Stress softening behaviour of HDNR bearings: modelling and influence on the seismic response of isolated structures

High-damping natural rubber (HDNR) bearings are extensively employed for seismic isolation of structures because of their low horizontal stiffness and high damping capacity. Filler is used in HDNR formulations to increase the dissipative capacity, and it induces also a stress softening behaviour, known as the Mullins effect. In this paper, a wide experimental campaign is carried out on a large number of virgin HDNR samples to better investigate some aspects of the stress softening behaviour, such as the direction dependence and recovery properties, and to characterize the stable and softening response under different strain histories. Test results are also used to define a model for simulating the response of HDNR bearings in shear that advances the state of the art in the description of the stress softening, which can be significant during the earthquake time history. The proposed model is used to analyse the seismic response of a simplified isolated structure modelled as an SDOF system under ground motions with different characteristics and by considering two different conditions for the bearings: one assuming the virgin (or fully recovered) rubber properties and the other assuming the stable (or fully scragged) rubber properties. The results show that, in the case of far-field records, the differences between the responses are limited although not negligible, whereas for near-fault records, modelling the bearings as being in their virgin state significantly reduces the effect of this kind of motion on isolated structures.