Influence of soil conditions on the optimal sliding friction coefficient for isolated bridges

The aim of this work is to evaluate the optimal properties of friction pendulum system (FPS) bearings for the seismic protection of bridge piers under earthquake excitations having different frequency characteristics representative of different soil conditions in order to reduce the seismic vulnerability of infrastructures increasing their safety level. A two-degree-of-freedom model is adopted to describe, respectively, the response of the infinitely rigid deck isolated by the FPS devices and the elastic behaviour of the pier. The FPS isolator behaviour is modeled through a widespread velocity-dependent rule. By means of a non-dimensional formulation of the motion equations, proposed in this study, a wide parametric analysis considering several structural parameters is performed to investigate their influence on the response parameters relevant to the performance assessment. Seismic excitations, modeled as time-modulated filtered Gaussian white noise random processes having different intensities and frequency contents, are considered. Specifically, the filter parameters, which control the frequency contents, are properly calibrated to reproduce stiff, medium and soft soil conditions, respectively. Finally, multi-variate non-linear regression relationships are derived to estimate the optimum values of the sliding friction coefficient able to minimize a percentile of the pier displacements relative to the ground as a function of the structural properties, of the seismic input intensity and of the soil condition.