This paper discusses hydrodynamic forces on bridge decks during a tsunami event. Two dimensional tsunami flow was reproduced by a water channel. Hydrodynamic forces (drag, lift and hydrodynamic moment) acting on bridge models with rectangular and channel section were measured. Flow patterns around the model when tsunami got over the model were investigated. The results showed that large hydrodynamic forces appeared when the tsunami got over the bridge deck model compared to the moderate wave. Strong instantaneous downward force acted shortly after a tsunami hit the model. Negative reaction force at the bridge shoe was also observed.

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Numerical simulation of tsunami wave force acting on a bridge was carried out, and discussion centered on a difference of a calculation result of two-dimensional model (shallow water equation) and three- dimensional model (CADMAS-SURF/3D). Three-dimensional model could reproduce a flow in circumference of the bridge precisely, and the inpact wave force acting on a bridge girder was predicted well. The two-dimensional model tends to underestimate inpact wave force because the reproduction precision of flow velocity is low. However, in three-dimensional models, the grid size of vertical direction should be set small enough to express the shape of bridge girder.

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This study describes on the adaptive flexibility of the equation on a tsunami wave force and pressure acting on a standing structures by use of some kinds of numerical models. As for conventional 2-D models on the basis of shallow water equation, there was a problem on a behavior of runup inland and spruch on the seawall, it was found that improvement of the numerical model is requrired. In addition, 3-D models on the basis of Navier-Stokes equation was effective for evaluation of the wave force and the wave pressure. However it was difficult to apply it to evaluate the whole town by the practical simulation because of a huge calculation time.

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This study performed numerical simulation using the SPH method to investigate the tsunami flow around a bridge girder and to evaluate hydrodynamic force acting on the girder. Hydraulic experiments were also conducted using 1/150 scale bridge model to calibrate the numerical simulations. Results showed that SPH can qualitatively predict the behavior of tsunami flow around bridge girder. The requirements concerning numerical stability and analytical accuracy for SPH method were clarified. This study reproduced experimental value about the horizontal drag, but the result of the vertical lift was not consistent with experimental value, further studies are needed to investigate this difference.

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