Nonlinear incremental analysis of fire-damaged r.c. base-isolated structures subjected to near-fault ground motions

Amplification of structural response of r.c. base-isolated structures is expected under near-fault ground motions, yet there is a lack of knowledge of their behavior in the case of fire. To investigate the nonlinear seismic response following a fire, an incremental dynamic analysis is carried out on five-storey r.c. base-isolated framed buildings with fire-protected High-Damping-Laminated-Rubber Bearings (HDLRBs), designed in line with the Italian seismic code. Horizontal components of near-fault ground motions characterized by forward-directivity or fling-step pulse-type are considered. The nonlinear seismic response of base-isolated structures in a no fire situation is compared with that in the event of fire, at 45 (i.e. R45) and 60 (i.e. R60) minutes of fire resistance, assuming both damaged (i.e. DS) and repaired (i.e. RS) stiffness conditions. Five fire scenarios are considered assuming the fire compartment confined to the area of the first level (i.e. F1), the first two (i.e. F1/2) and the upper (i.e. Fi, i=3-5) levels, with the parametric temperature-time fire curve evaluated in accordance with Eurocode 1. The nonlinear seismic analysis is performed by using a step-by-step procedure based on a two-parameter implicit integration scheme and an initial-stress-like iterative procedure. At each step of the analysis, plastic conditions are checked at the critical (end) sections of the girders and columns, where thermal mapping with reduced mechanical properties is evaluated with the 500 °C isotherm method proposed by Eurocode 2. A viscoelastic model with variable stiffness properties in the horizontal and vertical directions, depending on the axial force and lateral deformation, simulates the response of an HDLRB.