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For example, a 14-foot wide manufactured home having a 99-inch chassis I-beam spacing located in a 30 psf roof live load area and eight foot spacing of vertical support members has a 5,280 pound load at each point where the elevated foundation supports the manufactured home as shown in Figure 4~25. ^ 8' 5280# 5280# - 5280# - 5280# - , .I 5280# 5280# 8' 5280# 8' 5280# , I. , I. -v , .1 99" 14'-0' Figure 4.25 Vertical Member Load The design of an elevated foundation to resist these loads is dependent on the specific elevation technique selected, height above grade, size of foundation components, connections, and spac- ing of supporting members lengthwise along the manufactured home. Spacing of vertical members must not exceed the recommended spacing in the homeowner's installation manual or 10 feet, whichever is less. In addition, any re- quirements for perimeter blocking in the homeowner's installation manual must also be completed. Prior to selecting and designing a vertical member to elevate the manufactured home, the bearing capacity of the soil on the site must be determined. This is critical to assure that adequate support for any elevated foundation is provided by the soil. Estimating the bearing capacity of soil is difficult in that soil characteristics may be altered later by a change in moisture content. Soil classification of a general nature can be shown as follows: SOIL BEARING PRESSURE (psf) 600-1200 1500-2500 3000-4500 800-1600 1600-3500 2700-3000 2300-2700 3000-3300 2700-3000 Clay, soft Clay, firm Clay, stiff Loose sand, wet Firm sand, wet Packed earth Loose earth Gravel 1" Gravel 2'12" In general soils can be divided into five classifications; very soft, poor, average, good, and very hard. Bearing capacities are respectively 800 -1200 psf, 1200 - 2000 psf, 2000 - 3000 psf, 3000 - 4100 psf, and 4100+ psf. Where a general visual inspection and consulta- tion with a soils engineer is not sufficient to estimate the bearing capacity of the soil, a load test should be conducted. The load test should be conducted when the soil is wet. After the soil bearing capacity is determined, that value can be evaluated against the maximum vertical member load to determine the required bearing area of the vertical member as follows: Vertical Member Load (lb.) bearing = area Soil Bearing Capacity (pst) (sq. ft.) For example, if the previous example shown in Figure 4.25 was to be located where the soil bear- ing capacity was found to be 1800 psf, the required bearing area under each vertical member would be as follows: 5280 lb. _ 2 93 ft _ . sq. . 1800 psf A bearing pad 21 inches by 21 inches would be acceptable since it provides an area of 3.06 sq. ft. The following sections provide details on the design of components needed to resist these loads under various conditions. These include vertical member (piers, posts and piles), horizontal sup- ports, diagonal bracing, and connections asso- ciated with an elevated foundation design. Vertical Support Members Piers-Flood and wind loads are resisted by piers through their structural capacity. Cast-in-place concrete and built up concrete masonry unit or brick piers, because of their larger surface area and limited anchorage in the ground, are more susceptible to flood forces than posts or piles, 48