Numerical Investigation of Re-Centring Capability of Friction Pendulum Isolators

Self-centring capability after the seismic shaking is one of the fundamental functions required to seismic isolation systems. Low re-centring capacity may lead to serious damage and even structural collapse due to excessive cumulative displacements during mainshock-aftershock seismic sequences. Near fault quakes, frequently characterized by large pulse, may have a strong impact on the behaviour of the isolation systems leading in some cases, to instability phenomena. In this study some hundreds of nonlinear time-history analyses of SDOF systems were conducted within an extensive parametric study aimed to investigate the re-centring capability of Friction Pendulum System (FPS) isolators and its sensitivity to ground motions characteristics. A wide range of devices and earthquakes, characterized by different values of the isolator design-parameters (restoring stiffness and frictional damping) and characteristics of the ground motion (“pulse-level” and frequency content), has been considered. The latters have been quantified in terms of the “predominant period” of the ground motion and through the introduction of a “kinetic Pulse Index” defined in terms of the rate of transmission of the kinetic energy. Moreover, the results of a shake table investigation are reported in order to assess the consistency of the proposed numerical formulation with the experimental response of structures implementing FPS isolators. The preliminary results, discussed herein in terms of maximum and residual displacements of the isolators, show that both the mechanical properties of the isolator and the characteristics of the ground motion, but also their relationships, may have an important influence on the re-centring capability of the devices.