Fire is one of the most severe environmental hazards to which civil structures may be subjected during their lifetime. In recent decades, due to rapid development of urban ground-transportation systems, as well as increasing transportation of hazardous materials, bridge fires have become a growing concern. Steel structural members which are widely used in bridges exhibit lower fire resistance as compared to concrete members due to rapid degradation of strength and stiffness properties at elevated temperature. Therefore, behavior of steel girders under fire conditions is of critical concern from fire safety point of view.Unlike structural members in buildings, no specific fire resistance provisions (active or passive) are required for bridges as per specifications in AASHTO and other standards. Currently, there is no approach to evaluate fire resistance or residual capacity of steel girders after fire exposure. Also, fire provisions used for building elements might not be directly applicable to bridge elements due to different fire exposure scenario, load level, support conditions, and sectional characteristics. To overcome some of the current drawbacks, a research program involving both experimental and numerical studies on the fire response of steel bridge girders is undertaken. Three steel-concrete composite girders were tested under simultaneous loading and fire exposure to study the behavior of steel bridge girders. Test variables included; load level, web slenderness, and spacing of stiffeners. Results from fire tests indicate that typical steel girders can experience failure under standard fire conditions in about 30-35 minutes and the response is highly influenced by web slenderness, and type of fire exposure. As part of numerical studies a finite element model was developed in ANSYS for tracing thermal and structural response of steel bridge girders under fire conditions. Test data generated from fire experiments were utilized to validate the finite element model. The validated model was applied to carry out detailed parametric studies to quantify critical factors influencing fire response of steel bridge girders, namely fire scenario, exposure scenario, load level, span length, web slenderness, presence of stiffeners, and degree of axial and restraint stiffness. Also, as part of numerical studies, a methodology for evaluating residual capacity of fire exposed steel bridge girders was developed.Results from the parametric study show that steel bridge girders can experience failure in less than 20 minutes under severe fire exposures, such as hydrocarbon fires. Under such fire scenarios, failure through web shear buckling is the most dominant failure limit state especially when web slenderness exceeds 50. Fire resistance and failure mode is highly influenced by fire intensity, exposure scenario, web slenderness, load level, and span length. Results from the parametric studies are utilized to develop a strategy for enhancing fire resistance of steel bridge girders. The strategy mainly comprises of applying fire insulation, of different types and configurations, on steel bridge girders to achieve 1 to 2 hours of fire resistance. The information and strategy developed in this dissertation can be utilized to enhance fire resistance of steel girders and thus fire hazard to steel bridges can be mitigated to a great extent.
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