Fire-induced collapse of an I-95 overpass in Philadelphia: Causes, collapse mechanism, and mitigation strategies

The collapse of the I-95 overpass in Philadelphia on June 11, 2023, due to a tanker-induced fire, caused significant traffic disruptions to an important transportation system on the East Coast of the United States. The increasing frequency of similar bridge fire events over the last few years has ignited a debate to re-visit the fire performance of bridges and to account for fire hazards in bridge structures. From this lens, this paper employs the established methodology of fire-based importance factors to assess the vulnerability of the I-95 overpass and then examines the causes and mechanisms for its collapse. To complement this methodology, the paper also presents mitigation strategies, if included, that could have mitigated the collapse of this overpass. More specifically, a finite element model is applied to trace the response of an I-95 girder section under combined effects of thermal and mechanical loading, considering three representative fire scenarios: standard, hydrocarbon, and design fire. Results from numerical simulations show that the bridge experienced rapid temperature rise as in the case of a hydrocarbon fire, leading to the failure of steel girders within 15 min of fire exposure. The developed model was also applied to evaluate strategies that could have minimized the collapse of the overpass under similar fire incidents: composite action and fire insulation. Both alternatives contributed to increasing the fire resistance of the steel girder, with the latter option proving to be significantly more effective. Overall, the increasing frequency of bridge fire incidents in recent years demostrates the need to integrate fire hazards into the design of critical infrastructure, particularly critical bridges, to enhance their resiliency under possible fire scenarios.