Behaviour during seismic aftershocks of r.c. base-isolated framed structure with fire-induced damage

Fire following strong earthquakes is considered a very high-risk event and a fire-damaged structure could be subjected to seismic aftershocks. Most probably damage produced by the main shock will be absent in case a base-isolation system is adopted. To study the post-fire seismic performance a numerical study is presented on six-storey reinforced concrete (r.c.) framed structures, composed of a basement with elastomeric bearings and five storeys above the ground level. Three fire scenarios are compared, selecting the base-isolated level (i.e. F0) and first (i.e. F1) and fifth (i.e. F5) levels of the superstructure as fire compartments. A reduction of the mechanical properties of the fire-damaged elastomeric bearings is considered in accordance with a proposed 200 °C isotherm method. To this end, transient response of the elastomeric bearings corresponding to the Eurocode 1 time-temperature curve, is compared with steady response, defined as a heat conduction problem of a constant flux in an infinite circular cylinder. Reduced mechanical properties of r.c. cross-sections are considered by means of a thermal-mechanical mapping analysis, in accordance with the 500 °C isotherm method in the Eurocode 2. The nonlinear seismic analysis is performed by using a lumped plasticity model to describe the inelastic behaviour of the r.c. frame members; a nonlinear force-displacement law is considered for the elastomeric bearings, with coupling of horizontal and vertical motions. Near-fault seismic aftershocks, inducing severe demand in base-isolated buildings, are scaled within an incremental dynamic analysis procedure.