by tim nordstrom, pe, se -...
TRANSCRIPT
www.ncsea.com
Buckling Restrained Brace Frames 2015Presented by
TIM NORDSTROM, PE, SE
(435) 940‐9222 | [email protected]
1
www.ncsea.com
IntroductionAgenda• Introduction• History• Design Procedure• Specification• Submittal Review• Project Types
HANG ON WE’RE IN FOR A WILD RIDE!
2
www.ncsea.com
IntroductionAISC 341‐10 GLOSSARY:
“Buckling‐restrained brace.
A pre‐fabricated, or manufactured, brace element consisting of a steel core and a buckling‐restraining system as described in Section F4 and qualified by testing as required in Section K3.”
Acronyms BRB ‐ Buckling Restrained Brace
BRBF ‐ Buckling Restrained Braced Frame
3
www.ncsea.com
Introduction
How does a BRB work?
4
www.ncsea.com
History of the BRB
Where did the BRB come from?
“Properties of Brace Encased in Buckling‐Restraining Concrete and Steel Tube” (1988) Watanabe, A., Hitomi Y., Saeki, E., Wada, A., and Fujimoto, M.
Proceedings of Ninth World Conference on Earthquake Engineering
Vol. IV pp. 719–724
Japan Association for Earthquake Disaster Prevention
Tokyo‐Kyoto, Japan
5
www.ncsea.com
History of the BRB
First Building in the United States to use BRBs
Year 1999Plant and Environmental Science BuildingUniversity of CaliforniaDavis Campus
Manufactured by Unbonded Brace
6
www.ncsea.com
History of the BRBUS based BRB Manufactures are incorporated 2002 Star Seismic LLC 2002 CoreBrace LLC
2003 FEMA 450 NEHRPSection 8.6 Recommended Provision for Buckling–Restrained Braced Frames
2004 Steel tips for BRBF
7
www.ncsea.com
History of the BRB
2005 AISC 341‐05 ASCE 7‐05 IBC 2006
8
www.ncsea.com
History of the BRB2010 AISC 341‐10AISC Seismic Design Manual 2nd EditionIncludes BRBF Design Example
9
www.ncsea.com
BRBF Design Procedure Design Methods
Method Strength Variability Stiffness Variability
RecommendedUse
AREAbased
approach
Engineer specifies Core Area, Asc
Fysc = 39 – 46 ksi
Engineer Specified(± 10%)
New BRBF
STRENGTHbased
approach
Engineer specifies Req’d Strength, Pu
Pysc = Pu (+5%, ‐0%)
Engineer Specified(± 10%)
RetrofitsMulti‐Tier Brace
Frames
10
www.ncsea.com
BRBF Design ProcedureBRBF Design Procedure1. Preliminary Design Phase2. Consultation with Manufacturer3. Design Iteration4. Specification5. Brace Submittal
11
www.ncsea.com
BRBF Design Procedure1. Preliminary Design Phase
Structural Engineer of Recorda) Determine Base Shearb) Layout Braces/Frames c) Size Core Areas, Asc
i. Stiffness Factor, KF (Assumed)ii. Brace Overstrength Factors, and (Assumed)
d) Preliminary size Membersi. Beams ii. Columns (with orientation)
e) Check Drifts
12
www.ncsea.com
Computer ModelingETABS RAM Structural System SAP2000, RISA, STAAD
13
www.ncsea.com
Base ShearASCE 7‐10 Design Coefficients and Factors
Seismic Force Resisting System R W0 Cd
Height Limit, hn (ft)
Seismic Design Category
B C D E F
Buckling Restrained Braces 8 2½ 5 NL NL 160 160 100
BRBF + Height IncreaseNo torsion, single line <60% 8 2½ 5 NL NL 240 240 160
BRBF + Moment FramesMF can take 25% 8 2½ 5 NL NL NL NL NL
Non‐building Structure Type (ASCE 7 Chapter 15)
Buckling Restrained Braces Contact Us
14
www.ncsea.com
Base Shear
Response Spectra for three different systems
15
www.ncsea.com
Tdesign
Stiffer Braces-Higher Force
Softer Braces-More Drift
Tdesign
Softer Braces-More Drift
Stiffer Braces-Same Force,
Less Drift
Base Shear
Brace stiffness can impact design force levels
Computer modeling:Allow the computer to calculate the building period
16
www.ncsea.com
Brace LayoutTypical BRBF Configurations
TWO STORYX
INVERTED V(aka Chevron)
VSINGLE DIAGONAL“ZIP‐ZAG”
SINGLE DIAGONAL“ONE‐WAY”
17
www.ncsea.com
Brace Layout
Keep in mind: Try to achieve a redundancy factor, = 1.0 See ASCE 7‐10, Section 12.3.4
Try to keep brace angles between 30‐60 degrees
BRBF only needs a single brace.
