seismic evaluation and retrofit of concentrically braced ... · pdf fileseismic evaluation and...
TRANSCRIPT
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Seismic Evaluation and
Retrofit of Concentrically
Braced Frames
Jeffrey Berman, Dawn Lehman, and Charles RoederAndrew Sen (UW) Ryan Ballard (KPFF), Dan Sloat (Degenkolb) & Marsha Swatosh (CPL)
Lehigh NHERI Workshop 2016
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NCBFs IN THE FIELD
EERI Tohoku EQ Clearinghouse
EERI Tohoku EQ Clearinghouse
EERI Tohoku EQ Clearinghouse
Kelly et al. 2000
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Research Motivation
> Expectation of poor seismic behavior
but not well understood
> Substantial building stock of NCBFs
> Shifting cultural expectations of
seismic performance, resiliency
> Limited retrofit guidance
NSF and AISC Funded Studies
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Large-Scale Experiments Were Critical
Single-story, single-bay tests at UW to explore connection and brace
behavior
Two-story at NEES@Berkeley:
weak-axis columns and weak-beam chevron frames
Two-story tests at NCREE: composite
beams in weak-beam chevron frames
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Evolution of CBF Design
• Pre-1988: Non-Seismic (NCBFs)
• 1988-1997: CBFs – capacity based design
• 1997-Today: Special (SCBF) – additional detailing for regions of high seismicity
• 2008: Introduction of Balanced Design Procedure (BDP) for improved performance of SCBFs
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“Current” SCBF Design
6
NCBF Design
(pre-1988)
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SCBF System Behavior:
Elastic Response
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SCBF System Behavior:
Primary Yield Mechanism
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SCBF System Behavior:
Failure Mechanism
Failure Mode:
Brace Fracture
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Balanced Design Procedure (BDP)
SCBF System Behavior
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BDP-SCBF System Behavior:
Primary Yield Mechanism
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BDP SCBF System Behavior:
Secondary Yield Mechanism
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BDP SCBF System Behavior:
Primary Failure Mode
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System Behavior: Residual Capacity (Collapse-Resistance)
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Criterion Requirements
Bra
ce
Brace Slenderness KL/r < 100
Brace Compactness
SeismicallyCompact
Brace End Rotation Clearance
Required
Co
nn
ect
ion GP-to-Frame
Connection DesignDesign for Yield Strength of GP
Weld Metal Toughness
Minimum Toughness for Demand Critical Welds
Fram
e
Framing Member Compactness
Seismically Compact
Framing Member Design
Design for Expected Brace Capacity
SCBF Design with BDP
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Evaluation and Retrofit Methodology
Based on Demand/Capacity Ratios (DCRs) and Balanced Design
Evaluate the
FrameIdentify
Deficiencies
Determine
Frame
Performance
Select &
Design
Retrofit
Strategy
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Step 1:
Evaluate Building
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Background: Infrastructure Review(Sloat 2014)
Building Survey• 12 Buildings; 8 Connection Types• Designed Before 1988• Regions of High Seismicity
Survey Results• Non-Compact Braces• Connection Deficiencies• System Level Deficiencies
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Evaluate the FrameDemand-to-Capacity Ratios (DCRs)
Yielding Mechanisms
Failure Modes
Geometric Limits
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Evaluate the System
VERTICAL IRREGULARITIES
YIELDING-BEAM MECHANISM
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Step 2:
Identify Deficiencies
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Evaluate the Connection:Compute Demand/Capacity Ratios
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Yielding Mechanisms AISC DCR BDP DCR
Whitmore Yielding 0.8 0.7
GP Bolt Bearing 0.9 1.3
Beam Bolt Bearing 1.3 1.5
Prying Action 3.6
Failure Modes AISC DCR BDP DCR
GP-Brace Weld Fracture 0.7 0.7
Whitmore Fracture 0.6 0.5
GP Block Shear 0.7 0.6
GP-Beam Weld Fracture 0.5 1.1
GP Bolt Rupture 0.6 1.5
Geometric Limits
Brace Compactness Ratio 0.92
Slenderness 89.4
Identifying Deficiencies
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LOAD TRANSFER BEAMACTUATOR
OUT OF PLANE RESTRAINTS
SHEAR TRANSFER
STRONG FLOOR
STRONG WALL
CHANNEL ASSEMBLY
AXIAL LOAD
Experimental Setup
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ConnectionType
