Residual load capacity of fire-damaged rubber bearings for r.c. base-isolated buildings subjected to near-fault earthquakes

A base-isolation system of framed buildings frequently involves elastomeric materials (e.g. High-Damping-Laminated-Rubber Bearings, HDLRBs, and Lead-Rubber Bearings, LRBs), which are susceptible to deterioration when fire occurs. On the other hand, fire-damaged HDLRBs and LRBs may elongate the fundamental vibration period of a base-isolated building, shifting it into the resonant region under the long-duration horizontal pulses of near-fault ground motions. To study the nonlinear seismic response following fire in HDLRBs and LRBs, a numerical investigation is carried out on six-storey r.c. base-isolated buildings comprising a basement and five storeys above ground. Three fire scenarios are considered, with the fire compartment confined to the area of the base-isolated level (i.e. F0), first level of the superstructure (i.e. F1) and both levels (i.e. F0/1). Base-isolated structures in a no fire situation are compared with those in the event of fire, at different durations of fire resistance for the basement (i.e. R30) and the superstructure (i.e. R60). Different values of the opening factor are assumed in the basement so as to obtain the R30 fire resistance with different fire temperatures for HDLRBs and LRBs. To this end, transient analysis corresponding to the EC1 time-temperature curves in the basement is carried out by means of a finite element modelling of HDLRBs and LRBs. A reduction of the mechanical properties of the fire-damaged isolators is evaluated in line with a 200°C isotherm method. Reduced mechanical properties of r.c. cross-sections are considered by means of a thermal-mechanical mapping analysis, in line with the 500°C isotherm method proposed by Eurocode 2. A nonlinear incremental dynamic analysis of the base-isolated structures is carried out considering near-and far-fault earthquakes. The nonlinear seismic analysis uses a lumped plasticity model to describe the inelastic behaviour of the r.c. frame members; the response of an HDLRB is simulated with a viscoelastic model, with variable stiffness properties in the horizontal and vertical directions, while a bilinear model is considered for an LRB.