C

ompr

essi

on S

treng

thSlenderness Parameter λc

BRB Definitions Explained: Sleeved Column

Sleeve achieves π2EI/L2

Stress is zeroNo material stress limit

Fy A

π2 E I

L2Steel core achieves Fy Aλc ~ 0kl/r ~ 0

Brief History of BRBFs in US Codes

Historical Background1st BRBF paper: 2000 SEAOC ConventionBRBF design presentations:

SEAOC: 2001-2006NASCC: 2004, 2005Steel TIPS Seminars: 2004ASCE Structures Congress: 2005AISC braced frame seminars: 2005, 2006

BRBFs in U.S. to date: >100 bldgs, >15,000BRBs

Background (recent past/present)SEAOC/AISC BRBF committee

Background (present)

Sample BRBF Construction

Sample Construction

BucklingRestrained

Brace

Unbonded Brace

Buckling-Restrained Brace Types

PowerCat Brace

ACMEBracing

Company

Buckling-Restrained Brace Assembly

Core

Sleeve

Buckling-Restrained Brace Assembly

Buckling-Restrained Brace Mechanics

Unbonded Brace Type

DecouplingBucklingRestraint

Encasing mortar

Yielding steel core

Steel tube

Debonding material between steel core and mortar

Brief treatment on testing

Selected Testing Data

Literature Reference Test Type

Number of Tested Braces

Brace Strain

(%)

SIE, 1999

Uniaxial 3 2.1

SIE, 2001

Uniaxial 2 2.1

UC Berkeley, 2002

Frame (Subassemblage) 3 1.8 - 2.1

Merritt et al., 2003a

Subassemblage 6 2.4 - 2.7

Merritt et al., 2003b

Subassemblage 8 1.8 - 2.6

Merritt et al., 2003c

Uniaxial 2 1.6, 1.7

SIE, 2003

Subassemblage 4 1.6 – 3.0

BRB Tests Short Summary

• About 50+ different brace tests have been performed in support of US projects

• All tests results so far have met Appendix T’s acceptance criteria

• Tests have included Appendix T, moment frame, near-field, and fatigue displacement protocols

• Kinematic rotations of brace ends were not detrimental to brace performance

Building-Code Design

R Values

7 for Basic BRBF System

8 for BRBF System with Rigid Beam-Column Connections

8 for BRBF/SMF Dual System

ASCE 7 2005 (with Supplement 1)

Ωo Values

2 for Basic BRBF System

21/2 for BRBF System with Rigid Beam-Column Connections

21/2 for BRBF/SMF Dual System


Cd Values

51/2 for Basic BRBF System

5 for BRBF System with Rigid Beam-Column Connections

5 for BRBF/SMF Dual System


Height LimitsSeparated by Seismic Design Category:B&C D E FNL 160* 160 100 for Basic BRBF System

NL 160* 160 100 for BRBF System with RigidBeam-Column Connections

NL NL NL NL for BRBF/SMF Dual System

(NL = Not Limited)*Can be increased to 240 for regular buildings.


Coefficients for Determination of Approximate Period

Ta = Cr (H/ft.)x

Cr = 0.03 (ASCE to incorporate)x = 0.75(Similar to EBF)


Combined effect of R and T

Period

Des

ign

Bas

e Sh

ear SCBF

BRBF

SCBF Building

BRBF Building

Design Methodology

Design Procedure

Define appropriate BRB modelingDetermine required brace strengthCheck drift Determine brace displacements at 2.0 Δm

Compare required displacements to existing testsPlan and conduct new tests?

Determine adjusted BRB strengths at 2.0 ΔmRequires test data or manufacturer’s summary

Calculate required strength of columns, beams, and connections based on adjusted BRB strengths

BRBF Design Methodology• BRB is energy dissipater• Steel core material

specified as mild & ductile

• Design checks:• BRB φPn• Global drift • BRB deformation, ΔbM• Adjusted BRB strengths

• Beam Ru/φRn• Column Ru/φRn• Connections Ru/φRn

Analysis

Gravity LoadSize frame to resist 100% of gravity

All load combinationsDo not model braces as resisting gravity load

Check that braces do not yield under Live LoadSeismic Load

Size braces for seismic load onlyDo not model braces to resist gravity loadSize for 100% of seismic load?Or consider shear in columns

Found by analysisSize frame considering plastic mechanism

Design Summary

Design for seismic force from analysis; do not include gravity

Check to make sure live load does not cause (cyclic) yieldingBraces

Design for maximum brace forces, plus 100% of gravity

Design for 100% of loadFrame

1.2D + 0.5L + E1.2D + 1.6LSeismicGravity

Load Combination

Brace StiffnessKbr = P/Δ

Δ ~ PLy/AyE

Ly = 0.5-0.8 L(depending on brace type and configuration)

Kbr = 1.2 - 2.0 AyE /L

Flexibility L y

EA sc

L L y

EA nonyielding

BRB Modeling

Kbr = 1.3 AscE /L ? Kbr = 1.6 AscE /L ?

