Statistical Analysis of Rubber Compounds Material Tests for Seismic Isolation Bearings and Code Provisions Comparison

Rubber compounds are extensively used for manufacturing rubber bearings for seismic isolation. Current seismic codes prescribe the use of design properties of seismic isolators derived from experimental evaluations to account for the effects of strain rate representative of the design seismic condition, cyclic degradation, frequency and environmental conditions (especially the temperature variation). Moreover, ageing phenomena, which such devices undergo during their service life, should be taken into account. Such tests should be carried out on devices, even though the European Standard on seismic devices (EN 15129) allows for both high and low damping rubber bearings the use of data from material tests performed on the elastomer used in their manufacture, (although extrapolation the results of materials tests to the scale of isolation devices is not always straightforward). In this study a set of experimental data obtained from type tests performed by different manufacturers on both low and high damping compounds with nominal stiffness ranging from 0.4 MPa to 1.3 MPa is statistically analysed. The effects of shear deformation, cyclic degradation, ageing, frequency and temperature on equivalent shear stiffness and equivalent damping are shown, providing also a view of the intra-supplier variability and inter-supplier variability. As general modification factors, also known as λ-factors, are provided by the codes in case of unavailability of experimental data, a comparison between the codes values and the obtained experimental results is shown.