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Wave uplift pressures on horizontal platforms

French, Jonathan A. (1969) Wave uplift pressures on horizontal platforms. W. M. Keck Laboratory of Hydraulics and Water Resources, KH-R, 19. California Institute of Technology , Pasadena, CA. http://resolver.caltech.edu/CaltechAUTHORS:20121012-085505248

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Abstract

The major objective of the study has been to investigate in detail the rapidly-varying peak uplift pressure and the slowly-varying positive and negative uplift pressures that are known to be exerted by waves against the underside of a horizontal pier or platform located above the still water level, but not higher than the crests of the incident waves. In a "two-dimensional" laboratory study conducted in a 100-ft long by 15-in.-wide by 2-ft-deep wave tank with a horizontal smooth bottom, individually generated solitary waves struck a rigid, fixed, horizontal platform extending the width of the tank. Pressure transducers were mounted flush with the smooth soffit, or underside, of the platform. The location of the transducers could be varied. The problem of adequate dynamic and spatial response of the transducers was investigated in detail. It was found that unless the radius of the sensitive area of a pressure transducer is smaller than about one-third of the characteristic width of the pres sure distribution, the peak pressure and the rise-time will not be recorded accurately. A procedure was devised to correct peak pressures and rise-times for this transducer defect. The hydrodynamics of the flow beneath the platform are described qualitatively by a simple analysis, which relates peak pressure and positive slowly-varying pressure to the celerity of the wave front propagating beneath the platform, and relates negative slowly-varying pressure to the process by which fluid recedes from the platform after the wave has passed. As the wave front propagates beneath the platform, its celerity increases to a maximum, then decreases. The peak pressure similarly increases with distance from the seaward edge of the platform, then decreases. Measured peak pressure head, always found to be less than five times the incident wave height above still water level, is an order of magnitude less than reported shock pressures due to waves breaking against vertical walls; the product of peak pressure and rise-time, considered as peak impulse, is of the order of 20% of reported shock impulse due to waves breaking against vertical walls. The maximum measured slowly-varying uplift pressure head is approximately equal to the incident wave height less the soffit clearance above still water level. The normalized magnitude and duration of negative pressure appears to depend principally on the ratio of soffit clearance to still water depth and on the ratio of platform length to still water depth.


Item Type:Report or Paper (Technical Report)
Additional Information:Supported by The Sloan Fund for Basic Research and The National Science Foundation (Grant No. GK-1729).
Funders:
Funding AgencyGrant Number
Sloan Fund, CaltechUNSPECIFIED
NSFGK-1729
Record Number:CaltechAUTHORS:20121012-085505248
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20121012-085505248
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:34864
Collection:CaltechKHR
Deposited By: George Porter
Deposited On:12 Oct 2012 17:18
Last Modified:27 Dec 2012 02:51

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