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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

6. Conclusion

Understanding the forces acting on inundated bridge decks is a worthwhile pursuit to improve the durability of bridges which can, even if infrequently, be overtopped in flooding events. The study described in this report investigated the properties of three common bridge deck types by measuring the response of drag force, lift force, and overturning moment to changes in the inundation ratio.

Experiments on scale models of the three bridge deck types were performed with an ultra-precise force balance. These experiments defined the general response of the force coefficients to different flow conditions, inundation ratios, and bridge types. The following key conclusions can be made about the forces on bridge decks from the experiments:

Overall, the experimental results provide the bridge designer with a wealth of information on the bridges' response to hydrodynamic forces when inundated. The design charts, fitting equations, and critical coefficient values provide a great deal of useful information for designing more durable bridges. In the future, however, more research will be needed to investigate the forces on the bridge deck in the region of transition from the partially inundated to completely inundated case as the critical lift (or pull-down) and moment values occur in this region.

CFD modeling was also performed as an alternative method to estimate the forces acting on bridge decks. Two CFD software packages, STAR-CD® and Fluent®, were used to model the bridge deck. A wide range of simulation options were explored, and the models reproduced the flow conditions of the experiments reasonably well, as judged by velocity map comparisons with the PIV results.

The CFD models performed reasonably well at estimating the force coefficient for the six-girder bridge for certain ranges of inundation coefficients. The models performed fairly poorly in reproducing the critical coefficient values and, in some cases, failed to match the behavior of the coefficients' response to h* in general. The three-girder and streamlined bridge CFD results did not match the experimental values because they were calibrated using flow conditions from the six-girder experiments. Recalibrating the flow conditions for these models could lead to substantial improvements.

The CFD modeling showed great promise for estimating the performance of inundated bridge decks. While the models did not capture the full range of behavior shown in the experimental results, the estimates of forces showed enough similarity that further refinements may make the CFD models capable of producing a reliable projection of force coefficients for design purposes. The flexibility of CFD models to represent almost any scenario means that they have a definite upside over physical experimentation.