NIGHT SCHOOL SEISMIC MANUAL AISC

AISC Night School November 23, 2015

Application of the AISC Seismic Design Manual Session 8: Buckling Restrained Braced Frames and Quality Requirements

Thank you for joining our live webinar today. We will begin shortly. Please standby. Thank you. Need Help? Call ReadyTalk Support: 800.843.9166

AISC Night School – Seismic Design Manual

Today’s audio will be broadcast through the internet. Alternatively, to hear the audio through the phone, dial (855) 697-4479. Conference ID: 15640201


Copyright © 2015 American Institute of Steel Construction 1



Today’s live webinar will begin shortly. Please standby. As a reminder, all lines have been muted. Please type any questions or comments through the Chat feature on the left portion of your screen. Today’s audio will be broadcast through the internet. Alternatively, to hear the audio through the phone, dial (855) 697-4479. Conference ID: 15640201


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 addressed at the conclusion of this presentation.





Copyright Materials 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 American Institute of Steel Construction 2015 The information presented herein is based on recognized engineering principles and is for general information only. While it is believed to be accurate, this information should not be applied to any specific application without competent professional examination and verification by a licensed professional engineer. Anyone making use of this information assumes all liability arising from such use.


Course Description Session 8: Buckling Restrained Braced Frames and Quality Requirements November 23, 2015

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.





Learning Objectives • 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.


Application of the AISC Seismic Design Manual Session 8: Buckling Restrained Braced Braced Frames and Quality Requirements Presented by Thomas A. Sabol, Ph.D., S.E. Principal at Englekirk Institutional Los Angeles, CA


8




Chapter E

Application of the AISC Seismic Design Manual

Session 8 AISC Night School – Seismic Design Manual

9

Last Session • Special Concentrically Braced Frames • Examples from the Seismic Design Manual


10




F4 Buckling-Restrained Braced Frame (BRBF) Basis of Design Specially fabricated braces connected concentrically to beams and columns Eccentricities less than beam depth OK if considered in design and do not change source of inelastic deformation


11

Chapter F

F4.2 BRBF – Basis of Design BRBF are expected to withstand significant inelastic deformation (R = 8) in the links when subjected to the design earthquake. Bracing members shall be composed of a structural core and a system that retrains steel core from buckling


12




F4.2 BRBF – Basis of Design Braces shall be designed, tested and detailed to accommodate expected deformations Expected deformations are minimum 2% story drift or 2 x (design story drift), whichever is larger, in addition to brace deformations BRBF to be designed so that inelastic deformations under design earthquake occur as brace yielding in compression or tension AISC Night School – Seismic Design Manual

13

F4.2 BRBF – Basis of Design Typical brace behavior is asymmetric with respect to tension and compression and is subject to strength and stiffness degradation

P Ry Ag Fy

Pcr

Tension

Compression

Conventional brace behavior


14




F4.2 BRBF – Basis of Design P

Compression buckling of limited capacity tension member is resisted by “sleeve”

Ag Fy

“Sleeve”

Δ

Compression and tension performance are nearly identical

β Ag Fy

Balanced Hysteresis (Performance Definition)


15

F4.2 BRBF – Basis of Design Advantages of BRBF Balanced Hysteresis  Slightly Stronger in Compression

Ag Fy

Hysteretic Energy Dissipation Hysteretic Stability  Strength  Stiffness

Long Fracture Life


β Ag Fy

16




Chapter F

F4.2 BRBF – Basis of Design Performance Advantages of BRBF Story Mechanisms Uncommon  Fine-Tuning of Sizes  No Story Degradation  Distributed Yielding

Reduced Drift No Chevron Problems


17

Chapter F

F4.2 BRBF – Basis of Design Steel core shall be designed to resist entire axial force in the brace Core +

Sleeve = Buckling-Restrained Brace Assembly AISC Night School – Seismic Design Manual

18




Chapter F

F4.2 BRBF – Basis of Design

Encasing mortar

Yielding steel core Decoupling

Unbonding material between steel core and mortar Steel tube

Unbonded Brace Type

Buckling Restraint


19

F4.2 BRBF – Basis of Design – Brace Design Adjusted brace strength in compression: βωRyPysc Where:

β = compression adjustment factor ω = strain hardening adjustment factor Pysc = axial yield strength of steel core

Adjusted brace strength in tension: ωRyPysc Note: Ry need not be applied if Pysc established using coupon tests AISC Night School – Seismic Design Manual

