Performance Estimates for Seismically Isolated Bridges

Keywords: Seismic performance.  Seismic isolation systems.  Bridges.  Horizontal displacement.  Energy dissipation.  Ground motion.  Cyclic tests.  Numerical simulations.  Elastic response spectra.  Estimations.  Energy demands.

Abstract: This report presents an analytical study investigating the performance of an isolated bridge structure subjected to seismic excitation. Here performance is being assessed using the following descriptors: maximum horizontal displacements and cumulative energy dissipated by an individual seismic isolator. Twenty different isolation systems are considered with varied parameters, namely, characteristic strength and second-slope stiffness. Unidirectional and bi-directional response-history analysis was performed considering nonlinear systems and eight bins of earthquake ground motions. Results of the response-history analysis are being used to: (1) determine the increase in maximum horizontal displacement of a seismic isolator due to bi-directional seismic excitation utilizing a coupled plasticity model with a circular yield function to represent the seismic isolator elements, and (2) review the accuracy of the current AASHTO equation for the calculation of displacements in isolated bridge structures which assumes unidirectional seismic excitation and linearly increasing displacements for periods greater than 1-second, and (3) develop prototype testing requirements for seismic isolators in terms of an equivalent number of harmonic cycles to the maximum displacement and an equivalent testing frequency based on the observed energy demand imposed on individual isolators and isolation systems from numerical simulation of maximum earthquake events.