NCBF NCBF w/ Brace RetrofitNCBF w/
2 Retrofits
Welded
BoltedContinuous
Bolted Split
End Plate
Evaluation of NCBFsLFM
PFM
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Identify Deficiencies: Non-Compact Braces
HSS 5x5x3/8
DCR = 1.0
HSS 7x7x1/4
DCR = 2.3
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Identify Deficiencies: Deficient GP-to-Brace Weld
DCR = 0.7
DCR = 1.7
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Identify Deficiencies:
Deficient Bolts
Fracture DCR: 0.6Bearing/Fracture = 1.5
Fracture DCR: 1.0Bearing/Fracture = 2.2
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Identify Deficiencies: Deficient Shear Plate Weld
SCBF
DCR = 1.5
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Identify Deficiencies: Deficient GP-to-Beam Weld
DCR = 0.7
DCR = 1.1
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Step 3:
Determine Frame
Performance
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Determine Frame Performance
Type 1 Yield and Failure Hierarchy
PYMBrace Buckling/
Yielding
PYM
PFM
PFMBrace Fracture
(GP-Brace Weld orBolt Fracture)
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Determine Frame Performance
Type 2 Yield and Failure Hierarchy
PYMBrace Buckling/
Yielding
PFMBrace Fracture
SYM
Possible SYM:• GP Yielding• Beam/Col. Yielding• Bolt Bearing• Angle Prying• End Plate Prying• Knife Plate Yielding
SYMs
PYMPFM
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Determine Frame Performance
Type 3 Yield and Failure Hierarchy
Intro → Background → Experiments → Comparisons → Retrofit → Conclusion
PYMBrace Buckling/
Yielding
PFMBrace FractureSYM LFM
Possible LFM:• Interface Weld
Fracture• Bolt Rupture• Bolt Pull Through• Angle Fracture
PYM
SYMs
LFM
PFM
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Step 4:
Select Retrofit Strategy
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Priority Deficiency Deficiency severity
HighLocally slender HSS braces > 1.5
Brace-to-gusset plate welds > 1.3
ModerateGusset plate interface welds > 0.75 (BDP)
Gusset plate clearance < 2tp elliptical
Low
Gusset plate clearance < 4tp elliptical
Shear plate bolts > 1.2
Beam yielding (chevron) > 2.5
Minimal Whitmore yielding > 1.3
Retrofit Prioritization
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Deficiency Retrofit objective Retrofit strategy
Brace local slenderness
Improve brace deformation capacity
Replace brace (BRB, HSS, In-plane)
Fill brace with concrete
Brace-to-gusset weld
Develop brace capacity (Address in brace replacement)
Gusset plate interface welds
Mitigate demandsReplace brace (in-plane buckling)
Replace brace (BRB)
ReinforceAdd bolts
Overlay weld
Tested Retrofit Strategies
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ConnectionType
NCBF NCBF w/ Brace RetrofitNCBF w/
2 Retrofits
Welded
BoltedContinuous
Bolted Split
End Plate
Retrofit of NCBFsLFM
PFM
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Bolted-Bolted Split Double Angles w/ Weld Overlay
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Yielding Mechanisms AISC DCR BDP DCR
Whitmore Yielding 0.8 0.7
GP Bolt Bearing 0.9 1.3
Beam Bolt Bearing 1.3 1.5
Prying Action 3.6
Failure Modes AISC DCR BDP DCR
GP-Brace Weld Fracture 0.7 0.7
Whitmore Fracture 0.6 0.5
GP Block Shear 0.7 0.6
GP-Beam Weld Fracture 0.3 0.7
GP Bolt Rupture 0.4 0.6
Geometric Limits
Brace Compactness Ratio 0.92
Slenderness 89.4
Bolted-Bolted Split Double Angles w/ Weld Overlay
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Damage ProgressionDrift
Range
1. Brace Buckling 0.3%
2. Severe angle prying. 3.9%
3. Brace fracture. 5.1%
Bolted-Bolted Split Double Angles w/ Weld Overlay
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Concluding Thoughts
> Large-scale testing critical to determine yield mechanisms and
failure modes.
> Analysis alone would be insufficient
> New design and retrofit should maximize yielding by balancing the
brace capacity & secondary yield mechanisms with undesired
failure modes.
> NCBFs have low drift capacity because of non-compliant braces.
Advised retrofit: brace replacement (HSS, BRBs). Size brace for
connection DCRs < 1.
> Response of connection can determine the seismic performance of
the retrofitted system. In particular welded (E70T-11) connections
sustain early fracture. Weld overlays and supplemental bolts are
valid retrofit strategies.
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Questions
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FILL BRACE WITH CONCRETE &
ADD BOLTS to WELDED SHEAR PLATE
Concrete-Filled Brace
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Note that beam-to-gusset weld still vulnerable
Replace with BRB