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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
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Conference ID: 15640201
AISC Night School – Seismic Design Manual
AISC is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES). Credit(s) earned on completion of this program will be reported to AIA/CES for AIA members. Certificates of Completion for both AIA members and non-AIA members are available upon request.
This program is registered with AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product.
Questions related to specific materials, methods, and services will be addressedat the conclusion of this presentation.
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
This presentation is protected by US and International Copyright laws. Reproduction, distribution,display and use of the presentation without written permission of AISC is prohibited.
The information presented herein is based on recognized engineering principles and is for generalinformation only. While it is believed to be accurate, this information should not be applied to anyspecific application without competent professional examination and verification by a licensedprofessional engineer. Anyone making use of this information assumes all liability arising fromsuch use.
This session will define buckling-restrained braced frames and provide an overview of the design and test requirements per the AISC Seismic Provisions. A design example will be presented as part of the lecture. The session will then discuss Quality Control and Quality Assurance requirements per the Seismic Provisions.
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
• Become familiar with the basis of buckling restrained brace frame(BRBF) design.• Gain an understanding of the differences between BRBF and typical concentrically
braced frames.• Gain an understanding of the BRBF system and connection requirements per the
AISC Seismic Provisions.• Become familiar with the Quality Control and Quality Assurance requirements per
chapter J of the AISC Seismic Provisions.
Learning Objectives
AISC Night School – Seismic Design Manual8
Presented by Thomas A. Sabol, Ph.D., S.E.Principal at Englekirk InstitutionalLos Angeles, CA
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Braced Frames and Quality Requirements
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
F4.3 BRBF – AnalysisBRBF braces shall not be considered to resist
gravity forces
Required strength of columns based on load combinations including amplified seismic load
For amplified seismic load: effect of horizontal forces including overstrength Emh shall assume all braces achieve adjusted tension or compression strength
AISC Night School – Seismic Design Manual
Chapter F
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F4.3 BRBF – AnalysisBraces shall be classified as in either tension or
compression ignoring gravity loads
Analyses shall consider both directions of seismic loading
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
F4.5 BRBF – Members – Basic RequirementsDiagonal Braces
Steel Core Plates used in steel core 2 in. thick or greater shall
satisfy minimum CVN requirements of Section A3.3
Splices in steel core are not permitted
Buckling-restraining system shall consist of casing of steel core
In stability calculations, beams, columns and gussets connecting the core shall be considered parts of the system
AISC Night School – Seismic Design Manual30
Typically, Fysc (min) = 38 ksi
F4.5 BRBF – Members – Basic RequirementsAvailable Strength
Steel core designed to resist entire axial force in the brace
The brace design axial force, φPysc, and brace allowable axial strength, Pysc/Ω, in tension and compression, according to limit state of yielding shall be:
Pysc = Fysc Asc
φ = 0.9 (LRFD) Ω = 1.67 (ASD)
whereAsc = cross-sectional area of yielding segment of steel coreFysc = specified minimum yield strength of steel core or actual yield stress of core as determined by coupon test
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
F4.5 BRBF – Members – Basic RequirementsProtected Zones
Protected zones include steel core of braces and elements connecting core to beams and columns.
AISC Night School – Seismic Design Manual32
F4.6 BRBF – Members – ConnectionsDemand Critical Welds
Groove welds at column splices
Welds at the column-to-base plate connection unless column hinging at base can be shown to be precluded by conditions of restraint and absence of net tension (including amplified seismic loads)
Welds at beam-to-column connections per Section F4.6b(b)
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
F4.6 BRBF – Members – ConnectionsDiagonal Brace Connections
Required strength of brace connections in tension and compression (including beam-to-column connections, if part of braced frame) shall be 1.1 x (adjusted brace strength in compression)
When oversized holes are used, required strength for bolt slip limit state need not exceed that from required load combinations, including amplified seismic load
AISC Night School – Seismic Design Manual36
F4.6 BRBF – Members – ConnectionsDiagonal Brace Connections
Connection design shall consider local and overall buckling. Provide lateral bracing consistent with applicable testing.
