Seismic reliability-based design of inelastic superstructures equipped with unbonded FREIs

Unbonded fiber-reinforced elastomeric isolators (UFREIs) are composed of elastomer layers reinforced with flexible fibers and are not attached to their upper and lower supports. UFREIs are an effective seismic isolation technology for low-rise buildings in developing countries due to their low manufacturing cost and lightweight properties. The study aims to establish seismic reliability-based relationships between the ductility-dependent strength reduction factors (R) and the displacement ductility demand (μ) of nonlinear superstructures equipped with UFREIs. The yielding strength capacities of the inelastic superstructures are designed according to established relationships, aligning with the life safety limit state (LS) and the seismic hazard of a reference site in Los Angeles, California. The base-isolated superstructure is modeled as a two-degree-of-freedom (2DF) system where a bilinear hysteretic behavior is assigned to the superstructure, and a non-iterative model is adopted for the UFREI response. The proposed non-iterative model consists of a nonlinear elastic element defined by a fifth-degree polynomial, acting in parallel with a hysteretic element described by the Coulomb friction model. Incremental dynamic analyses (IDAs) are conducted to calculate the superstructure displacement response of inelastic 2DF systems subjected to a set of far-field ground motion records. Then, the seismic fragility curves are created using the IDAs results by calculating the probabilities of exceeding appropriate LS thresholds in terms of the displacement ductility for the superstructure. Seismic reliability curves are derived from the convolution integral of the obtained seismic fragility curves and the seismic hazard curve of the reference site. Finally, in reference to the probability of exceeding the life safety LS within 50 years, linear regressions that relate displacement ductility demand to the strength reduction factors are presented. According to the seismic reliability results, designing base-isolated superstructures requires very low strength reduction factors to maintain an elastic response, especially in cases with a high mass ratio and low superstructure periods. The findings provide valuable guidelines on the maximum strength reduction factors for inelastic superstructures equipped with UFREIs.