LONG-TERM SEISMIC RESPONSE OF BUILDINGS WITH ISOLATION DEVICES AFFECTED BY DETERIORATION EFFECTS

Base-isolation technique has long been used worldwide but seismic codes generally do not take into account ageing and environmental effects on the evolution and extent of the deterioration of elastomeric (e.g. high-damping-rubber bearings, HDRBs) and friction (e.g. flat sliding bearings, FSBs) isolators during their lifetime and on their impact on the nonlinear seismic behaviour of the superstructure. The aim of the present work is to investigate the effects of variability of the mechanical properties of elastomeric and sliding bearings on the nonlinear dynamic response of base-isolated reinforced concrete (r.c.) framed structures, focusing the attention on ageing and air temperature effects. First, six-storey base-isolated r.c. framed structures are designed in accordance with the current Italian code in a high-risk seismic zone, considering nominal values of mechanical properties of the isolation system. Specifically, nine structural configurations are examined, considering two alternative arrangements: i) elastomeric base-isolation (EBI) systems, with three different elastomer compounds for the HDRBs (i.e. soft, normal and hard); ii) hybrid elastomeric-sliding base-isolation (ESBI) systems, with the aforementioned elastomer compounds of the HDRBs and two friction coefficients (i.e. low and medium) of the FSBs. 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, are also considered. The results of nonlinear time-history analysis are used to build fragility curves based on simple regression in the logarithmic space of structural response versus seismic intensity.