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
Refer to Brace BRB-1 in Figure 5-70. Design a buckling-restrained brace to resist the resulting axial loading, PQE = 113 kips.
SDM Example 5.5.1
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Note: There are two of these frames in the direction under consideration
BRB-1
AISC Night School – Seismic Design Manual
Note: Some printings of the SDM text incorrectly have
“1.5” on page 5-419
Frame configurations and preliminary loads have been sent to a BRB manufacturer
Elastic stiffness of the braces have been found to be 1.28 times higher than the stiffness of the yielding core area alone, if it were extended from work-point to work-point (KF = Kactual/Kcore= 1.28) .
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
These stiffness factors may be used to determine the horizontal load distribution on each story.
The applicable building code specifies the use of ASCE 7 for calculation of loads. According to AISC Seismic Provisions Section F4.3, buckling-restrained braces should not be considered as resisting gravity forces.
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AISC Night School – Seismic Design Manual
Allow for material variability of 42 ksi ± 4 ksi.
From an elastic analysis, the first-order interstorydrift is ΔH = 0.223 in.
Assume that the ends of the brace are pinned and braced against translation for both the x-x and y-y axes.
38 ksiyscF =min 46 ksiyscF =max
SDM Example 5.5.1
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
AISC Specification Appendix 8 is used to addresssecond-order effects. The required second-orderaxial strength is:
2r nt ltP P B P= + (Spec. Eq. A-8-2)
SDM Example 5.5.1
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AISC Night School – Seismic Design Manual
For the calculation of B2:
To determine Pstory, use an area of 9,000 ft2 on each floor and the surface gravity loads given in the BRBF Design Example Plan and Elevation section. Use load combinations that include seismic effects.
2
11 ( . Eq. A-8-6)
α1 story
e story
B SpecP
P
= ≥−
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
The total story shear, H, with two bays of bracing in the direction under consideration where each braced frame is designed to resist the seismic loads shown in Figure 5-70.
SDM Example 5.5.1
2(54.0 kips 49.0 kips 32.0 kips 16.0 kips)
302 kips
14.0 ft
1.0 for braced framesM
H
L
R
= + + +===
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54k x 2
49k x 2
32k x 2
16k x 2Note: There are two of these frames in the
direction under consideration
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
Considering second-order effects, the required compressive and tensile strengths of the brace are:
Determination of the brace area required to resist the required brace strength must use the minimum yield of the core material, Fysc min.
LRFD ASD
( )1.02 147 kips
150 kips
u uP T===
( )1.03 103 kips
106 kips
a aP T===
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AISC Night School – Seismic Design Manual
LRFD ASD
( )2
150 kips0.90 38 ksi
4.39 in.
usc
ysc
PA
F=
=
=
minminφ
( )
2
Ω
1.67 106 kips
38 ksi 4.66 in.
asc
ysc
PA
F=
=
=
minmin
For the limit state of tensile or compressive yielding, set the required strength equal to AISC Seismic Provisions Equation F4-1 and solve for Asc min:
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
In design practice, either LRFD or ASD design should be used consistently. The two methods give slightly different results here.
In order not to show two separate designs, the LRFD result will be used.
Try a BRB with a core area, Asc, of 4.50 in.2
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AISC Night School – Seismic Design Manual
Note that while BRB manufacturers can fabricate a BRB with the accuracy to which the core can be cut (generally ± 1/8 in. in width) it is common to round the required core area up to standard increments.
Generally, it is good practice to specify core areas in:
When specifying BRB area greater than required, the EOR must account for the increased demand that the specified area will place on the structure, because the beams and columns are designed to be stronger than the adjusted brace strength.
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AISC Night School – Seismic Design Manual
For LRFD, the available axial strength for the limit state of tensile or compression yielding is:
n ysc scP F A=min minφ φ
( )( )20.90 38 ksi 4.50 in.
154 kips >150 kips o.k.
