47.5 MB - The Whole Building Design Guide

2.3.2.1 Elevation ConsiderationsThe selection of the appropriate method ofelevating a critical facility in a V Zone floodhazard area depends on many factors, includingcost, desired level of safety andproperty protection, and the nature of theflood hazard area. The NFIP regulations andthe building codes require the elevation ofthe bottom of the lowest horizontal structuralmember of the lowest floor (includingbasement) to be at or above the DFE (plusfreeboard, where required). Given the importanceof critical facilities, elevation to orabove the 0.2-percent-annual-chance flood(500-year) elevation is appropriate andstrongly recommended.Buildings in V Zones must be elevated usingopen foundations, which consist of verticalload bearing members (columns, piers, pilings,and shear walls) without solid walls connecting the verticalmembers. The design of the vertical members must also accountfor hydrodynamic loads and debris impact loads. Flood loads onshear walls are reduced if the walls are oriented parallel to the anticipateddirection of flow. Since erodible soils may be present andlocal scour may occur, both conditions must be accounted for indesigns of load-bearing members and foundations.The area under elevated buildings in V Zones may be used onlyfor limited purposes: parking, building access, and limited storage.Owners and designers are cautioned that enclosures below theDFE are exposed to flooding. Areas under elevated buildings maybe open or enclosed by lattice walls or screening. However, if areasare enclosed by solid walls, the walls must be specifically designedto break away under certain flood loads to allow the free passageof floodwaters under the building. Breakaway walls are non-loadbearing walls, i.e., they do not provide structural support for thebuilding. They must be designed and constructed to collapseunder the impact of floodwaters in such a way that the supportingfoundation system and the structure are not affected.Communities that participate in the NFIPwill require that a registered professionalengineer or architect develop or reviewthe structural design, specifications, andplans, and certify that the design andmethods of construction to be used arein accordance with accepted standardsof practice. The standards of practicerequire that the foundation and structureattached thereto is anchored to resistflotation, collapse, and lateral movementdue to the effects of wind and water loadsacting simultaneously on all buildingcomponents. Water loading values shallbe those associated with the base floodconditions, and wind loading values shallbe those required by applicable State orlocal building codes and standards.MAKING CRITICAL FACILITIES SAFE FROM Flooding2-69