Bivariate structural-fire fragility curves for simple-span overpass bridges with composite steel plate girders

This study demonstrates the development of bivariate fragility curves for a fire-exposed simple-span overpass bridge prototype with composite steel plate girders. The fire and resulting heat transfer to the girders are modeled using the computationally efficient Modified Discretized Solid Flame (MDSF) model, developed previously by the authors. Several input parameters to model the thermo-structural response of the girders (particularly the material strength and applied loading) are stochastically selected for Monte Carlo Simulation via Latin Hypercube Sampling. The thermo-structural response of the composite steel girders is calculated using uncoupled, reduced-form finite element analyses. The structural-fire response of the prototype bridge girder is iteratively calculated for a large suite (∼1,000 iterations) of fire scenarios and then categorized into escalating damage levels based on the maximum deflection reached during the fire event. The damage from each fire scenario is correlated to two measures of fire hazard intensity: the peak heat release rate, and the total thermal energy imparted along the girder span. Bivariate fragility curves that correlate the two intensity measures to each damage level via a cumulative normal distribution function are obtained for the prototype bridge with span length varying from 12.2 to 42.7 m. An illustrative example uses these fragility curves to assess fire-induced damage for the two overpass spans in the MacArthur Maze interchange in Oakland, CA that collapsed due to a 2007 tanker truck fire.