INCREMENTAL DYNAMIC ANALYSIS OF FIRE-EXPOSED BASE-ISOLATED R.C. FRAMED BUILDINGS SUBJECTED TO NEAR-FAULT GROUND MOTIONS

Abstract eng:
It is well known that near-fault ground motions are characterized by long-duration horizontal pulses, which can become critical for base-isolated reinforced concrete (r.c.) structures when the pulse intensity is so strong that the superstructure undergoes plastic deformations. There is, however, a lack of knowledge on the seismic response in case of fire; moreover, an amplification of the structural damage is expected in case of fire-exposed baseisolated structures which are not designed to withstand fire. To investigate the nonlinear seismic response following a fire, an incremental dynamic analysis is carried out with reference to five-storey base-isolated r.c. buildings with fire-protected High-Damping-LaminatedRubber Bearings (HDLRBs), designed according to the Italian seismic code. Horizontal components of seven near-fault ground motions have been selected according to the design subsoil class C adopted for the test structures. More specifically, the nonlinear seismic response of base-isolated structures in a no fire situation is compared with that in which fire occurs, 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 according to 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.

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