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