Fire−induced Thermo−structural Response of Reinforced Concrete Beams in Underground Parking Lots

This study analyzed the thermo−structural response of reinforced concrete beams exposed to multi−vehicle fires in underground parking lots and proposed a regression−based model for predicting the time−deflection response from fire temperature histories. The heat release rate model and ceiling time−temperature model for multi−vehicle fires developed in a previous study were used as input thermal loads for structural analysis. Ceiling temperature histories for 96 fire scenarios, generated by combining the peak heat release rate of a single−vehicle fire, time to reach the maximum heat release rate, duration of the fully developed stage, and fire spread time, were applied to coupled temperature−displacement finite element analyses of an RC beam. The analysis results showed that, as the single−vehicle peak heat release rate increased, both the ceiling peak temperature and midspan deflection increased. In contrast, as the fire spread time increased, the peak temperature decreased, whereas the high−temperature duration and the area under the time−temperature curve increased. Accordingly, the deflection of the reinforced concrete beam could not be explained solely by either the peak temperature or the cumulative thermal exposure; rather, it was governed by the combined effects of both factors. Based on these results, a maximum deflection prediction equation was proposed by introducing a fire amplification factor into the ambient−temperature immediate deflection, and a model was developed to estimate the deflections at key characteristic time points using deflection ratios relative to the maximum deflection. The proposed maximum deflection model showed a coefficient of variation of 0.109 compared with the finite element analysis results, and the predicted characteristic deflection points also exhibited trends similar to those of the numerical results. Therefore, the proposed model can be used to provide a simplified estimate of the fire-induced deflection of RC beams under multi−vehicle fire conditions in underground parking lots and to assess structural fire risk at the preliminary stage of performance−based fire safety design.