Seismic behaviour of a RC frame isolated by HDNR bearings under increasing intensity levels

High Damping Natural Rubber (HDNR) bearings are characterized by stiffness and damping capacity that significantly depend on the shear deformation amplitude. More in details, at low deformations stiffness and damping increase, whereas at large deformations the stiffness remarkably increase but the damping capacity decreases. Additionally, this kind of bearings show a loading hysteresis dependence, due to the internal damage of the rubber occurring as the deformation history progresses. This effect, also known as stress-softening, becomes significant for large deformation amplitudes and represents a source of uncertainty, which has recently caused a limitation of the use of this kind of isolators. However, consequences of this nonlinear behaviour of HDNR bearings on the response of isolated structures are not comprehensively investigated, primarily because advanced models have been only recently developed. In this paper some investigations are carried out by using a nonlinear constitutive law recently developed by some of the authors describing the behaviour of a HDNR with significant stress-softening complying with the limits of European code on anti-seismic devices. Analyses are carried out on a multi-degree of freedom system by considering different seismic intensity levels and different response parameters, including floor response spectra. A linear visco-elastic model calibrated at each seismic intensity level is also adopted in the analyses. The obtained results show that some response amplifications happen due to the higher modes of the superstructure, which are underestimated by linear models and may cause damages to non-structural components and equipment