18
www.ncsea.com
Brace LayoutSingle Diagonal Frame: Zig‐Zag vs. One‐WayConnection Count Load Path Length
19
www.ncsea.com
Brace LayoutTwo Story X vs. ChevronsConnection Count Load Path Length
20
www.ncsea.com
Brace Layout
Reduce the # of braces in the upper levels?
Brace layout is flexible
21
www.ncsea.com
Size BracesBRB Available Strength
= 0.9Fysc = 39 ksi for AREA based designFysc = 42 ksi for STRENGTH based designPu = demand load on the brace
Solve for Asc .= .
Round off core areas:Asc = 1.0 to 2.0 (0.1 in2 increments)Asc = 2.0 to 6.0 (0.25 in2 increments)Asc = 6.0 to 20 (0.5 in2 increments)Asc > 20 (1 in2 increments)
BRB Range Available Minimum Maximum ??
Pysc 35 kips 2,000 kips
Asc 1.0 sq in 50 sq in
22
www.ncsea.com
Preliminary BRB FactorsBRB PRELIMINARY design factors
KF – Stiffness modification factor
Overstrength factors– Tension 1.50 – Compression 1.65
Core Area, Asc (in2)Length between Work‐Points, Lwp‐wp < 18 ft 20 – 30 ft > 30 ft
1.0 to 6.0 1.7 1.5 1.26.5 & up 2.0 1.5 1.3
GROSSLYSIMPLIFIEDStiffnessModificationFactor(KF)Forpreliminarydesign
23
www.ncsea.com
Member Requirements
Columns & BeamsHighly Ductile Members (hd) see AISC 341 Section F4.5aWide FlangeHSS
• Round• Square• Rectangular
See Table D.1 of AISC 341‐10 for more information on Highly Ductile Members
24
www.ncsea.com
Drifts and Brace DeformationCheck Story Drifts
Drift limits set by code, design objective etc…• ASCE 7‐10 New Buildings• ASCE 41 Retrofits• Building/Structure Functionality
Increase in BRB core area or additional braces may be required to meet drift requirements.
max
25
www.ncsea.com
BRBF Design Procedure2. Consult with BRB Manufacturer
Structural Engineer of Recorda) Sends info shown in graphic to BRB manufacturer
BRB Manufacturera) Brace Core Areasb) Brace Stiffness, (KF)c) Overstrength
i. Tension, ii. Compression,
26
www.ncsea.com
Verify Sufficient TestingAISC 341‐10 Section F4. (3) Conformance Demonstration“The design of braces shall be based upon results from qualifying cyclic tests …”
(a) Tests … for research or … for other projects
(b) Tests … specifically for the project
AISC Section K3.3:
50% < Pysc < 120%Extrapolation beyond the limitations stated in this section is permitted subject to qualified peer review and approval by the authority having jurisdiction.
27
www.ncsea.com
Stiffness VerificationBrace Stiffness (K) and/or Brace Stiffness Modification Factor (KF)
KF
28
www.ncsea.com
Brace Stiffness
Brace same STRENGTH different STIFFNESS
Yielding Core Length
Yielding Core Length
BRB Manufacturers have some flexibility with stiffness of the braces. Strain levels must be checked!
29
www.ncsea.com
Stiffness Controlled Stiffness Not Controlled
Cm
Cr
Cm
Cr
Brace Stiffness
30
www.ncsea.com
Brace Overstrength FactorsOver‐strengths and Where do they come from?
AISC 341‐10 Section F 4.2
Braces shall be designed, tested and detailed to accommodate expected deformations.
Brace expected deformations are the larger Story Drift of :
2bm > 2% of the Story Height or
2x Design Story drift
+ brace deformations due to gravity loading.
