Wave uplift pressures on horizontal platforms

摘要

The major objective of the study has been to investigate in detailudthe rapidly-varying peak uplift pressure and the slowly-varying positiveudand negative uplift pressures that are known to be exerted byudwaves against the underside of a horizontal pier or platform locatedudabove the still water level, but not higher than the crests of the incidentudwaves.ududIn a "two-dimensional" laboratory study conducted in a 100-ftudlong by 15-in.-wide by 2-ft-deep wave tank with a horizontal smoothudbottom, individually generated solitary waves struck a rigid, fixed,udhorizontal platform extending the width of the tank. Pressure transducersudwere mounted flush with the smooth soffit, or underside, ofudthe platform. The location of the transducers could be varied.ududThe problem of adequate dynamic and spatial response of theudtransducers was investigated in detail. It was found that unless theudradius of the sensitive area of a pressure transducer is smaller thanudabout one-third of the characteristic width of the pres sure distribution,udthe peak pressure and the rise-time will not be recordedudaccurately. A procedure was devised to correct peak pressures andudrise-times for this transducer defect.ududThe hydrodynamics of the flow beneath the platform are describedudqualitatively by a simple analysis, which relates peak pressureudand positive slowly-varying pressure to the celerity of the wave frontudpropagating beneath the platform, and relates negative slowly-varyingudpressure to the process by which fluid recedes from the platformudafter the wave has passed. As the wave front propagates beneath the platform, its celerity increases to a maximum, then decreases. Theudpeak pressure similarly increases with distance from the seawardudedge of the platform, then decreases.ududMeasured peak pressure head, always found to be less than fiveudtimes the incident wave height above still water level, is an order ofudmagnitude less than reported shock pressures due to waves breakingudagainst vertical walls; the product of peak pressure and rise-time,udconsidered as peak impulse, is of the order of 20% of reported shockudimpulse due to waves breaking against vertical walls. The maximumudmeasured slowly-varying uplift pressure head is approximately equaludto the incident wave height less the soffit clearance above still waterudlevel. The normalized magnitude and duration of negative pressureudappears to depend principally on the ratio of soffit clearance to stilludwater depth and on the ratio of platform length to still water depth.