Effects of permanent offset on the response of the Curved Surface Sliders

The large worldwide diffusion of isolation systems with Curved Surface Sliders (CSS), also known as the Friction Pendulum System®, requires detailed knowledge of their behavior and improved modelling capability under seismic conditions. Restoring capability after the earthquake is one of the fundamental functions required to seismic isolation systems. It is noted the dependency of residual displacements on ground motions characteristics and isolator mechanical properties, namely the radius of curvature and the coefficient of friction. Current standards on antiseismic devices (EN15129, AASHTO) and design codes (Eurocode 8) provide criteria to ensure good restoring capability. An analytical model has also been formulated based on numerical analysis to predict the residual displacement of the isolation system. In mainshock-aftershock sequences it is possible that at the occurrence of the aftershock the isolation system present an offset from its original configuration as a result of the main shock. The main concern is whether or not an increase in the maximum or residual displacement consequent to the displacement accrual starting from the offset position may lead, under certain conditions, to exceed the displacement capacity of CSS device and the structural integrity or compromise the serviceability of the structure. In this study, some hundreds of nonlinear time-history analyses of SDOF systems were conducted within an extensive parametric study aimed to investigate the effects of a non-centered initial position on the CSS response, in terms of maximum and residual displacements. Five different initial offsets were considered coupled with a wide range of devices and earthquakes, characterized by different values of the isolator design parameters and characteristics of the ground motion. Twenty different CCS isolators were considered varying five radii of curvature (from 2 200 mm to 5 000 mm) and four friction laws, covering the current design practice. Twenty-four natural ground motion time histories were selected from a database and classified in terms of the predominant period of the quake and the level of impulsivity (to this scope, a “Pulse Index” was formulated based on the rate of transmission of the kinetic energy). The effects of an initial offset are discussed herein in terms of maximum and residual displacements of the isolators. The displacement responses obtained in presence of the offset are eventually compared with the relevant response provided by the isolation system that moves from its centred configuration.