Effects of the long-term behaviour of isolation devices on the seismic response of base-isolated buildings

Seismic isolation is widely used around the world for the protection of buildings, but its long-term behaviour was not considered in detail in past seismic design of isolated structures. Nevertheless, the American seismic code has recently introduced an explicit procedure aimed at evaluating upper-bound and lower-bound values of isolation system properties. Yet there are few studies on the evolution and extent of the deterioration of elastomeric (e.g., high-damping rubber bearings) and sliding (e.g., flat sliding bearings) isolators during their lifetime and on their impact on the seismic behaviour of the superstructure. To investigate this problem, six-storey-reinforced concrete buildings, base isolated with high-damping rubber bearings acting alone or in combination with flat sliding bearings, are designed without considering the variability of mechanical properties of the isolation system due to ageing and air temperature. Then, based on experimental results from accelerated ageing tests at high temperature, mathematical models are implemented to account for oxidation of elastomers and friction changes. The variability of mechanical properties at different mean temperatures, due to seasonal thermal variations, is also considered. Finally, fragility curves are developed for the base-isolation system and superstructure on the basis of nonlinear dynamic analysis of the degraded test structures.