Seismic resistance parameters of reinforced concrete frame buildings after fire considering multiple fire exposure scenarios

Introduction. The consequences of devastating earthquakes demonstrate that the issue of seismic resistance of reinforced concrete frames after fire is highly relevant. The computational models used to address such problems must account for the nonlinear characteristics of concrete and reinforcement, variability of fire exposure, and design accelerograms. This study aims to identify the most hazardous fire exposure scenarios (from the standpoint of seismic resistance).

Materials and methods. A flat five-story reinforced concrete frame was selected as the research object under various fire exposure scenarios. Fire exposure was applied according to the standard temperature-time curve (ISO 834) with a duration of 120 minutes. Five scenarios were considered. A preliminary thermal analysis was conducted in PC SolidWorks
to determine temperature fields in element cross-sections. Subsequently, nonlinear seismic analysis of the five frames was performed in PC OpenSees. The analysis was carried out in the time domain using the Loma Prieta (1989) accelerogram. Bilinear and trilinear material diagrams were used. Cross-sections were modelled using a fiber model.

Results. A FEM was conducted of the dependence of seismic resistance parameters of reinforced concrete frames on various fire exposure scenarios. The periods of the first mode increased by up to 11.2 % compared to the control frame, roof-level displacements increased by up to 32.2 %, maximum inter-story drifts increased by up to 34.0 %, and maximum bending moments in frame elements increased by up to 26.3 % due to force redistribution. Failure mechanisms changed — plastic hinges formed not only in beams but also in columns; for most frames considered, deformation levels in elements reached critical values.

Conclusions. The most dangerous scenario, which most significantly reduces the seismic resistance of the reinforced concrete frame, is the occurrence of fire on the first floor. The percentage of damaged vertical load-bearing elements is particularly significant here — higher percentages lead to a more substantial reduction in seismic resistance. In some cases, such as localized fire damage on intermediate floors, the frame’s seismic resistance decreases only slightly. During the design stage of buildings in seismic zones, it is recommended to consider the most hazardous fire exposure scenarios. Special emphasis should be placed on enhancing the fire resistance of vertical load-bearing structures in lower floors and reducing the size of fire compartments.

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