Torsional response of fire-damaged base-isolated buildings with elastomeric bearings subjected to near-fault earthquakes

Information is lacking on the nonlinear dynamic response of r.c. base-isolated buildings where asymmetries are due to fire-induced lateral stiffness eccentricities in the frame members of the superstructure or in the base-isolation system. To study the seismic response following fire, a numerical investigation is carried out on four-, six- and eight-storey r.c. base-isolated structures composed of a basement and three, five and seven storeys above ground level, respectively. The base-isolation system is constituted of High-Damping-Laminated-Rubber Bearings, inserted on the top of the columns of the basement. Different values of the isolation ratio, defined as the ratio between the fundamental vibration period of the base-isolated and fixed-base structures above the isolation system, and plan-aspect ratio, defined as the ratio between the plan dimensions of the building, are considered. Fire scenarios correspond to the fire compartment confined to the whole (i.e. symmetric case) and half (i.e. asymmetric case) area of the base-isolation level (i.e. basement, F0) and superstructure (i.e. first level, F1) at 30 and 60 min of fire resistance, respectively. A thermal–mechanical mapping analysis is carried out, with reduced mechanical properties of r.c. cross-sections, in line with the 500 °C isotherm method proposed by Eurocode 2, and HDLRBs, in accordance with a proposed 200 °C isotherm method. Finally, the nonlinear dynamic analysis is carried out with reference to near-fault earthquakes selected and scaled on the design hypotheses adopted for the test structures.