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Larry Fahnestock, PhD, PE
University of Illinois at Urbana-Champaign
Researchers Workshop: Advanced Simulation for Natural Hazards Mitigation
December 5-6, 2016
Full-Scale Testing of Low-Ductility Braced Frames in the Lehigh Experimental Facility
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• University of Illinois at Urbana-Champaign– Larry Fahnestock (PI)
– Josh Sizemore (RA, PhD student)
• Tufts University / LeMessurier Consultants– Eric Hines (Co-PI)
– Cameron Bradley (RA, PhD student)
– Jessalyn Nelson (RA, MS student)
• École Polytechnique Montréal– Robert Tremblay (Co-PI)
– Thierry Beland (RA, PhD student)
– Ali Davaran (post-doctoral researcher)
NEESR: Reserve Capacity in New and Existing Low-Ductility Braced Frames(Grant No. CMMI-1207976)
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Research Objective
• Develop a simple yet rigorous design approach for concentrically-braced frame (CBF) buildings in moderate seismic regions that economically provides reliable seismic stability
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Research Motivation
• CBFs are the predominant steel system used in moderate seismic regions
• Minimal to no seismic detailing and proportioning are required
• Inelastic response is expected to be nonductile, but little experimental data
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Historical Perspective
• CBFs have exhibited nonductile behavior in large earthquakes (1994 Northridge and 1995 Kobe), yet not collapsed
• Commonly attributed to lateral resistance from outside the primary CBF –reserve capacity
Rai and Goel
(2003)
(EERI)
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Fundamental Paradigm
• Primary system (CBF) behavior is relatively unimportant for seismic stability of low-ductility frames
• Secondary system behavior (reserve capacity) –development of a predictable mechanism or sequence of mechanisms – is critical
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Experimental Needs
• Full-scale system testing
• Data on behavior of low-ductility CBFs
• Characterization of reserve capacity in CBFs
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Braced Frame Tests• Full scale
• Lower two stories of three-story prototypes
• Frame 1:
– R = 3 chevron
– No seismic requirements
• Frame 2:
– R = 3.25 OCBF split-X
– Ductile detailing (b/t, KL/r)
– Ad hoc capacity design (beams, columns and connections)
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Test Setup
R = 3Chevron
North
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Loading Scheme
• Quasi-static loading
• Increasing amplitude cyclic protocol
• Mixed-mode control based on top drift
• Loading beam system wrapped around test frame
• Load always applied by pushing on the test frame
• Loading beams not attached to test frame
• Test frame beams braced laterally by loading beams
• Test frame beams free to move vertically
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Laboratory Instrumentation
• 2 load cells (actuators)
• 2 load cells (reactions)
• 4 load cell pins (reactions)
• 8 string potentiometers (brace axial displacements)
• 18 inclinometers (connection rotations)
• 22 LVDTs (displacements, connection rotations)
• 80 strain gages (internal forces)
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Frame 1 (R = 3)
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Frame 1 (R = 3) Overall Behavior
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Frame 1 (R = 3) Initial Behavior
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Frame 1 (R = 3) Initial Behavior
Upper storybrace buckling
(2) (1)
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Frame 1 (R = 3) Upper Story Behavior
North brace initial
buckling
South brace final state
(2)(1)
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Frame 1 (R = 3) Upper Story Behavior
Kres
(1)(2)
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Frame 1 (R = 3) Secondary Behavior
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Frame 1 (R = 3) Secondary Behavior
• Adjust loading
• Fracture lower story brace end connection (weld)
• Observe reserve capacity mechanisms
– Brace reengagement
– Long-link eccentrically-braced frame (EBF) behavior
(1) Weld fracture(2) Compression reengagement
(3) Long-link EBF and beam hinge
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Frame 1 (R = 3) Secondary Behavior
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Test SetupOCBFR = 3.25Split-X
North
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Frame 2 (OCBF)
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Frame 2 (OCBF) Overall Behavior
(1) Beam yielding
(2) Upper story south brace buckling
(3) Lower story south brace buckling
(4) Upper story north brace-gusset weld fracture
(5) Lower story beam-gusset weld fracture
(4)
(5)
(1)
(2)
(3)
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Frame 2 (OCBF) Brace Buckling (2)
Upper Story South
(1) Beam yielding
(2) Upper story south brace buckling
(3) Lower story south brace buckling
(4) Upper story north brace-gusset weld fracture
(5) Lower story beam-gusset weld fracture
(2)
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Frame 2 (OCBF) Brace Buckling (3)
Lower Story South
(1) Beam yielding
(2) Upper story south brace buckling
(3) Lower story south brace buckling
(4) Upper story north brace-gusset weld fracture
(5) Lower story beam-gusset weld fracture
(3)
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Frame 2 (OCBF) Weld Fractures
(4)
(5)
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Frame 2 (OCBF) Weld Fracture (4)
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Frame 2 (OCBF) Weld Fracture (5)
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Frame 2 (OCBF) Overall Behavior
(1) Beam yielding
(2) Upper story south brace buckling
(3) Lower story south brace buckling
(4) Upper story north brace-gusset weld fracture
(5) Lower story beam-gusset weld fracture
(4)
(5)
(1)
(2)
(3)
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Experimental Summary
• OCBF b/t requirements provided ductile brace buckling
• R = 3 exhibited brittle brace buckling (high b/t)
• Chevron can mobilize some level of EBF action
• Split-X redundancy can lead to multi-story brace failure
• Brace compression reengagement capacity can be significant
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Project Summary
• Full-scale testing in Lehigh EF provided valuable new experimental data
• Tests were used to verify numerical modeling framework, which was then employed for extensive static and dynamic simulations
• Fundamental understanding of seismic stability for low-ductility braced frames is now established
• A new braced frame design approach is being proposed for incorporation in AISC Seismic Provisions
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Larry Fahnestock, PhD, PE
University of Illinois at Urbana-Champaign
Researchers Workshop: Advanced Simulation for Natural Hazards Mitigation
December 5-6, 2016
Full-Scale Testing of Low-Ductility Braced Frames in the Lehigh Experimental Facility