Seismic response assessment of base-isolated bridges accounting for the stress-softening behaviour of high damping natural rubber bearings

In the last few decades, high damping natural rubber (HDNR) bearings have been extensively employed for seismic isolation of bridges and buildings because of their low horizontal stiffness and high damping capacity, which allows shifting the vibration period of the isolated structure away from where the earthquake input has the highest energy content and at the same time controlling the motion of the system. In HDNR material, filler is added to the natural rubber in order to improve its properties such as stiffness and dissipative capacity. The addition of the filler induces also a stress-softening behaviour, known as "Mullins effect". This effect makes the response of HDNR bearings path-history dependent and thus may influence the seismic performance of isolated systems. Published literature has shown that the initial "virgin" properties of the material are recovered. Accordingly, current seismic codes make the assumption that "Mullins effect" is a reversible phenomenon. Thus, they suggest simplified methods based on the use of property modification factors for the bearing model properties to account for this effect in the seismic response evaluation of structures isolated with HDNR bearings. The present paper is a study of the consequences of such strain-history dependent behaviour on the seismic response of structural systems isolated with HDNR bearings. In particular, the bearing behaviour is described by a non-linear processdependent constitutive model based on one previously developed. This model allows separation of the contribution to the response due to the path-history dependent component from the contribution resulting from the stable (scragged) component. The model properties are calibrated against double-shear tests carried out on HDNR compounds commonly employed for seismic isolators. In particular, the response of a realistic isolated bridge is analyzed under ground motions with different characteristics 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 obtained for the two different conditions are compared to evaluate the influence of the Mullins effect on the bridge performance. This also permits an evaluation of the accuracy of simplified bearing models and relevant property modification factors suggested by the codes for the preliminary design and analysis of bridges isolated with HDNR bearings.