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Publication Number:  FHWA-HRT-09-028    Date:  May 2009
Publication Number: FHWA-HRT-09-028
Date: May 2009


Hydrodynamic Forces on Inundated Bridge Decks

1. Introduction

When a bridge crossing a waterway is partially or entirely submerged during a flood event, its deck may be subjected to significant hydrodynamic loading. The proper estimation of loading exerted by the flow on the structure is important for design and evaluation of vulnerability. This report uses a combination of reduced scale experiments and computer modeling to investigate the forces on inundated bridges.

The analysts at Argonne National Laboratory (Argonne), a U.S. Department of Energy laboratory, are working in collaboration with the researchers at the Federal Highway Administration's (FHWA) Turner-Fairbank Highway Research Center (TFHRC) to study computational fluid dynamics (CFD) techniques for simulating open channel flow around inundated bridges. The reduced scale experiments conducted at the TFHRC J. Sterling Jones Hydraulics Laboratory established the foundation of validated computational models of the same phenomena. The overall objectives of the study for which this report is based was to investigate the forces on inundated bridges and to establish validated computational practices to address the research needs of the transportation community in bridge hydraulics.

Bridges are a critical component of the nation's transportation network. Evaluation of a bridge's stability during and after flooding events, including the structural response of the bridge, is critical to highway safety. During a flood or tsunami, highway bridges over large waterways may become partially or completely submerged. Flood flows add significant hydrodynamic loading on bridges, resulting in possible shearing or overturning of the bridge deck and failure of the bridge superstructures. Traditionally, bridge analysts and designers have relied on expensive scaled experiments to provide estimates of the flow field and structural response. With the rapid development of supercomputing technology, commercial CFD code provides a quick, economic way to study these systems. The availability of parallel computers and analysis capabilities of commercially available software provides an opportunity to shift these evaluations into the CFD domain. When validated using the broad experimental database, the use of CFD simulations will allow expanded parametric analysis and provide a means of directly evaluating the effects of scaling.

The general external flow characteristics of a submerged body depend on the shape of the body. Generally, streamlined bodies (i.e., airfoils, streamlined cars) have little influence on the fluid around them compared with the effect from blunt bodies (i.e., triangle shapes and square-bodied bridge decks). The drag and lift on a bridge deck depend on many variables, notably the height of the free-surface level in relation to the bridge deck and the Reynolds number (Re) or Froude number (Fr), both of which describe the amount of turbulence in the flow and the degree to which the flow is critical. There are different mechanisms that have to be considered between the partially inundated case and completely inundated case. For a partially inundated deck, accurate estimates of hydrodynamic loading must take into account not only the forces from the upstream flow field but also the influence of free surface due to the hydrodynamic force from waves. For the completely inundated deck neglecting the effect of wave forces, the mean flow field upstream dominates the hydrodynamic loading on the bridge deck.

For a body moving through a fluid or a body immersed in a moving fluid, the interaction between the body and the fluid surrounding it produces forces at the fluid-body interface. The forces acting parallel to the free-stream direction due to the influence of viscosity may be called wall shear stresses, and the force acting normal to the free-stream direction due to pressure may be called normal stresses. The resultant force of shear stress and pressure distribution in the velocity's direction is termed as drag, and the resultant force normal to the direction of velocity is termed lift.

As flow separates from the leading edge corner for a bridge deck, large numbers of vortices form at different scales along the surface of the bridge deck. Eventually, they shed from the trailing edge in a process called the vortex shedding phenomenon. The center of a vortex, or a vortex core, has a local minimum pressure. Thus, the formation and development of the vortices tend to dominate the progression of the drag, lift, and moment on the surface of the bridge deck. Different deck geometries submerged by the flow have different flow fields and distributions of vortices along the surface of the decks leading to the alterations of forces or the force coefficients with the change in the geometry.

The applicability of commercial CFD software to predict flow field and evaluate drag and lift forces is being investigated. CFD provides a prediction of fluid flow by means of numerical modeling and software tools. It enables scientists and engineers to perform experiments (i.e., computer simulations) in virtual flow laboratory and significantly reduces the amount of experimentation and overall cost. CFD is a highly interdisciplinary research area that lies at the interface of physics, applied mathematics, and computer science.

The CFD-based simulations can be used for a range of hydraulics research, including the assessment of lift and drag forces on flooded bridge decks, shape optimizations to minimize pressure scour and pier erosion, analysis of sediment transport and its influence on scour, evaluation of active or passive countermeasures for damage mitigation, and consideration of environmental issues such as fish passage through culverts. Currently, the applicability of the commercial CFD software for prediction of these phenomena is being investigated, and the agreement between the code predictions and experimental data from TFHRC flumes is presented in this report.