20




Chapter F

F4.2 BRBF – Basis of Design – Brace Design Compression stress adjustment factor, β, shall be calculated as ratio of maximum compression force to maximum tension force of test specimen per Section K3.4c In no case shall β < 1.0


21

Chapter F

F4.2 BRBF – Basis of Design – Brace Design Strain hardening adjustment factor, ω, shall be calculated as ratio of maximum tension force per Section K3.4c (for expected deformations) to measured yield force RyPysc Where core material does not match prototype, ω shall be based on coupon test


22




Chapter F

F4.3 BRBF – Analysis BRBF 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


23

Chapter F

F4.3 BRBF – Analysis Braces shall be classified as in either tension or compression ignoring gravity loads Analyses shall consider both directions of seismic loading


24




F4.3 BRBF – Analysis Exceptions:  May neglect flexural forces from seismic drift, but moment from load applied to the column between points of lateral support must be considered  Required strength of columns need not exceed lesser of • Forces from foundation uplift • Forces from nonlinear analysis per Section C3. AISC Night School – Seismic Design Manual

25

Chapter F

F4.4 BRBF – System Requirements V- and Inverted V-Braced Frames  Required strength of beams, connections and supporting members shall be determined assuming braces support no gravity loads  Vertical load effect on beam shall be determined using adjusted brace strength


26




Chapter F

F4.4 BRBF – System Requirements V- and Inverted V-Braced Frames  Beams shall be continuous between columns and braced per moderately ductile requirements in Section D1.2(a)  See discussion of SCBF bracing requirements  For purposes of brace design, calculated maximum deformation of braces shall be increased to account for beam vertical deflection AISC Night School – Seismic Design Manual

27

Chapter F

F4.4 BRBF – System Requirements K-Braced Frames  Not permitted


28




Chapter F

F4.5 BRBF – Members – Basic Requirements Diagonal 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 Manual

29

F4.5 BRBF – Members – Basic Requirements Available 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

where

(ASD)

Typically, Fysc (min) = 38 ksi

Asc = cross-sectional area of yielding segment of steel core Fysc = specified minimum yield strength of steel core or actual yield stress of core as determined by coupon test AISC Night School – Seismic Design Manual

30




F4.5 BRBF – Members – Basic Requirements Protected Zones Protected zones include steel core of braces and elements connecting core to beams and columns.


31

F4.6 BRBF – Members – Connections Demand 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) AISC Night School – Seismic Design Manual

32




F4.6 BRBF – Members – Connections Beam-to-Column Connections Where brace or gusset connects to both members at beam-to-column connection:  Connection shall be “simple” per Specification Section B3.6a with required rotation of 0.025 rad…or…


33

F4.6 BRBF – Members – Connections Beam-to-Column Connections Where brace or gusset connects to both members at beam-to-column connection:  …or…Designed for moment to resist lesser of: •

1.1 x (beam expected flexural strength = RyMp)

•

Sum of 1.1 x (column expected flexural strength = Σ (RyFyZ)


34




F4.6 BRBF – Members – Connections Diagonal 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 Manual

35

F4.6 BRBF – Members – Connections Diagonal Brace Connections Connection design shall consider local and overall buckling. Provide lateral bracing consistent with applicable testing.


36




F4.6 BRBF – Members – Connections Column Splices  Comply with Section D2.5  When groove welds are used, they shall be CJP  Column splice shall develop at least 50% of the flexural strength of smaller member  Required shear strength shall be ΣMpc/Hc where Mpc = FycZc of spliced columns and Hc is clear height of column (including slab) AISC Night School – Seismic Design Manual

37

SDM Example 5.5.1

Example 5.5.1: BRBF Brace Design

This example shows the information needed by and produced by the engineer of record working with a brace manufacturer


38




SDM Example 5.5.1 Given: Refer to Brace BRB-1 in Figure 5-70. Design a buckling-restrained brace to resist the resulting axial loading, PQE = 113 kips. Note: There are two of these frames in the direction under consideration

BRB-1


39

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) . Note: Some printings of the SDM text incorrectly have “1.5” on page 5-419


40




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, bucklingrestrained braces should not be considered as resisting gravity forces.


