Distribution characteristics of fire temperature field under I-type composite beam bridge

Abstract

In recent years, the I-type composite beam bridge has been adopted for green and low-carbon benefits. However, a fire beneath the bridge can expose steel girders to elevated temperatures, potentially leading the bridge to collapse. The complex spatial structure, consisting of main girders, transverse beams, and small longitudinal girders, influences fire smoke spread and temperature field. This research focused on the distribution of the fire temperature field. First, a detailed fire dynamics model was established and verified by similar tests. Then, parameters such as vehicle types, ambient winds, and vertical clearances were analyzed. Finally, a prediction formula was established. Results indicate that the fire temperature field is mainly influenced by smoke spread. The temperature is highest at the direct flame site, followed by bottom temperature of the bridge deck. It decreases as smoke spreads. The temperature at the bottom of the bridge deck is nearly uniform across each cross-bridge region divided by the main girders and small longitudinal girders. It decreases exponentially along the longitudinal direction as the distance from the fire source increases. Larger vehicle fires, lower wind speeds, and smaller vertical clearance lead to higher temperature. The bottom flange temperature of the exposed main girder reaches 1200 °C. Meanwhile, the bottom temperature of the bridge deck reaches 1000 °C. When the wind speed reaches 3 m/s or the vertical clearance reaches 22.5 m, the main girder and bridge deck temperatures are below 300 °C. A prediction formula considering these factors agrees well with the simulation results is proposed.