2bm
31
www.ncsea.com
PyscPysc
Pysc
Brace Overstrength FactorsOver‐strengths and Yielding Core Strain at a deformation of 2bm, = 0.82%
840
1,120820 1.33
1,1801,120 1.05
32
www.ncsea.com
BRB Manufacturer ConsultBRB Manufacturer Design Verification/Iteration
33
www.ncsea.com
BRB Manufacturer Consult
34
www.ncsea.com
Member Requirements
AISC 341‐10 Section F4.2a“…brace connections and adjoining members shall be designed to resist forces calculated based on adjusted brace strengths.”
Load Combo Example:1.2 0.2 0.2
Pysc
Pysc
Pysc
Pysc
Brace Loads Columns
35
www.ncsea.com
Member Requirements
V‐ and Inverted V‐Braces Frames
Beam ‐ Highly Ductile (F4.5a)
Beam Bracing Moderately Ductile Members (F4.4a.(2))
PyscPysc
36
www.ncsea.com
BRBF Design ProcedureIf Necessary…3. Design Iteration. SEoR reanalyzes
a) Finalize member sizes:i. BRB core area, Asc ii. BRB Stiffness (K) or Stiffness Factor, KFiii. Beam and Column Sizes
Typically, the sooner the BRB manufacturer is brought into the project, fewer iterations are done.
37
www.ncsea.com
BRBF Design Procedure4. Specification by Structural Engineer of Record
a) Buckling Restrained Braces (BRB) core area, Asc (in2)b) Yield Strength, Fysc 39 ksi – 46 ksic) Overstrength, and d) BRB Stiffness (K) or Stiffness Factor, KF tolerance (+/‐ 10%)e) Brace Deformation and Connection Rotationf) Testing per AISC Provisiong) Connection Design Responsibilities
38
www.ncsea.com
BRB Schedule and Notes
BRB SCHEDULE NOTES:
1. Buckling Restrained Braces (BRB) are to be tested per AISC 341‐10. BRB Manufacturer shall submit proof of each braces compliance with load and strain range.
2. Demand Strength, Pu, is the minimum code level available strength reqruied for the brace, using LRFD force levels. Where, Pu < Pysc = 0.9 Asc Fysc.
3. Fysc is the actual yield strength of the BRB core determined by coupon testing. a) Acceptable Range: 39 ksi < Fysc < 46 ksi. b) Charpy testing reqruied for core material 2” and thicker.
4. Brace Stiffness,
a) Acceptable range, ±10% b) KF & Asc are shown in the table abovec) Lwp‐wp is the work‐point to –work‐point length along the brace.
5. Brace elongation shall be calculated as the maximum story drift of either: 2% of the story height or 2x the design story drift.
6. Values shown for and are maximums and shall not be exceeded.
39
www.ncsea.com
BRBF Beam‐Column‐Brace ConnectionsAISC 341‐10 Seismic Provisions for Structural Steel BuildingsSection F4.6b Beam‐to‐Column ConnectionsTwo Connections Options:1. Simple 2. Moment
40
www.ncsea.com
BRBF Beam‐Column‐Brace ConnectionsSimple Connections(a) The connection shall be a simple
connection meeting the requirements of Specification Section B3.6a where the required rotation is taken to be 0.025 rad
41
www.ncsea.com
BRBF Beam‐Column‐Brace ConnectionsMoment Connections(b) The connection shall be designed to resist a
moment equal to the lesser of the following:i. A moment corresponding to the expected
beam flexural strength multiplied by 1.1 (LRFD) or by 1.1/1.5 (ASD), as appropriate. The expected beam flexural strength shall be determined as RyMp.
ii. A moment corresponding to the sum of expected column flexural strengths multiplied by 1.1 (LRFD) or by 1.1/1.5 (ASD), as appropriate. The sum of expected column flexural strengths shall be Σ(RyFyZ).
42
www.ncsea.com
BRBF Specification (+)PINNED CONNECTION ENDS
Preferred by many Architects for AESS applications.
CostsManufacturing $↑Installation $↑
Used for Large Capacity BRBPysc > 750 kipsAsc > 20 in2
Often times gussets are field welded to accommodate the 1/32” pin tolerance.
