Numerical assessment of concave sliding Isolator's mechanical behavior under bi-directional motion

In this contribution, some aspects of the mechanical behavior of friction pendulum (FP) isolators are investigated from a numerical point of view. Attention is focused on the distinctive features observed in FP bearings when subjected to multi-directional excitations like those occurring in a real earthquake scenario. Due to the peculiarities related to the kinematics of the device and the curvature of the sliding surface, neglecting the bi-directional interaction and considering a uni-directional idealization would lead to underestimate the displacements expected during seismic events and to overestimate the dissipative capacity of the isolator accordingly. Furthermore, the resultant horizontal force of the FP bearing may differ markedly from what expected (and predicted by most numerical models) in the uni-directional case. This deviation is mainly ascribed to the geometrical features of the device itself. Additionally, the force-displacement behaviour of the FP bearing depends upon the coefficient of sliding friction, which is not constant as postulated by the simplified Coulomb model but varies during the course of an earthquake with sliding velocity, vertical load and, above all, repetition of cycles and consequent heating phenomena arising at the sliding interface. Among these effects, frictional heating has been recognized as the most important factor that affects the FP maximum displacement because it may induce significant friction degradation, consequently it should be properly considered in a truthful model of the FP bearing via a sophisticated coupled thermo-mechanical analysis. The aim of this paper is to scrutinize the above physical phenomena by means of a 3D finite element model. The developed model allows us to closely analyze the kinematics of the device, to assess the contact force distribution between the sliding surfaces and to examine certain nonlinear phenomena that become manifest especially during bi-directional excitations.