Seismic reliability assessment of equivalent perfectly elastoplastic base-isolated structures

This paper deals with seismic reliability of nonlinear structural systems equipped with friction pendulum isolators (FPS) for an intermediate isolation degree. The FPS behaviour is described by a bilinear model, dependent on velocity, while isolated structures are described by a 2dof model with a perfectly elastoplastic behaviour. A parametric study is carried out for different elastic and inelastic building properties, seismic intensity levels and considering an intermediate isolation degree and assuming the friction coefficient as a random variable. Employing a set of natural seismic records and in compliance with seismic hazard of L‘Aquila site (Italy) according to NTC08, the inelastic characteristics of the superstructures are designed as the ratio between the average elastic responses and increasing strength reduction factors. Using Incremental Dynamic Analyses (IDA) and assuming different values of the limit states for the isolation level and for the superstructure, the seismic fragility curves are evaluated. The reliability curves of the inelastic base-isolated structural systems, with a design life of 50 years, are derived integrating the fragility curves with the seismic hazard curve related to L‘Aquila site (Italy). The reliability curves are seismic reliability-based design (SRBD) abacuses, useful to define the FPS properties and superstructure properties.