=
=
(Spec. Eq. D2-1)
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
Verify with the brace manufacturer that the stiffness factor KF = 1.28 is acceptable for a 4.50 in.2 brace of this length.
The remainder of the brace design is performed by the BRB manufacturer.
Overstrength factors, β and ω, along with available stroke, the maximum deformation capability of the brace, must be provided by the brace manufacturer in order to design the columns and beams of the BRBF and to determine the BRB applicability to the design.
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AISC Night School – Seismic Design Manual
The final part of the brace design is establishing the expected deformation of the brace and using this deformation to determine forces that the brace imposes on the columns, beams and connections.
AISC Seismic Provisions Section F4.2 requires consideration of deformations at the greater of 2% drift or two times the design story drift.
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
The design story drift is defined in the AISC Seismic Provisions Glossary as the calculated story drift including the effect of expected inelastic action.
As given, the first-order interstory drift is ΔH = 0.223 in. This drift does not include the redundancy factor, ρ.
Note that ASCE 7 Section 12.3.4.1 permits ρ to be taken equal to 1 for drift calculations.
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AISC Night School – Seismic Design Manual
The design story drift including inelastic action is:
Twice the story drift including inelastic action is:
( )
ΔΔ
5.0 0.223 in.
1.01.12 in.
d H
e
CI
=
=
=
(ASCE 7 Eq. 12.8-15)
( )2Δ 2 1.12 in.
2.24 in.
=
=
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
Consulting with the brace manufacturer, the ω andβ factors corresponding to this level of strain aredetermined to be: ω = 1.36 and β = 1.1
Alternatively, according to AISC SeismicProvisions Section F4.3 and ASCE 7 Chapter 16,brace deformation is permitted to be determinedfrom a nonlinear analysis in lieu of the expecteddeformation requirements in AISC SeismicProvisions Section F4.2 illustrated here.
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AISC Night School – Seismic Design Manual66
End of SDM Example 5.5.1
End of Example
Example 5.5.1: BRBF Brace Design
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
The expectation is that will result in more appropriate inspection programs (e.g., engineers won’t invoke Seismic Provisions Chapter J requirements for gravity systems)
AISC Night School – Seismic Design Manual70
J5 Inspection Tasks
Tasks identified as:
• Observe (O): observe on a random, daily basis
• Perform (P): inspection shall be performed prior to final acceptance of the item
• Document (D): prepare reports indicating work has been performed in accordance with contract documents
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
• New reporting site (URL will be provided in the forthcoming email).• Username: Same as AISC website username.• Password: Same as AISC website password.
CEU/PDH Certificates
AISC Night School – Seismic Design Manual
One certificate will be issued at the conclusion of all 8 sessions.
8-Session RegistrantsCEU/PDH Certificates
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
The final exam will be issued on Tuesday, December 1. Final exam must be submitted by December 11.
8-Session RegistrantsFINAL EXAM
AISC Night School – Seismic Design Manual
Access to the quiz: Information for accessing the quiz will be emailed to you by Thursday. It will contain a link to access the quiz. EMAIL COMES FROM [email protected]
Quiz and Attendance records: Posted Tuesday mornings. www.aisc.org/nightschool -click on Current Course Details.
Reasons for quiz:
•EEU – must take all quizzes and final to receive EEU•CEUs/PDHS – If you watch a recorded session you must take quiz for CEUs/PDHs.•REINFORCEMENT – Reinforce what you learned tonight. Get more out of the course.
NOTE: If you attend the live presentation, you do not have to take the quizzes to receive CEUs/PDHs.
8-Session RegistrantsQuizzes
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AISC Night SchoolNovember 23, 2015
Application of the AISC Seismic Design ManualSession 8: Buckling Restrained Braced Frames
Access to the recording: Information for accessing the recording will be emailed to you by this Wednesday. The recording will be available for two weeks. For 8-session registrants only. EMAIL COMES FROM [email protected].
CEUs/PDHS – If you watch a recorded session you must take AND PASS the quiz for CEUs/PDHs.
8-Session RegistrantsRecording
AISC Night School – Seismic Design Manual82
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