SEISMIC VULNERABILITY IN CASE OF FIRE OF EXISTING R.C. FRAMED BUILDINGS: MODELLING AND NONLINEAR DYNAMIC ANALYSIS

Abstract eng:
The seismic vulnerability of reinforced concrete (r.c.) framed buildings designed with inadequate seismic zone classifications and seismic code provisions is recognized to be a serious problem. Nevertheless, knowledge on the seismic response in the case of fire is lacking and an amplification of the structural damage is expected in the case of fire-exposed existing structures not designed to withstand fire. To evaluate the nonlinear seismic response following a fire, a numerical investigation is carried out with reference to a five-storey r.c. framed building, which, primarily designed according to the previous Italian seismic code (DM96) for a medium risk zone, needs to be considered as in a high-risk seismic zone provided by the current Italian seismic code (NTC08). More specifically, the nonlinear seismic response of the test structure 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 damaged (i.e. DS) and repaired (i.e. RS) stiffness conditions. Five fire scenarios have been considered on the assumption that the fire compartment is 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 dynamic analysis is performed through 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 a thermal mapping with reduced mechanical properties is evaluated in line with the 500°C isotherm method proposed by Eurocode 2. Two sets of seven real motions, whose response spectra match on average NTC08 spectra for a subsoil class B, are considered with regard to highrisk seismic zone and two topographic classes.

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