47.5 MB - The Whole Building Design Guide

Debris velocity: The velocity of the debris depends on the natureof the debris and the velocity of floodwaters. For the impact loadcomputation, the velocity of the water-borne object is assumed tobe the same as the flood velocity. Although this assumption is reasonablefor smaller objects, it is conservative for large objects.Debris impact duration: Duration of impact is the elapsed timeduring which the impact load acts on the building or building element.The duration of impact is influenced primarily by thenatural frequency 2 of the building or element, which is a functionof the building’s stiffness. Stiffness is determined by theproperties of the material, the number of supporting members(columns or piles), the height of the building above the ground,and the height at which the element is struck. Despite all the variablesthat may influence duration of impact, early assumptionssuggested a 1-second duration. A review of results from severallaboratory tests that measured impacts yielded much briefer periods,and ASCE 7 currently recommends a duration of 0.03second.2.1.2.7 Erosion and Localized ScourErosion generally refers to a lowering of the ground surface as aresult of a flood event. Erosion may occur in riverine and coastalflood hazard areas. In coastal areas, erosion may affect the generalground surface and may cause a short-term or long-term recession ofthe shoreline. Erosion should be considered during load calculations,because it increases the local flood depth, which in turn influencesload calculations. In areas subject to gradual erosion of the ground surface,additional foundation embedment depth can mitigate the effects.However, where waterways are prone to changing channels andwhere shoreline erosion is significant, engineered solutions areunlikely to be effective. Avoidance of sites in areas subject to activeerosion is the safest and most cost-effective course of action.Localized scour results from turbulence at the ground levelaround foundation elements. Scour occurs in both riverine andcoastal flood hazard areas, especially in areas with erodible soils.2. The frequency at which an object will vibrate freely when set in motion.MAKING CRITICAL FACILITIES SAFE FROM Flooding2-27