Thermo-structural response of a prestressed bulb-tee girder bridge to severe fire: Case study and parametric analysis

To enhance safety and longevity of highway infrastructure, there is a need for detailed investigation and a better understanding of the failure modes and vulnerability of prestressed (PS) concrete girder bridges under fire hazards. This study uses the I-85 highway overpass that collapsed under fire in 2017 in Atlanta, GA as a validation case for numerical simulation of PS concrete bridge girders. The model is then parametrically analyzed to investigate the sensitivity of design choices to fire resistance. Thermal spalling, which significantly affects the thermal resistance of the concrete, is incorporated into the simulation process. The numerical results show that the failure mode of the I-85 bridge span is dominated by flexural runaway and is shown to correlate with the fracture location and the collapse time observed during the event. It is found that the moment capacity of the PS girder largely depends on the strand proximity to the surface of the girder section where heat is applied. Harping strands into the web region creates flexurally vulnerable locations off the midspan, as the strands at those locations experience elevated temperatures at thinner web locations where spalling of the concrete cover is likely. The preservation of concrete cover will significantly increase the time to collapse, as it mitigates the temperature increase of the strands. The fire resistance of the PS girder bridge can be enhanced by preventing the loss of concrete cover via adding polypropylene fibers to the concrete mix or by minimizing the placement of strands near the concrete surface. • Thermal-structural analysis of prestressed concrete bridge under fire, accounting for spalling and strand heating effects. • Concrete bridge flexural failure validated by I-85 Atlanta fire event in 2017. • Realistic fire load model representing the solid fuel combustion under the bridge. • Spalling severity influences the structural response of prestressed concrete girders subjected to fire hazard.