The seismic performance of existing RC frame buildings seismically retrofitted by base isolation is examined. Two different types of isolation systems are considered, i.e.: rubber-based and friction-based isolation systems, respectively. Two different performance levels are examined, namely: Global collapse prevention and Usability preventing damage. Seismic performance is assessed by multi-stripe nonlinear time-history analysis, considering earthquake intensity levels with return period ranging from 10 to 100000 years. Results point out that seismic isolation works effectively in limiting damage well beyond the design limit state level while it shows a limited margin with respect to collapse beyond the design limit state level.

Double Curved Concave Surface Sliders (DCCSS) are seismic isolators based on the pendulum principle widely used worldwide. Coherently with European code, DCCSS do not include any mechanical elements as end-stopper. In case of displacement higher than those associated with the design earthquakes, the inner slider runs on the edge of the sliding surfaces beyond their geometric displacement capacity keeping the ability to support gravity loads. In this paper, the advanced modelling and risk analysis of reinforced concrete (RC) base-isolated buildings designed for medium and high seismicity zones according to the Italian code has been assessed considering new construction and existing structures retrofitted using the seismic isolation technique. Pushover analyses and nonlinear dynamic analyses including inelastic superstructure behaviour and the over-stroke displacement of the isolation system have been carried out. Annual rates of failure are computed for Usability-Preventing Damage (UPD) related to the superstructure inter-storey drift and for Global Collapse (GC) associated with the ultimate displacement of the DCCSS. Moreover, the ultimate displacement is assumed with an extra-displacement of more than 30% of the maximum geometrical displacement. Results pointed out that in the case of new buildings the GC and UPD conditions occur almost at the same seismic intensity, while for the cases of the existing building, the UPD is the dominant limit state, being reached at an intensity level lower than GC.