43
www.ncsea.com
BRBF Specification (+)WELDED CONNECTION ENDS
Ideal for retrofit construction
Built in erection tolerance of 2”
CostsManufacturing $↓Installation $↑
44
www.ncsea.com
BRBF Specification (+)BOLTED CONNECTION ENDS
Preferred by many erectors
Manufacturing cost $↑Installation (no welding) $↓
45
www.ncsea.com
BRBF Specification (+)Protected Zones: AISC 341‐10 Section F4.5c
46
www.ncsea.com
BRBF Specification (+)Casing Shape No Preference
Square/Rectangular
Round
Casing Size Limit No Limit
10” Maximum Width
12” Maximum Width
14” Maximum Width
Other:
47
www.ncsea.com
BRBF Specification (+)Casing finishesPainted, Galvanized or Stainless Steel
48
www.ncsea.com
BRBF Specification (+)AESS‐Architecturally Exposed Structural Steel
Please make AESS requirements CLEAR!
STRUCTURAL DRAWINGS point to AESS requirements
49
www.ncsea.com
BRBF Specification (+)Slab interference
50
www.ncsea.com
BRBF Specification (+)Slab interference
51
www.ncsea.com
BRBF Design Procedure5. Brace Submittal
a) Buckling Restrained Braces (BRB) Shop Drawingsb) BRB calculations:
i. Brace strength ii. Brace stiffness verificationiii. Overstrength factorsiv. Connection Calculations
a. Brace to Gussetb. Gusset (if applicable)c. Gusset to base plate / beam / column (if applicable)
c) Test Documentationd) SEoR review
52
www.ncsea.com
Brace SubmittalReviewing the BRB Submittal:
Frame Elevations Shop Drawings Calculations Brace Test Reports Core Material Tests
Create a Checklist of the important items for the project.
AESS Casings etc….
53
www.ncsea.com
Project TypesWarehouses
Why?
• Very long braces
• Flexible layout
54
www.ncsea.com
Project TypesMedical Facilities
• Hospitals• Medical Offices• Clinics
Poplar Bluff Regional Medical CenterPoplar Bluff, MO
1) TRUE FALSE
55
www.ncsea.com
Project TypesSchools
• Primary Education• University Buildings• Laboratories
Primary SchoolSanta Clara CA
56
www.ncsea.com
Project TypesSmall Structures
Skykomish Transfer Facility, Skykomish, WA
57
www.ncsea.com
Project Types
Parking Structures
John Wayne Airport Parking Structure COrange County, CA
Legacy Emanuel Parking Structure, Portland, OR
58
www.ncsea.com
Project TypesRetrofitsReplace existing braces with BRB Single BRB as a buttress
(continued operation during retrofit)
VA Medical Center, Seattle, WA Confidential Power Station, CA
Brace Capacity ≈1,100 kips (4,900 kN)
59
www.ncsea.com
Project TypesRetrofits Continued…BRBs across horizontal joints between adjacent structures to prevent pounding
Nursing Tower and CLC, Seattle VA Hospital, Seattle, WA
60
www.ncsea.com
Project TypesRetrofits Continued…High‐rise retrofit, in this case, using BRB to tri existing moment frames together.
Changes the structural response of the building frame.
This retrofit was done a minimal number of floors.
140 New Montgomery, San Francisco, CA
61
www.ncsea.com
Project Types
Civil Structures: Bridges and Dams
Casad Dam, Bremerton WA Harbourside Pedestrian Bridge, North Vancouver British Columbia, Canada
62
www.ncsea.com
Project TypesOutrigger SystemsHigh‐rise Buildings
One Rincon Tower, San Francisco, CAWaMu Tower, Seattle, WA
63
www.ncsea.com
Project TypesEvent Centers• Stadiums
• Arenas
• Theaters
• Convention Centers
Rio Tinto Stadium, Salt Lake City, UT
64
www.ncsea.com
Project TypesIndustrialBuilding Like Structures
Pipe Racks
Material Handling
Plum Point Energy Station Osceola, AR
65
www.ncsea.com
Project TypesMulti‐Tier Braced Frames(MTBF)
Seahawks Practice FacilityRenton, WA
66
www.ncsea.com
PowerCat 350 test at UC San DiegoStep 1) AISC Qualification
→ Step 2) Northridge Simulation
→ Step 3) 100 cycles at 1.5 x MCE
Final ThoughtsNote:
AISC Requires Cumulative Inelastic Deformation (CID)
≥ 200 times yield deformation
This brace withstood 1800 CID without failure
67
www.ncsea.com
NOT BRBs!
BRBs
Final ThoughtsBuckling Restrained Braces are tested, manufactured assemblies.
BRBs are proprietary
BRBs can be competitively bid
BRBs can comply with Buy America or Buy American Requirements
68
