Partial safety factor for seismic design of isolation systems consistent with second-generation Eurocode 8

A seismically isolated structure comprises the isolation system, the substructure and the superstructure. The design of the former is displacement-based, i.e., demand and capacity of the devices are in terms of displacement, subject to force constraints such as the transmission of vertical loads. Design of the latter is traditionally force-based, i.e., based on strength verifications of members. Design of the substructure is in between, with the sizing of the rattle-space being displacement-based and that of connection elements being force-based. International codes adopt different safety margins in the design of each of these three components. Sometimes, this leads to structures that, even though performing better than their fixed-base counterparts at lower intensities up to the design one, have a lower safety margin with respect to collapse. This paper focuses on the partial safety factor to be adopted in the design of the isolators’ displacement capacity. Values are derived to meet the Near Collapse reliability target set in the code, employing the same theoretical framework used to derive partial factors for displacement-based design of structural members in fixed-base structures, for the second-generation Eurocode 8. Even though seismic isolation can provide better-than-code performance, admitting some degree of damage allows relaxing design requirements to just match the regular code performance objective. Numerical assessment via multiple-stripe inelastic response history analyses, capable of considering the full range of phenomena occurring at higher than design seismic intensities, including device and super-structure damage, is thus used to show how the partial safety factor value can be reduced as a function of admissible (controlled) damage in the isolators.