BRB Modeling (Nonlinear)

Isotropic and kinematic strain hardeningDifference in tension/compression valuesModified DRAIN, PERFORM 3D

Steel Core Material

• Specifications• ASTM A36 Grade 36/42 • JIS G3136 SN400B

• Wide range of yield strength not desired• Solution: supplementary yield strength

requirements verified by coupon tests• Current practice: material procured based on

MTRs, coupon tests performed prior to fabrication

Preliminary BRB Design

ysc

usc F

PA

φ≥

θcos2 F

Pu= Assume braces

resist 100% of story shear

Design braces to calculated capacity(Pu = φPn = φFyscAsc)

F

θ

BRB Axial Deformation Check

Compute elastic story drift ΔXExtract from analysis program ΔbX = Δbrace at ΔXstory drift

BRB Axial Deformation Check

ΔbX is computed at largest elastic story drift (ρ = 1.0 for drift)

Compute ΔbM = Cd ΔbX = Δbrace at ΔM story driftCompute max. brace strain εMAX= 2.0ΔbM / Lysc

εMAX cannot exceed maximum value testedIf εMAX exceeds tested values, resize BRB

BRB Axial Deformation Comparison

For a ASCE 7 earthquake (2/3 of MCE)2.0 Δbm ~ 10 Δby (elastic methods, Ch. 16)Mean = 9-11 Δby (Sabelli, Fahnestock)

For a 2%/50 year eventNot addressed in codesMean = 17-19 Δby (Sabelli, Fahnestock)

• Ductilities underestimated but not forces• Solution: fabricate BRBs to Δby larger than

predicted by elastic methods

Plastic Mechanism

All braces yieldingTension or compressionStrain Hardened“Adjusted strength”

= Maximum forceBased on first mode

BRB Adjusted Strength

Compression: βωRy Fysc AscTension: ωRy Fysc AscAdjusted for Various Factors

ω Strain-Hardeningβ Compression OverstrengthRy Material Overstrength

If Fy is used as core yield strength Fysc, Ry is > 1.0If Fysc is taken from material coupon test, Ry = 1.0.


FactorsFactors Taken from Test Results within 2.0

Δm.Compression Strength Adjustment Factor

β = Cmax/Tmax

Strain-Hardening Adjustment Factor ω = Tmax/FyA

Provided by brace manufacturers

BRB Uniaxial Test Results

Hysteresis courtesy of SIE, Inc.

BRB Adjusted Strength (example)

εMAX = 0.98 % at 2.0ΔbM

Go to graph from BRB manufacturer and obtain:ω = 1.22ωβ = 1.25β = ωβ/ω

= 1.25/1.22 = 1.03


Case at inverted-V beam

ωRyFyscAsc

βωRyFyscAsc

βωRyFyscAsc

ωRyFyscAsc

βωRyFyscAsc

ωRyFyscAsc

βωRyFyscAsc

ωRyFyscAsc

βωRyFyscAscωRyFyscAsc

Frame Design: Model BRB Forces Directly

Combine with 1.2D + 0.5 L + 0.2 Sds D

Column flexural forces not calculated

E = 1 ksi

α = 1/oF

ΔT = ωRyFysc

Tension

ΔT = βωRyFysc

Compression

Axial force approximationColumn flexural forces not calculated

Combine with 1.2D + 0.5 L + 0.2 Sds D

Frame Design: Model BRB forces Using Temperature

Specification, Other Issues

Use of Proprietary BRBs

Engineer Specifies:Brace StrengthBrace Core Area (or stiffness)Maximum and Minimum Fy

Displacement rangeManufacturer Provides:

Braces that meet the specificationTest data that qualifies the braces

Typical Specification of BRB Size- ASC

Uncertainty in strength (example)Calculations

φPn = 0.9Aysc (38 ksi)Ry = 46 ksi/38 ksi = 1.21

DrawingsAsc = 8.5 in.2 (for example)

Specifications38 ksi ≤ Fysc ≤ 46 ksi

ManufactureAsc = 8.5 in.2

323 kips ≤ Pysc ≤ 391 kips

Proportioning of strength likely similar to design

Alternate Specification of BRB Size- Pysc

Uncertainty in stiffness (example)Calculations

φPn = 0.9Asc Fysc where Fysc is measured during manufacture and Asc is adjusted accordingly

Ry = 1.0Asc = φPn /0.9 (44 ksi) [reasonably low stiffness for analysis]

DrawingsPysc = 323 kips (= Pu /φ)

Specifications38 ksi ≤ Fysc ≤ 46 ksi

ManufacturePysc = 323 kips7.0 in.2 ≤ Asc ≤ 8.5 in.2

Proportioning of stiffness likely similar to design

Construction AdministrationGeneral contractor

BRBDetailer

Steel fabricator

Fabricator Detailer

DrawingExchange

Coordinated submittals:BRBs, gusset plates, frames

Code Issues

• BRB is a better brace that doesn't buckle.• BRB is a performance-specification item.• Single diagonals in one direction and stacked

chevron allowed without penalty.• BRB and gussets often need not be fireproofed.• If manufactured in approved shop, inspections

may be waived.• Non-structural attachments to casing not

prohibited.

Gusset Connections

Sample Connections

Alternative Connections

Courtesy ofSTAR Seismic

Courtesy ofCoreBrace

Direct bolting of core

Direct welding of core

Gusset Plate Design Issues

•Adjusted BRB strengths readily determined from backbone curve (first validation of methodology)

•Frame fixity must be acknowledged in analyses

•Recognize that cyclic testing of gusset plates not fully developed

•Avoid unnecessary connection restraint

Beam (or column) yield at <1%Rotation ductility not testedThese issues apply to all gussets at large drift

SCBF and OCBF drift likely to be greater than BRBFEBF rotations may be much greater

Potential Connection Issues

Courtesy of K.C. Tsai

Potential Connection Issues

Pin Connection

Courtesy of

L. Fahnestock

Summary

BRBF Design Summary

• BRB is energy dissipater

• Check BRB ductility demands

• Check surrounding elements for adjusted BRB strengths

Overall Summary

BRBs harness steel ductility to provide member ductility

BRBF provide a ductile system ifConnection failure is precludedBraces are proportioned to earthquake demandFrame is designed for plastic mechanismBraces are properly specified.

Thank You

buckling-restrained braced frames - seaot · buckling-restrained braced frames (brbfs) • code...

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