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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
Presentation Outline
• Comprehensive Study Overview
• Bridges Studied
• Loading and Instrumentation Apparatus
• Selected Results
• Anticipating Seismic Performance
• Summary, Conclusions, Recommendations
• Acknowledgements
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
Seismic Isolation Concept
(Kelly, 2001)
Seismic Isolation Concept (cont’d)
(Kelly, 2001)
Bridges’ Plan View
Bridges’ Elevation; Brg. Schedule
Bridge Cross Sections
Bridge Bearing Replacement (Abut.)
Bridge Bearing Replacement (Pier)
Installing Isolation Brgs at Abutments
Southbound Abutment Isolator
Kelly, 2001
Southbound Pier Isolator
Northbound Expansion Bearing (non-seismic)
Northbound Bridge Bearings
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
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
Loading Scheme
Loading Scheme (cont’d)
Loading Scheme (cont’d)
Loading Scheme (cont’d)
Loading Scheme (cont’d)
Loading Scheme (cont’d)
(Under-Bridge) Loading Scheme!
Loading Scheme (cont’d)
Loading Scheme: The Importance of Genuinely Quick Release!
Loading Scheme: Mechanical Fuse
Loading Scheme: Mechanical Fuse in Place
Selected Instrumentation
Selected Instrumentation (cont’d)
Selected Instrumentation (cont’d)
Selected Instrumentation (cont’d)
Selected Instrumentation (cont’d)
Selected Instrumentation (cont’d)
Selected Results: Free Vib’n
Pre-Retrofit in Frequency Domain
1st 2 Pre-Retrofit Mode Shapes
SAP Model
DRAIN-2DX Model
3rd Pre-Retrofit Mode Shape
Seismic Isolation Concept: Recall!
Sa
T T
Sd
ξ +
Post- Retrofit Time-
History at Abutment
Post-Retrofit SB (Linear: “Portion 2”)
Post-Retrofit SB cont’d (Linear: Portion 2)
Post-Retrofit NB (Linear: Portion 2)
Post-Retrofit NB cont’d (Linear: Portion 2)
Selected NB Post-Retrofit T-H’s
Post-Retrofit SB cont’d
Post-Retrofit SB (Nonlinear: Portion 1)
Time, Temperature, Setup Effects (SB)
Old Steel Bearings: Not So Bad After All!
C/D vs. PGA, Pre-Retrofitted
C/D vs. PGA, Post-Retrofitted
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
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
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