Numerical simulation of prestressed concrete bridges exposed to potential bushfire

Abstract

The hazardous effects of wildfire on bridge structures have not attracted as much attention as building fires. In this study, the effects of bushfire on prestressed concrete bridges were analysed through a sequentially-coupled numerical approach comprising fire dynamics simulation, nonlinear thermal analysis and structural analysis. The adiabatic surface temperature distribution of the bridge was first obtained with a fire simulating model by the computational fluid dynamics (CFD) software Fire Dynamics Simulator. The surface temperature was then applied as boundary condition to a thermal model to get the internal temperature distribution. The mechanical response of the bridge was finally studied using the software ABAQUS (Figure 1). The time-equivalence method was used to simplify the transfer from the CFD model to ABAQUS. The thermal and mechanical properties were assumed to be temperature-dependent in all the simulations. For fire simulation, the grid size, simulation domain and clear space were investigated to ensure the accuracy of results. Based on the fire model verified, sensitivity studies were carried out to evaluate the effects of wildfire taking into account fire intensity, front width, spread rate, fire height and bridge headroom. A structural model was verified using experimental results from a fire test on a post-tensioned concrete beam available in the literature. Results show that the finite element model can well predict the increase of temperature in prestressing tendon and the mid-span deflection. Evaluation of the damage to structural members subjected to wildfire shows that bushfire may not cause bridge collapse, but potential damage may still result in excessive deflection that makes replacement of structural member necessary.