47.5 MB - The Whole Building Design Guide

Floodwalls that protect essential and critical facilities usually aredesigned for the 0.2-percent-annual-chance flood (500-year) andhave freeboard to increase the factor of safety.2.3.1.2 Elevation ConsiderationsThe selection of the appropriate method“Lowest floor” is the floor of the lowestof elevating a critical facility in an A Zoneenclosed area (including the basement). flood hazard area depends on many factors,An unfinished or flood-resistant enclosure, including cost, level of safety and propertyusable solely for parking of vehicles,protection determined as acceptable risk, natureof the flood hazard area, and others.building access, or storage in an areaother than a basement, is not the lowest Methods of elevation are described below.floor, provided the enclosure is built in The minimum elevation requirement is thatcompliance with applicable requirements. the lowest floor (including the basement)must be at or above the DFE (plus freeboard,if desired or required). Table 2-1 in Section2.1.3.4 summarizes the elevation requirements in ASCE 24. Giventhe importance of critical facilities, elevation of the lowest floor toor above the 0.2 percent-annual-chance flood (500-year) elevationis crucial.For elevation methods other than fill, theASCE 7 outlines methods to determine area under elevated buildings in A Zonesdesign loads and load combinations in may be used only for limited purposes:flood hazard areas, including hydrostatic parking, building access, and limited storageloads, hydrodynamic loads, wave(crawlspaces are treated as enclosures, seeloads, and debris impact loads. ASCE below). Owners and designers are cautioned24 addresses design requirements forthat enclosures below the design flood elevationare exposed to flooding and thestructures in flood hazard areas.contents will be damaged or destroyed byfloodwaters. The walls surrounding an enclosuremust have flood openings that areintended to equalize interior and exterior water levels duringrising and falling flood conditions, to prevent differential hydrostaticpressures that could lead to structural damage. Theenclosed area must not contain utilities and equipment (includingductwork) below the required elevation.2-58 MAKING CRITICAL FACILITIES SAFE FROM Flooding