Fire represents a significant hazard to bridges, often resulting in damage or collapse of structural members. Typically, bridge fires result from crashes or overturns of vehicles carrying large amounts of flammable materials near bridges. These fires have become a growing concern over the last decade due to increasing urbanization and transportation of hazardous materials. Characterized by the rapid onset of very high temperatures (above 1000°C), these fires significantly affect the stability and integrity of structural members. Despite these risks, current bridge codes and standards do not specify any fire safety features in the design and construction of bridges, leaving critical transportation infrastructure vulnerable to fire hazard.While there has been some research in recent years on the fire response of steel and composite bridges, there have been limited studies on the fire problem in concrete bridges. Further, prestressed concrete girders, designed with slender cross-sections to reduce self-weight and span longer distances, can experience faster degradation during fire exposure due to rapid temperature propagation within the girder cross-section. Although conventional concrete members have good fire response properties, newer concrete types such as High-Strength Concrete (HSC) and Ultra-High Performance Concrete (UHPC) experience faster degradation of mechanical properties at elevated temperatures and are also more susceptible to fire-induced spalling. To address some of the identified knowledge gaps, experimental and numerical studies on the fire response of concrete bridge girders have been carried out. As part of the experimental work, pore pressure measurements in concrete at elevated temperatures were conducted to evaluate the mechanisms that lead to fire-induced spalling in concrete. Also, shear strength tests were carried out to assess the degradation of shear strength with temperature in UHPC. Complementing the experimental studies, a comprehensive finite element-based numerical model was developed to trace the response of concrete bridge girders under fire conditions. The model accounts for varying fire scenarios, loading conditions, and temperature-dependent thermal and mechanical properties of steel and concrete, and was validated with data from fire tests. To develop typical bridge fire scenarios, fire dynamics simulations were carried out and incorporated into the model. A set of parametric studies were undertaken to evaluate the effect of critical parameters on the fire response of concrete bridge girders. Results demonstrate that smaller concrete sections present lower fire resistance due to their lower thermal mass, and that I-shaped concrete girders are susceptible to shear failure from the high temperatures in their webs. Other design parameters, such as span length and concrete strength, also significantly affect the fire performance of concrete bridges. In addition, fire simulations have shown that bridge fires present high severity and are influenced by the bridge geometrical features. Based on these findings, recommendations to improve the fire design of bridge girders have been proposed. For conventional concrete bridge girders, increasing cross-sectional size and limiting exposure of the web near the supports to the high temperatures can improve fire performance. Spalling can be reduced in UHPC members through addition of polypropylene fibers. Additionally, parameters for assessing the fire resistance of bridge girders, such as failure criteria and a bridge fire curve that accounts for the thermal gradient along the girder length were proposed. The developed numerical tool is also applied to analyze the fire-induced collapse of the I-95 overpass in Philadelphia on June 11, 2023.
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