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The following section provides calculational pro- cedures and design methods which can be used to determine appropriate strategies to withstand or counteract these forces. Note, as previously stressed, elevation of the manufactured home above anticipated flooding significantly reduces or eliminates the flood loading on the manufactured home. This, in turn, greatly enhances the probability for survival of a flood occurrence. B. EVAWATION OF ELEVATED FOUNDATIONS An elevated foundation must be adequately sup- ported in the ground, tied effectively to the manufactured home, and have the ability to with- stand flood and wind induced loads, dead loads, roof live loads, and floor live loads. As previously discussed, the imposition of hydrostatic, hydro- dynamic, and impact forces on the manufactured home will add additional loads to the elevated foundation. The magnitude of these loads, as shown in Figure 4.21, is sufficient to seriously damage a manufactured home not elevated above anticipated flooding. w... .I....C W "., t I ~ =Choy"no:lI t t '" ..:::,.,,. ":;'''''' Figure 4.21 Magnitude of Loads Ground anchors and tie-downs can hold the manufactured home in position only under ex- tremely low velocity and depth conditions dur- ing flooding. From a design standpoint, the prac- tical limitations on anchoring and tie-down to counteract flood forces can be summarized as follows: Water Velocity Tie Angle To Vertical Maximum Flood Height Above Floor o to 5 fps o 30 45 10" 9" 8" 5 to 10 fps o 30 45 8" 7" 6" 10 fps None-Manufactured home must be elevated above anticipated flooding. The above information indicates that flood heights of more than one fool will result in forces that cannot be effectively counteracted by ground anchors and tie-downs. Because of this, and also because even a small amount of depth above the floor can substantially damage a manufactured home, the use of anchoring alone, without addi- tional elevation is not effective in minimizing future flood losses. The optimum strategy is to elevate the manufactured home to a height where no flood forces are anticipated to affect the manufactured home. Appendix E provides additional details on ground anchor spacings necessary to resist buoyancy and drag forces and should be con- sulted for further information. A review of this appendix will serve to validate the limitations of ground anchoring and tie-downs in flooding conditions. Piers, posts, and piles can be used to elevate a manufactured home above anticipated flood levels. When the manufactured home is elevated above anticipated flooding, flood forces act only on the elevated foundation. The elevated fo'un- dation, however, must support the manufac- tured home, and withstand all other anticipated forces. As previously discussed, a registered design professional may be required to design the elevated foundation depending upon state or local regulations and the height above grade. Design Forces and Loads The flood forces to be considered in the design of an elevated foundation are buoyancy, lateral, impact, and drag forces as discussed in Section A of this chapter. Generally, when compared to forces imposed by wind, dead load, roof live load, and floor live load, the flood forces on the founda- tion are small if the manufactured home is elevated above the anticipated flood level. As the manufactured home elevation is increased, the 45