Full-scale experimental tests on unbonded fiber reinforced elastomeric isolators under bidirectional excitation

Unbonded fiber reinforced elastomeric isolators (UFREIs) represent a lower-cost alternative to conventional steel reinforced elastomeric isolators (SREIs), in which the internal steel shims are replaced with fiber reinforcement and the installation is accomplished without anchorage bolts or dowels, but only relying on frictional mechanisms and interfacial roughness. In the literature, experimental tests on UFREIs were generally presented under unidirectional seismic excitation and considering small-scale devices. In this contribution, the hysteretic behavior of UFREIs is investigated experimentally by testing full-scale devices (diameter 620 mm) not only under unidirectional loading, but also including bidirectional orbits, thus quantifying the degree of biaxial coupling. A simple phenomenological model is then set up to numerically simulate the experimental outcomes. This model consists of a set of nonlinear springs arranged in a circular configuration. The spring parameters are calibrated by either curve-fitting 1D test results or considering 2D tests, thus selectively incorporating the lateral coupling. This model makes it possible to investigate the influence of bidirectional interaction of UFREIs on the seismic performance of base-isolated structures. Nonlinear time-history analysis on a regular three-dimensional reinforced concrete building with UFREIs modeled through the proposed approach demonstrates that the lateral coupling of the UFREIs leads to lower seismic response, of around 10-20%, in comparison to a simpler, uncoupled model that ignores the bidirectional interaction of the isolation devices.