Performance estimates in seismically isolated bridge structures
Andrew S Whittaker2004, Engineering Structures
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Abstract
An analytical study investigating the performance of seismically isolated bridge structures subjected to earthquake excitation is summarized. Here, performance is assessed using the following descriptors; maximum isolator displacement and energy demand imposed on individual seismic isolators. Nonlinear response-history analysis is employed considering 20 different isolation systems and three bins of earthquake ground motions. Results of these analyses are used to: (1) review the accuracy of the current AASHTO equation for the calculation of displacements in seismically isolated bridge structures, and (2) determine the increase in maximum horizontal displacement of a seismic isolator due to bidirectional seismic excitation, and (3) review the current AASHTO prototype testing requirements for seismic isolators under seismic loading conditions. The current AASHTO equation for calculating maximum isolator displacements is shown to underestimate median maximum horizontal displacements determined from bidirectional nonlinear response-history analysis. Maximum isolator displacements determined from bidirectional seismic excitation are shown to be significantly larger than those considering unidirectional seismic excitation. Two factors contributing to the increase in maximum isolator displacement are identified; additional displacement demand from a second (orthogonal) component, and the coupled response of seismic isolators. The current prototype testing requirements for seismic loading specified by the AASHTO are shown to result in energy demands that are inconsistent with those determined from numerical simulation of maximum earthquake excitation. An improved prototype testing protocol for seismic isolators subjected to seismic loading is proposed. #
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