41

SDM Example 5.5.1

Allow for material variability of 42 ksi ± 4 ksi. Fysc min = 38 ksi Fysc max = 46 ksi From an elastic analysis, the first-order interstory drift 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. AISC Night School – Seismic Design Manual

42




Solution: The governing load combinations in ASCE 7 including seismic effects are: LRFD ASD LRFD Load Combinations 5 and 6 from ASCE 7 Section 12.4.2.3 (including the 0.5 factor on L permitted in Section 12.4.2.3)

ASD Load Combinations 5 and 8 from ASCE 7 Section 12.4.2.3

( 1.2 + 0.2SDS ) D + ρQ + 0.5L + 0.2S ( 1.0 + 0.14SDS ) D + H + F + 0.7ρQE ( 0.6 − 0.14SDS ) D + 0.7ρQ + H ( 0.9 − 0.2SDS ) D + ρQ + 1.6H E

E

E


43

The required compressive and tensile strengths of the brace are: LRFD Pu = Tu = ρPQE = 1.3(113 kips) = 147 kips


ASD Pa = Ta = 0.7ρPQE = 0.7 ( 1.3 ) (113 kips) = 103 kips

44




SDM Example 5.5.1

Required Strength

Consider second-order effects AISC Specification Appendix 8 is used to address second-order effects. The required second-order axial strength is:

Pr = Pnt + B2Plt

(Spec. Eq. A-8-2)


45

For the calculation of B2: B2 =

1 ≥1 αPstory 1− Pe story

(Spec. Eq. A-8-6)

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.


46




LRFD Pstory

ASD

[1.0 + 0.14(1.0) ]  [1.2 + 0.2(1.0) ]      2 ×  68 psf + 3 ( 85 psf )   Pstory = 9,000 ft ×  68 psf + 3 ( 85 psf )   2  = 9,000 ft      +0 psf + 0.5 ( 3 )( 50 psf )   +0 psf + 0 psf + 0 psf   +0.2 20 psf  × (1 kip/1,000 lb) ( )   [1.0 + 0.14(1.0) ]  × (1 kip/1,000 lb)   [1.2 + 0.2(1.0) ]  + ×175 lb/ft ( 4 )( 390 ft )       + ×175 lb/ft ( 4 )( 390 ft )   ×(1 kip/1,000 lb)    (1 kip/1,000 lb) × = 3,630 kips   = 5,160 kips


47

SDM Example 5.5.1

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. H = 2(54.0 kips + 49.0 kips + 32.0 kips + 16.0 kips) = 302 kips L = 14.0 ft RM = 1.0 for braced frames Note: There are two of these frames in the direction under consideration AISC Night School – Seismic Design Manual

54k x 2 49k x 2 32k x 2 16k x 2

48




From an elastic analysis, the first-order interstory drift is ΔH = 0.223 in. Pe story = RM = 1.0

HL ΔH

(Spec. Eq. A-8-7)

302 kips ( 14.0 ft )

0.223 in. ( 1 ft/12 in. )

= 228,000 kips


49

Using AISC Specification Equation A-8-6: LRFD ASD α = 1.00

B2 =


1 1.00(5,160 kips) 1− 228,000 kips = 1.02 =


α = 1.60 B2 =


1 1.60(3,630 kips) 1− 228,000 kips = 1.03 =

50




Considering second-order effects, the required compressive and tensile strengths of the brace are: Pu = Tu LRFD Pa = Ta ASD = 1.02 ( 147 kips )

= 1.03 ( 103 kips )

= 150 kips

= 106 kips

Determination of the brace area required to resist the required brace strength must use the minimum yield of the core material, Fysc min. AISC Night School – Seismic Design Manual

51

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: LRFD ASD Pu ΩPa Asc min = Asc min = φFysc min Fysc min =

150 kips 0.90 ( 38 ksi )

= 4.39 in.2 AISC Night School – Seismic Design Manual

=

1.67 ( 106 kips )

38 ksi = 4.66 in.2 52




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


53

SDM Example 5.5.1 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: • 0.25 in.² increments for 0 in.² < Asc ≤ 5.00 in.² • 0.50 in.² increments for 5.00 in.² < Asc ≤ 10.0 in.² • 1.00 in.² increments for 10.0 in.² < Asc ≤ 20.0 in.², • 2.00 in.² increments for Asc > 20.0 in.² • (or maintaining increment amounts in the range of 5% to 10% of the total amount). AISC Night School – Seismic Design Manual

54




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.


