Parametric analysis of optimum isolator properties for bridges susceptible to near-fault ground motions

This paper examines the selection of optimum isolator properties, namely characteristic strength, Qd, and post-elastic stiffness, kp, for bridges located in near-fault regions. First, a two-phased sensitivity analysis is conducted to evaluate the influence of bridge, isolator, and near-fault ground motion parameters on optimum levels of Qd and kp based on minimizing maximum isolator displacement and force. In the first phase of sensitivity analyses, a screening via design of experiments principles is performed to assess the statistical significance of various parameters on the optimum isolator properties. The second phase includes rigorous sensitivity analyses to assess the trends in optimum Qd and kp as a function of the bridge, isolator, and near-fault ground motion parameters. Next, nonlinear time history analyses of typical seismically isolated bridges are conducted for a suite of near-fault ground motions across a range of values of the identified parameters to enable the development of parametric equations for optimum Qd and kp to minimize isolator force or displacement. The parametric equations are validated using an alternate suite of near-fault ground motions. Furthermore, the dispersion about the predictive equations are quantified and assessed. It is observed that the developed equations produced reasonable estimates of optimum isolator properties with a relatively consistent dispersion across the modeling parameters. Moreover, it is observed that for near fault ground motions with high intensity and strong directivity, supplemental energy dissipation devices are required to minimize the isolator displacements.