This paper deals with seismic reliability of nonlinear structural systems having an intermediate isolation degree and equipped with friction pendulum system (FPS) isolators, by adopting an equivalent inelastic two-degree-of-freedom model and considering the friction coefficient as a random variable. Employing a set of natural seismic records and L’Aquila (Italy) as reference site, the inelastic characteristics of the superstructures are designed according to NTC08 for increasing strength reduction factors and for an intermediate value of the isolation degree. Using Incremental Dynamic Analyses (IDA) and assuming different values of the limit state thresholds for the single concave sliding bearings and for the superstructure, the seismic fragility curves are evaluated. Finally, the reliability curves of the inelastic base-isolated structural systems, with a design life of 50 years, are derived and represent seismic reliability-based design (SRBD) abacuses, useful to define the system properties.