55

For LRFD, the available axial strength for the limit state of tensile or compression yielding is: φPn min = φFysc min Asc

(Spec. Eq. D2-1)

= 0.90 ( 38 ksi ) ( 4.50 in.2 ) = 154 kips >150 kips o.k.


56




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. AISC Night School – Seismic Design Manual

57

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.


58




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.


59

The design story drift including inelastic action is: Cd Δ H (ASCE 7 Eq. 12.8-15) Δ= Ie

5.0 ( 0.223 in. ) 1.0 = 1.12 in. =

Twice the story drift including inelastic action is: 2Δ = 2 ( 1.12 in. ) = 2.24 in. AISC Night School – Seismic Design Manual

60




2% drift corresponds to a deflection of: Δ = 0.02H = 0.02(14.0 ft) = 0.280 ft Δ = 0.280 ft ( 12 in./1 ft ) = 3.36 in.


61

In this case, 2% drift governs. The brace spans 14.0 ft vertically and 12.5 ft horizontally. The brace deformation can be calculated to be:  Δ br =   

( 14.0 ft ) + ( 12.5 ft + 0.280 ft ) 2

( 14.0 ft ) + ( 12.5 ft ) 2

2

2

−  ( 12 in./1 ft )  

= 2.25 in.


62




Consulting with the brace manufacturer, the yield length for this brace is determined to be 70% of the work-point length. The yield length is the length over which the core is expected to yield, and is typically equal to the length of casing. Ly ≥ 0.7L = 0.7

( 14.0 ft ) + ( 12.5 ft ) ( 12 in./1 ft ) 2

2

=158 in. AISC Night School – Seismic Design Manual

63

The strain is therefore: Δ br 2.25 in. ε= = 158 in. Ly = 1.42%

Determination of the strain and the yield length is typically performed by the brace manufacturer and is shown here for illustrative purposes only.


64




Consulting with the brace manufacturer, the ω and β factors corresponding to this level of strain are determined to be: ω = 1.36 and β = 1.1 Alternatively, according to AISC Seismic Provisions Section F4.3 and ASCE 7 Chapter 16, brace deformation is permitted to be determined from a nonlinear analysis in lieu of the expected deformation requirements in AISC Seismic Provisions Section F4.2 illustrated here.


65

End of SDM Example 5.5.1

Example 5.5.1: BRBF Brace Design

End of Example AISC Night School – Seismic Design Manual

66




J1 Scope Quality Control and Quality Assurance AISC emphasizes reliance on visual inspection rather than an over-reliance on nondestructive testing (NDT) There are no QA/QC examples in the SDM, but if the built structure doesn’t follow the design requirements, all the examples in the world won’t make any difference


67

J1 Scope Quality Control Provided by the fabricator/erector Quality assurance Provided by others (e.g., third-party deputy inspectors) NDT shall be provided by QA agency except as permitted by Specification Section N7.


68




J1 Scope Specification Chapter N (new for AISC 360-10) contains universal QA/QC requirements Seismic Provisions Chapter J contains “seismic only” QA/QC requirements 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)


69

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


70




J6 (J7) Welding (Bolting) Inspection and NDT Seismic Provisions Chapter J contains tables listing inspection/NDT tasks for welding and bolting for different stages of construction: Prior to welding (bolting) During welding (bolting) After welding (bolting) Distinguishes inspection work by QA and QC personnel Also has a list of other inspection tasks (e.g., contour and finish on the RBS cut) AISC Night School – Seismic Design Manual

71

J6 (J7) Welding (Bolting) Inspection and NDT


72






73



74




`

Questions?


75

Individual Webinar Registrants CEU/PDH Certificates Within 2 business days…

• You will receive an email on how to report attendance from: [email protected]. • Be on the lookout: Check your spam filter! Check your junk folder! • Completely fill out online form. Don’t forget to check the boxes next to each attendee’s name!





Individual Webinar Registrants CEU/PDH Certificates Within 2 business days…

• New reporting site (URL will be provided in the forthcoming email). • Username: Same as AISC website username. • Password: Same as AISC website password.


8-Session Registrants CEU/PDH Certificates One certificate will be issued at the conclusion of all 8 sessions.





8-Session Registrants FINAL EXAM The final exam will be issued on Tuesday, December 1. Final exam must be submitted by December 11.


8-Session Registrants Quizzes 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 Registrants Recording 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.


Thank You Please give us your feedback! Survey at conclusion of webinar.


82


NIGHT SCHOOL SEISMIC MANUAL AISC

Recommend Documents