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Wave slamming on a horizontal plate Bhat, Shankar Subraya


The design of coastal and offshore structures requires a thorough understanding of environmental loading, particularly due to waves. Structural elements such as decks located in the splash zone encounter intermittent contact with the water, and the loads associated with the water impact may be several times larger than those experienced by elements when fully submerged. These forces may give rise to localized damage and to fatigue problems. Such structures should clearly be designed to account for wave impact, in addition to more general wave loading. Several studies have reported the related problems of ship bottom slamming, missile entry and sea plane landing. Although previous studies have contributed to an improved understanding of wave impact, there is still considerable uncertainty in the estimation of impact loads on structural elements near the water surface. In this context, the present study has been carried out to examine the wave loads on a fixed horizontal plate located near the still water level. Experiments were conducted in the wave flume of the Hydraulics Laboratory of the Department of Civil Engineering at the University of British Columbia. A plate, 60.0 cm long, 20.0 cm wide and 6.25 mm thick, was instrumented with load-cells to measure the vertical force on the plate due to waves. The plate was supported by two vertical rods through the load-cells which were connected to a cross shaft mounted on bearings at the ends. Tests were conducted over a range of wave periods and wave heights in combination with different plate clearances above the still water level. The vertical reactions at the two supports were measured, and the time histories of vertical force and its line of action are thereby obtained. The wave surface elevations at the leading and rear end of the plate were measured with the plate absent. Results are presented in the form of force time histories, their lines of action and the associated water surface elevation. An analysis of these time histories is carried out to obtain various parameters of wave impact which include, the peak upward and downward force, their lines of action and times of occurrence, and the associated wetted lengths. The influence of incident wave parameters on these is investigated. Video images are studied to understand the impact process and to identify the difficulties involved in the investigation. An attempt is also made to predict the vertical force based on the hydrodynamic impact, drag and buoyancy forces.

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