Seismic reliability of base-isolated structures with friction pendulum isolators (FPS)

The friction pendulum system (FPS) is becoming a widely used technique for seismic protection and retrofit of buildings, bridges and industrial structures due to its remarkable features such as the stability of physical properties and durability respect to the elastomeric bearings. Experimental data also showed that the coefficient of friction depends on several effects (i.e., sliding velocity, apparent pressure, air temperature, cycling effect) so that it can be assumed as a random variable. The aim of the study consists in evaluating the seismic reliability of base-isolated structures with FP isolators considering both isolator properties (i.e., coefficient of friction) and earthquake main characteristics as random variables. Assuming appropriate density probability functions for each random variable and adopting the LHS method for random sampling, the input data set has been defined. Several 3D nonlinear dynamic analyses have been performed considering both the vertical and horizontal components of each seismic excitation to evaluate the system response. In particular, monovariate and multivariate probability density and cumulative distribution functions have been defined and, considering the limit state thresholds and domains defined respectively on mono/bi-directional displacements, assumed as earthquake damage parameter (EDP) according to performance-based seismic design, the exceeding probabilities have been evaluated. Estimating the reliability of the superstructure, substructure and isolation level led to define a reliability-based abacus to design the FP system.