Drag-Induced Displacement of a Simply Supported Bridge Span during Hurricane Katrina

Abstract

Coastal bridges are vulnerable to large hydrodynamic loads during extreme events. During the 2004 and 2005 hurricane seasons, these loads exceeded the capacity of bridge spans resulting in bridge failures across the Gulf Coast region. This case study provides a forensic evaluation of a simply supported bridge span that failed during Hurricane Katrina (2005). The damaging coastal processes were simulated using sophisticated hydrodynamic models [advanced circulation (ADCIRC) + simulating waves nearshore (SWAN), XBeach] that reasonably reproduced the water levels, waves, and velocities at the bridge location. The bridge’s vulnerability to trapped-air effects, wave-induced loads, and drag forces was evaluated using the model results. All of these loads exceeded the structural capacity at the span that failed. However, a detailed evaluation of the modeling results suggests that this bridge span failed due to large drag-induced loads as a result of flow convergence near the bridge abutment. The superelevation of the bridge spans may have contributed to the damage by causing localized increases in flow velocity below the span beams but also possibly mitigated wave-induced damage. This may be the first known case in which drag-induced loads contributed to bridge failure during a hurricane. As such, high velocity flows may also pose a hazard to bridges and overpasses located in coastal floodplains.