Influence of Dynamic Properties Variability of Elastomeric Bearings on the Seismic Response of Base-Isolated Structures

The response of bae-isolated structures is affected by the variability of dynamic properties of isolation bearings due to environmental effects, such as the temperature and ageing, and/or behavioral effects. For elastomeric bearings made by High Damping Rubber (HDR), the main behavioral effects are the strain amplitude and the strain rate dependence of the shear response and the load history dependence. They can be all included in advanced numerical models, alternatively current seismic codes allow to adopt less accurate models in combination with property modification factors (also called λ-factors) defined for both behavioral and environmental effects. These factors should be experimentally assessed during the qualification process of bearings but, in some cases such as in the European context, the product standard on anti-seismic devices (EN 15129) allows the manufacturers to deduce these variabilities from tests performed on material specimens. In this paper a statistical analysis is carried out on a large set of material tests carried out according to EN 15129 on low and high damping rubber compounds provided from different suppliers. Based on the results, new λ-factors for stiffness and damping are proposed and compared with those deduced from the “λ-factors” suggested in the informative annexes of EN 15129 and EC8-part 2. In the last part of the paper some preliminary considerations are made on the influence of such variability on the vulnerability assessment of base-isolation structures.