| dc.description.abstract | This thesis examines the interaction of flood flow with bridge superstructures by considering the effects of the flow parameters, bridge geometry, debris, and bridge substructures. The hydrodynamic forces are expressed by using force and moment coefficient charts to portray the effect of bridge submersion, bridge position with respect to the channel bed, flood velocity, and bridge geometry.
Of the 580,000 bridges in the United States, 83% span streams and rivers that may subject them to floods and debris loads. Although bridges are designed to withstand hydrodynamic forces, the most frequent causes of failure are attributed to hydraulic events, including floods, debris, and drifts. The flood-induced hydrodynamic forces cause shear and overturn on bridge decks and may cause them to fail; therefore, the ability to resist hydrodynamic forces is a key factor in the design and construction of safe bridges.
Laboratory bridge models were developed to investigate the range of submergence (inundation ratio, h*), flood velocity (Froude Number, Fr), and proximity to the channel bed (proximity ratio, Pr) of hydrodynamic force coefficients of bridge superstructures. The effects of the bridge geometry (i.e., width, height, and shape) were studied to determine their influence on the hydrodynamic force and moment coefficients and revealed that there were minimal to no effects on the drag coefficients (CD), lift coefficients (CL), and moment coefficients (CM) when a proximity ratio of 3 or greater was selected. The experimental results revealed that an increase in Fr slightly reduced the force and moment coefficients, especially near the partially-inundated-to-fully-inundated transition (h* = 1). It was also observed that the coefficients of drag, lift, and moment was significantly affected by the inundation ratio (h^*), especially in the transition from partially- to fully-submerged states, i.e., h^*= 0.75 – 1.25. The experimental results showed that the force and moment coefficients can be affected by the width of the deck (aspect ratio, Ar), the height of the deck (blockage ratio, Br), and shape of the bridge superstructure.
Flow visualization, using the particle image velocimetry (PIV) technique, was used to investigate the flow structures of submerged and partially submerged bridge superstructures. Features of flow kinematics such as flow separation lines, reattachment points, wake width, and reattachment length were investigated in detail and correlated to the trends of hydrodynamic force coefficients. The drag force coefficients were directly proportional to the width of the wake region, and the lift force coefficients were dependent on the difference between the length of the flow separation at the top and bottom of the deck.
An attempt was also made to investigate the effects of debris and the presence of substructures on force coefficients. For this purpose, experiments were performed with two types of debris: flat plate and wedge debris. The results indicated that flat plate debris increased the drag force coefficients significantly, and wedge debris increased the overturning moment and moment coefficients. The results of the substructure model showed that the hydrodynamic force coefficients were conservative for partially submerged cases but not for fully submerged cases. This knowledge and understanding will help to create improved guidelines and standards for designing river-crossing bridges. Lessons from this study can be applied to the design of resilient bridges, retrofit of existing bridges, and more. | |
