Seismic performance assessment of isolated bridges for different limit states

This study aims to evaluate the seismic performances of bridges isolated by the friction pendulum system (FPS) bearings considering the seismic hazard of Sant’Angelo dei Lombardi site (Italy), to provide useful and preliminary recommendations in terms of health assessment for design or retrofit of new or existing bridges, respectively. Single- and two-degree-of-freedom models are considered to describe the isolated bridge behavior taking into account an infinitely rigid deck and the isolated bridge behavior having an infinitely rigid deck with the elastic pier, respectively. In both models, a velocity-dependent rule for the FPS isolators is assumed. Seismic excitations are properly modeled as non-stationary stochastic processes having different intensities corresponding to different limit states and with frequency contents related to the medium soil condition, representative of the soil type in Sant’Angelo dei Lombardi site (Italy). The statistics of deck and pier responses of the isolated bridge are evaluated for different system parameters such as mass ratio, isolation period, pier period and friction coefficient of the FPS considering both Life Safety and Collapse Prevention limit states according to Italian seismic codes. The results, deriving mainly from the two-degree-of-freedom (2dof) model analyses, show that particular values of the friction coefficient allow to minimize the response of the pier depending on the different system properties and the different limit states. In particular, the optimum friction coefficient of the FPS ranges from 0.01 to 0.04 and from 0.01 to 0.05 for Life Safety and for Collapse Prevention limit state, respectively, depending on the structural properties