COMPARATIVE STUDY OF THE WIND AND EARTHQUAKE DYNAMIC RESPONSES OF FIRE EXPOSED STEEL FRAMED BUILDINGS

Wind design of steel-framed buildings aims to provide appropriate stiffness and strength properties of the structural elements, so as to obtain an elastic behaviour, at the same time as controlling floor displacements and accelerations. On the other hand, in a seismic design it is commonly accepted that the structural members can undergo inelastic deformations under strong ground motions, provided that these deformations be kept within an acceptable threshold; moreover, deformability thresholds are also imposed at the serviceability limit state. Nevertheless, knowledge on the wind and seismic responses in the case of fire is lacking and an amplification of the structural response is expected in the case of existing structures exposed to fire. To evaluate the wind and seismic responses following a fire, a numerical investigation is carried out with reference to the steel framed structure of a ten-storey office building, which was designed for a low-risk zone under the former Italian seismic code and in line with Eurocodes 1 and 3. More specifically, the dynamic response of the test structure in a no fire situation, along the in-plan principal directions, is compared with what would happen in the event of fire, at 500°C, 550°C and 600°C fire temperatures, hypothesizing a reduction of stiffness and strength due to fire. Four 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=5, 10) levels, with the parametric temperature-time fire curve evaluated in accordance with Eurocode 1. Dynamic analyses are carried out in the time domain using a step-by-step initial stress-like iterative procedure. Along-wind loads are considered assuming, at each level, time histories of the wind velocity for two return periods (i.e. Tr=10 or 50 years), based on an equivalent spectrum technique. Real accelerograms, whose response spectra match those adopted by Italian seismic code for a mediumrisk seismic zone and a medium subsoil class, are considered to simulate the seismic loads.