A Localized Fire Model for Predicting the Surface Temperature of Box Girder Bridges Subjected to Tanker Truck Fire

The quantification of thermal action is important to the analysis of structural-fire performance of bridges. This study evaluates the parameters for the localized fire model adopted in SFPE Handbook for application to the fire scenario of a tanker truck burning beneath a bridge. Modification is applied first to the flame length and then to the distribution of gauge heat flux using the simulation results of various fire models established in Fire Dynamics Simulator considering parameters, including the sectional dimensions of bridge, bridge headroom, truck size and heat release rate. Spatially varied gauge heat flux or adiabatic surface temperature of the bridge can be predicted with this fire model. Implementation of this modified fire model in structural-fire analysis is illustrated with a sequentially coupled thermo-mechanical modelling of a post-tensioned segmental concrete box girder bridge exposed to tanker truck fire. The adiabatic surface temperature calculated from the modified fire model is applied as thermal boundary to the bridge. Simulation results show that, although the global structural responses are seldom influenced by fire, localized damage in concrete and tendon may result. The prestress in tendons near mid-span may be reduced even after the bridge is cooled down to ambient temperature, which may adversely affect its load-carrying capacity. The damage to concrete may also induce localized separation between adjacent segments, possibly affecting the durability of tendons.