Fire risk methodology for fires impacting bridge structures

Bridges are generally designed to withstand traffic and wind loads, but fire, despite being a lowprobability hazard, can cause highly dangerous consequences; thus, the fire risk may not be negligible. Moreover, current design standards do not take account for fire, making them vulnerable to this action. In fact, a fire can lead to bridge collapse or excessive deformation, disrupting the infrastructure network. Notable fire incidents include the I-95 overpass collapse in Philadelphia (2023) and the I-95 overpass fire in Norwalk (2024). In this respect, this work presents a methodology for evaluating highway bridge fire risk using a calibrated Highway Safety Manual (HSM) method to determine the probability of fire occurrence and computational fluid dynamics coupled with thermomechanical finite element models to estimate the consequences of a fire under an overpass. One of the most useful and widely recognised methods for evaluating accidents and risk factors in highway engineering is represented by the HSM, which includes statistically derived regression models that allow the estimation of the expected number of crashes along the infrastructure. The HSM model was calibrated with real data by taking into account the geometric characteristics of the selected highway, the traffic levels, and the fire accidents that occurred over a thirteen-year period. The fire accidents were analysed in terms of location, damage level, vehicle type, presence of an overpass, type of fire load and ignition causes. Then, the calibrated model was applied to predict the expected number of accidents that result in fires under an overpass with a 5-year future projection once the evolution of annual average daily traffic was established by analysing the traffic time series of each motorway segment. A significant increase in predicted fires for heavy traffic segments is noted. The consequences were determined by considering two different overpass types: a girder overpass with a composite steel-concrete structure and a steel arch overpass. Different plausible vehicle fire scenarios were simulated using the Fire Dynamics Simulator. Then, a series of thermomechanical analyses were performed using SAFIR software to investigate the structural fire response and the potential loss of functionality. The research considered direct and indirect costs. Eventually, the fire risk was computed. The results provide valuable information that can be used by road operators and bridge management specialists to identify appropriate fire mitigation strategies (e.g. intumescent paint) to be potentially applied to strategic bridges.