Numerical study of submergence-induced forces on a maintenance bridge

The US Army Engineer Research and Development Center–Coastal and Hydraulics Laboratory conducted a numerical study for a proposed maintenance bridge over the Trinity River in downtown Fort Worth, Texas, that will be submerged in flood conditions. The purpose of this study was to determine the hydrodynamic loads on the structure. The study found that, due to a combination of peak velocities and water depth, a return period of 500 yr generates the largest drag forces, while the higher depth and discharge of the standard project flood generate larger lift forces. Two flow structures were observed with shallow submergence: The first resulted in a jet parallel to the deck with a recirculation extending from railing to railing and the second resulted in a plunging jet over the deck with smaller but stronger recirculation. These patterns resulted in significantly different loads on the structure. The study analyzed the bridge’s deck slope to the flow as a possible variable affecting the loads. Observed changes related mostly to the flow pattern predicted for different flow configurations. Finally, it was observed that an open railing provides the best possible conditions in terms of loading; therefore, minimizing the frontal area of the railing is recommended.