Statistical modelling of HDNR bearing properties variability for the seismic response of isolated structures

This paper reports some results of an ongoing Research Project called RINTC aimed at computing the risk of collapse of buildings conforming to the Italian Seismic Design Code. The project involves different areas of application (reinforced concrete, masonry, steel buildings, etc.) including reinforced concrete (RC) buildings equipped with isolation systems. In particular, this paper focuses on seismic isolation systems based on High Damping Natural Rubber (HDNR) bearings, which are widely employed for buildings and other structures. The aim of the paper is to evaluate the response dispersion due to the uncertainties in the seismic input as well as the variability of the isolation system properties. The study proposes a model to define the production variability of the bearing properties, taking into account the tolerance allowed in factory production control tests (FPCT) by the European code on anti-seismic devices (EN15129). To this purpose, in the first part of the paper, experimental results of groups of HDNR bearings belonging to different batches (classes) has been analysed, focusing on the values of shear stiffness and damping coefficient at design deformation and their correlation inside and between device groups. Both the intra-class and inter-class variability affecting the HDNR isolator properties are evaluated, by using a proper statistical model. Successively, the effect on the properties variability of the FPCT acceptance criteria provided by the European code is evaluated. In the second part of the paper, results of multi-stripe analyses carried out on a base isolated prototype consisting of a 6-storey RC building are presented for increasing ground motion intensities. In particular, several varied parameters of bearings are sampled starting from mean properties and by using the statistical model calibrated from test data. The influence of the bearings parameters variability on the most interesting engineering demand parameters (EDPs) and on collapse modalities is evaluated and discussed