This report presents a case study on the seismic response of a newly constructed freeway over-crossing that is equipped with elastomeric bearings and fluid dampers. The 91/5 overcrossing, shown in Figure A, is located in Orange County, California, and is the first reinforced concrete bridge in the United States equipped with fluid dampers.
First, the bridge is decomposed into its main substructural components such as approach embankments, pile foundations, center bent, abutments, deck, and the seismic protection system that consists of isolation bearings and fluid dampers. Subsequently, the mechanical behavior of each substructural component is examined an d expressed by macroscopic force-displacement laws represented in the form of equations or graphics. The overcrossing is modeled with a simple stick model that synthesizes th e individual mechanical behavior of the various substructural elements. The modal analysis of the overcrossing is conducted within the context of equivalent linear analysis. Seismic response analysis is conducted in the time domain to capture the nonlinear behavior of the protective system. Finally, an in-depth parametric study is presented of the nonlinear seismic response of the isolated bridge accounting for the effects of
soil-structure interaction. The various response quantities presented are compared with the corresponding response quantities of a hypothetical bridge with integral abutments. Advantages and challenges in the two design configurations are identified and discussed.
The study concludes that the bridge with sitting abutments results in large displacements and accelerations at the deck ends. Supplemental damping reduces both displacements and accelerations, yet the response of the bridge with integral abutments appears to outperform the response of the bridge with sitting abutments. Soil-structure interact ion is responsible for increasing substantially both displacements and forces at the end abutments.
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