Fragility analysis of RC seismically-isolated structures with residual mechanical properties after fire exposure

In seismically isolated reinforced concrete (r.c.) structures exposed to fire, significant damage to r.c. frame members of the superstructure and irreversible deterioration in the isolation system with elastomeric bearings (e.g. High-Damping-Rubber Bearings, HDRBs, and Lead-Rubber Bearings, LRBs) can occur. Note that the residual mechanical properties after fire can be lower or higher than during fire, depending on the maximum fire temperature and the heating and cooling rates. Earthquakes following fire may find load capacity notably reduced vis-a-vis the no fire condition, with an amplification in the structural response in the fire compartment. To evaluate seismic vulnerability following fire, a fragility analysis is performed on a six-storey r.c. seismically isolated framed structure designed in line with the current Italian code. Two fire scenarios are simulated assuming the fire compartment is confined to the area of the isolated level (i.e. F0) and the first level of the superstructure (i.e. F1). In particular, the nonlinear seismic response of the test structure in the no fire situation is compared with those during exposure to the heating phase (i.e. at 45, R45, and 60, R60, minutes of fire resistance) and after the cooling phase (i.e. R-c(olling) fire resistance). An isotherm method characterized by a fire-damage threshold calibrated on the basis of experimental results is proposed to evaluate reduced mechanical properties of HDRBs and LRBs. Finally, the residual seismic load capacity of the r.c. cross-sections after fire is assessed by means of experimental laws representing heating and cooling effects.