Accrual of displacements for sliding isolators with curved surfaces

For base-isolated structures, fulfillment of the non-collapse requirement demands that at the ultimate limit state, the ultimate capacity of the isolating devices in terms of strength and deformability is not exceeded. A criterion for establishing the displacement capacity required by isolation systems, accounting for the effects of both non-seismic and seismic actions, as well as for possible accrual of displacements during the lifetime of the structure, in case of system with poor restoring capability, is provided in the Eurocode 8. However this criterion is rather empirical and its validity for isolation devices endowed with high nonlinear behavior has been never proved in practice. The study addresses the concern of displacement accrual for sliding isolators with curved surfaces, investigating in a parametric study the effect of a non-seismic offset displacement on the total displacement induced by the design earthquake. Nonlinear History Response Analyses were conducted, accounting for five offset displacements, combined with a wide range of devices and earthquake characteristics. Twenty-five different isolators were obtained from five radii of curvature (from 2200 mm to 5000 mm) and five friction coefficients, covering the design practice. Twenty-four natural ground motion histories were selected from a database according to their predominant period and pulse-like behavior. The Eurocode provision is checked against the observed data, an improved formulation of the Eurocode’s displacement capacity for sliding isolators with curved surfaces is proposed, together with a criterion to evaluate the capability of the devices to provide a seismic response independent of the non-seismic displacement