1 RE-CENTRING CAPABILITY OF FRICTION PENDULUM SYSTEM: PARAMETRIC INVESTIGATION Emanuele GANDELLI 1 , Maria Pina LIMONGELLI 2, Virginio QUAGLINI 3 , Paolo DUBINI 4 , Giacomo VAZZANA 5 and Gianluigi FARINA 6 ABSTRACT Self-centring capability after the seismic shaking is one of the fundamental functions required to seismic isolation systems. Low re-centering capacity may lead to serious damage and even structural collapse due to excessive cumulative displacements. Near fault quakes, frequently characterized by large pulse, may have a strong impact on the behavior of the isolation systems leading in some cases, to instability phenomena. In this paper are reported some preliminary results relevant to an extensive parametric study aimed to investigate the re-centering capability of Friction Pendulum System (FPS) isolators and its sensitivity to pulse-like ground motions. A wide range of devices and ground motions, characterized by different values of the isolator characteristic parameters (equivalent radius Req and coefficient of friction eq) and of the pulse-like characteristics of the seismic ground motion has been considered. The latters have been calculated 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. The preliminary results, discussed herein in terms of maximum and residual displacements of the isolators, show that both the mechanical properties of the isolator (restoring stiffness and frictional damping) and the characteristics of the ground motion, but also their relationships, may have an important influence on the recentering capability of the devices. INTRODUCTION Re-centering related to a seismic isolation device represents the capability of the same to recover its original configuration after a seismic event. The parameter used to quantify this aspect is the residual displacement of the device at the end of the seismic shaking (often measured as a percentage of the maximum one). Re-centering capability must be carefully taken into account particularly for devices with low restoring forces since, in certain cases, the lack of recentering capability may lead to serious damages and even structural collapse due to excessive cumulative displacements. The evaluation of the 1 MSc, Politecnico di Milano, Milan, emanuele.gandelli@polimi.it 2 PhD, Politecnico di Milano, Milan, mariapina.limongelli@polimi.it 3 PhD, Politecnico di Milano, Milan, virginio.quaglini@polimi.it 4 PhD, Politecnico di Milano, Milan, paolo.dubini@polimi.it 5 MSc, Politecnico di Milano, Milan, giacomo.vazzana@polimi.it 6 BSc, Politecnico di Milano, Milan, gianluigi.farina@mail.polimi.it