Study on the Evaluation of Temporal Change in Horizontal and Vertical Tsunami Forces Acting on a Bridge Superstructure

Bridge superstructures were washed away in the 2011 Tohoku earthquake tsunami. To determine whether and when bridge superstructures are washed away, it is essential to evaluate the temporal change in horizontal and vertical tsunami forces. In this study, tsunami force acting on bridge superstructures and its estimation formulae are investigated from hydraulic model experiments and numerical simulations. From experimental results, it is found that it can be determined from a comparison between the temporal change in horizontal tsunami force and that in static friction force taking into account vertical tsunami force whether and when a bridge superstructure move. Furthermore, a drag force equation with the projection area taking into account water surface elevation and a Morison equation in which inertia force is added to the drag force equation are proposed for horizontal tsunami force, and an estimation formula composed of buoyancy force and downward lift force is proposed for vertical tsunami force. From a comparison with numerical results, it was revealed that the estimation formulae can predict the temporal change in the horizontal and vertical tsunami forces except for short wave period, suggesting that it can be evaluated using the estimation formulae whether and when a bridge superstructures moves.