Study on flame height and temperature distribution of double-deck bridge fire based on large-scale fire experiments

Vehicle fire is one of the important causes of bridge fires. This study conducted large-scale actual fire experiments by simulating heat release rates (HRR) of oil tank truck fires with 50 MW, 100 MW, and 200 MW in rectangular oil pans of varying sizes. The flame shape and spatial temperature distribution at different HRRs were studied by controlling variables such as wind speed, fire source location, and layer spacing. The results indicated that the actual HRR of the large-scale fire exceeded the anticipated value. In large-scale fire experiments, flame splitting into two main peaks was observed along the long side of the rectangular oil pan due to the coupling effect of ambient wind, long side, and sidewall effect of the diagonal web member. The average flame height was determined through image processing, and subsequently, a formula for predicting flame height during bridge fires under varying heat release rates was developed through the use of dimensionless analysis and feature length analysis. The spatial temperature distribution in the vertical direction of the axis of the center of the fire source, the temperature distribution of the diagonal web member and the bottom of the dense crossbeams, which are critical components of the bridge, were also analyzed. Furthermore, the conclusion that the flame in the lower bridge space which split into two main peaks is validated by the results. The results of this study can provide important theoretical support for investigating bridge fire resistance strategies.