The major objective of the study has been to investigate in detailthe rapidly-varying peak uplift pressure and the slowly-varying positiveand negative uplift pressures that are known to be exerted bywaves against the underside of a horizontal pier or platform locatedabove the still water level, but not higher than the crests of the incidentwaves.
In a "two-dimensional" laboratory study conducted in a 100-ftlong by 15-in.-wide by 2-ft-deep wave tank with a horizontal smoothbottom, 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, ofthe platform. The location of the transducers could be varied.
The problem of a d equate dynamic and spatial response of thetransducers was investigated in detail. It was found that unless theradius of the sensitive area of a pressure transducer is smaller thanabout one-third of the characteristic width of the pressure distribution,the peak pressure and the rise-time will not be recordedaccurately. A procedure was devised to correct peak pressures andrise-times for this transducer defect.
The hydrodynamics of the flow beneath the platform are described qualitatively by a si1nple analysis, which relates peak pressureand positive slowly-varying pressure to the celerity of the wave frontpropagating beneath the platform, and relates negative slowly-varyingpressure to the process by which fluid recedes from the platformafter the wave has passed. As the wave front propagates beneath theplatform, its celerity increases to a maximum, then decreases. Thepeak pressure similarly increases with distance from the seawardedge of the platform, then decreases.
Measured peak pressure head, always found to be less than fivetimes the incident wave height above still water level, is an order ofmagnitude less than reported shock pressures due to waves breakingagainst vertical walls; the product of peak pressure and rise-time,considered as peak impulse, is of the order of 20% of reported shockimpulse due to waves breaking against vertical walls. The maximummeasured slowly-varying uplift pressure head is approximately equalto the incident wave height less the soffit clearance above still waterlevel. The normalized magnitude and duration of negative pressureappears to depend principally on the ratio of soffit clearance to stillwater depth and on the ratio of platform length to still water depth.
