Temperature Calculation Method for Hanger Systems Exposed to Multigrade Vehicle Fires

Abstract

Extensive vehicle fires threatening suspension bridges in the last decade have aroused an emerging demand to estimate the fire safety of hanger systems. One of the paramount tasks for evaluating the fire resistance of hangers is to calculate their temperatures. However, as plans always lag behind events, no competent temperature calculation methods are available now for engineers, whether vehicle fire models, thermal boundaries on hanger surfaces, or cross-sectional heat transfer formulations. This paper addresses these three issues and then proposes an integrated framework for assessing the thermal response of hanger systems subjected to vehicle fires. Vehicular fires were classified into five grades, adapted to cars, buses, light goods trucks, medium goods trucks, and tanker trucks, respectively. This hierarchy was validated using 16 full-scale vehicle fire tests in opening or tunnel conditions. An empirical formulation of thermal radiation developed for pool fires was adopted to envelope the heat flux applied by passenger vehicle fires on the hanger surface and was validated using five tests. Radiation from truck fires was constructed using a cuboid solid radiative flame. The spatial heat fluxes surrounding Grade-5 vehicle fires typified by tanker fires were computed using the computational fluid dynamics method validated by two large-scale outdoor trench fires. With these quantitative thermal boundaries on hanger exteriors, the transient temperature across the hanger section was derived using the lumped capacitance method, validated by finite-element models in various scenarios. As a multigrade temperature estimation methodology proposed for hangers subjected to vehicle fires, this method is easy to implement. It can serve in future structural fire design of critical suspension bridges with high exposure risks to vehicle fires.