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Quick-Release Field Experiments on Seismically Isolated Bridges
Stuart S. Chen, Ph.D., P.E.Associate Professor, University at
Buffalo (SUNY), Fulbright Senior Lecturer, ITU
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Presentation Outline
• Comprehensive Study Overview
• Bridges Studied
• Loading and Instrumentation Apparatus
• Selected Results
• Anticipating Seismic Performance
• Summary, Conclusions, Recommendations
• Acknowledgements
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Comprehensive Study!: Overview
• Two Similar Bridges, Before and After Seismic Retrofit with Isolation Bearings
• Field (System Quick-Release) and Laboratory (Component Reverse-Cyclic) Experiments
• Comparing Analytical vs. Experimental Dynamic Behavior
• Inferring Seismic Performance from Calibrated Models
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Seismic Isolation Concept
(Kelly, 2001)
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Seismic Isolation Concept (cont’d)
(Kelly, 2001)
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Bridges’ Plan View
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Bridges’ Elevation; Brg. Schedule
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Bridge Cross Sections
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Bridge Bearing Replacement (Abut.)
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Bridge Bearing Replacement (Pier)
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Installing Isolation Brgs at Abutments
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Southbound Abutment Isolator
Kelly, 2001
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Southbound Pier Isolator
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Northbound Expansion Bearing (non-seismic)
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Northbound Bridge Bearings
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Objectives
• Measure and assess in-situ dynamic behavior and performance of typical slab- on- girder bridge subjected to transverse quick-release loading (steel brgs vs. seismic isolation brgs vs. standard laminated elastomeric brgs)
• Quantify in-situ dynamic performance change attributable to bearing retrofits
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Objectives, cont’d
• Quantify Winter-Weather Effects on In-Situ Performance of Isolation and Elastomeric Bearings
• Assess Relative Merits of Several Modeling Approaches (at varying levels of complexity) in predicting transverse transient dynamic behavior of straight skewed slab-on-girder bridge structures
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Loading Scheme
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Loading Scheme (cont’d)
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Loading Scheme (cont’d)
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Loading Scheme (cont’d)
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Loading Scheme (cont’d)
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Loading Scheme (cont’d)
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(Under-Bridge) Loading Scheme!
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Loading Scheme (cont’d)
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Loading Scheme: The Importance of Genuinely Quick Release!
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Loading Scheme: Mechanical Fuse
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Loading Scheme: Mechanical Fuse in Place
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Selected Instrumentation
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Selected Instrumentation (cont’d)
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Selected Instrumentation (cont’d)
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Selected Instrumentation (cont’d)
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Selected Instrumentation (cont’d)
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Selected Instrumentation (cont’d)
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Selected Results: Free Vib’n
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Pre-Retrofit in Frequency Domain
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1st 2 Pre-Retrofit Mode Shapes
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SAP Model
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DRAIN-2DX Model
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3rd Pre-Retrofit Mode Shape
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Seismic Isolation Concept: Recall!
Sa
T T
Sd
ξ +
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Post- Retrofit Time-
History at Abutment
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Post-Retrofit SB (Linear: “Portion 2”)
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Post-Retrofit SB cont’d (Linear: Portion 2)
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Post-Retrofit NB (Linear: Portion 2)
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Post-Retrofit NB cont’d (Linear: Portion 2)
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Selected NB Post-Retrofit T-H’s
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Post-Retrofit SB cont’d
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Post-Retrofit SB (Nonlinear: Portion 1)
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Time, Temperature, Setup Effects (SB)
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Old Steel Bearings: Not So Bad After All!
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C/D vs. PGA, Pre-Retrofitted
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C/D vs. PGA, Post-Retrofitted
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Summary, Conclusions, Recommendations• Transverse Quick-Release Experiments are
successfully executed using the deployed apparatus
• Replacement of the original steel bearings with seismic isolation bearings leads to substantial behavior changes (period shift, etc.) in the transverse direction, as predicted. However,– Due to reserve strength of columns and steel bearings,
actual seismic resistance of the retrofitted southbound bridge is not greatly increased by its seismic isolation retrofit, and
– The torsional behavior induced in the retrofitted northbound bridge nullifies the otherwise seismically beneficial effect of its standard (non-seismic) elastomeric bearing retrofit
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Summary, Conclusions, and Recommendations (cont’d)
• Neither a 3D Model nor formal structural identification methods produce any significant advantages over 2D modeling as long as nonlinear behavior is incorporated into the model (DRAIN-2DX)
• Cold-weather effects in western NY are not sufficiently severe to have a significant influence on LRB seismic isolation behavior
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Acknowledgements
• J. B. Mander, D. Wendichansky, G. Pekcan, D.-K. Kim, I.-S. Ahn, L. Zhang, P. Dreyer, etc.
• New York State Dept. of Transportation, Union Concrete and Construction Corp.
• U.S. D.O.T. / Federal Highway Administration funding through NCEER/MCEER at the University at Buffalo