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<br />Figure 3.12 Cast-in-place Pier
<br />
<br />The dimensions, reinforcement, and spacing of
<br />cast-in-place concrete piers depend on the manu-
<br />factured home support spacing requirements as
<br />well as anticipated flood and wind loads,
<br />Pier footing sizes are a direct function of soil
<br />bea~ing capacity and loading, [)_epth of pier
<br />footings depends on local frost penetration levels
<br />and expected flooding, wind, and erosion levels,
<br />Local codes may provide specific requirements for
<br />the size and depth of footings based on local soil
<br />conditions, At a minimum the bottom of the footing
<br />must be at least 30 inches below grade or to the
<br />frost line, whichever is greater.
<br />As with all elevation techniques, footings in
<br />areas where the soil bearing capacity is not known
<br />can be unstable and should, therefore, be de-
<br />signed with the guidance of a soils engineer.
<br />Design calculations, suggested construction
<br />details, and anchoring criteria for pier foundations
<br />are presented in Chapter IV, "Design of Eleva-
<br />ted Foundations."
<br />
<br />
<br />Posts-Posts may be used to elevate a
<br />manufactured home above an anticipated flood
<br />level. The height of the posts, design re-
<br />quirements, and the need for additional anchor,
<br />ing will be determined by anticipated flooding con-
<br />ditions, wind loads, and the type of terrain and soil
<br />on the manufactured home site,
<br />
<br />As, shown in Figure 3,13, posts replace the
<br />blocking arrangement described for a conventional
<br />installation and carry the weight of the manufac,
<br />tured home by bearing, on undisturbed soil or a
<br />concrete bearing pad,
<br />
<br />
<br />Figure 3.13 Post Foundation
<br />
<br />Lateral forces from moving water, wind, and
<br />debris are resisted by embedment in the soil, post
<br />size, horizontal cross beams, and by the connec-
<br />tion of the steel I-beams of the manufactured home
<br />chassis to the foundation, Posts are a suitable
<br />elevation method in areas where the soil has the
<br />bearing capacity to resist the manufactured home
<br />foundation loads and where high velocity flood-
<br />water is not anticipated, Recommended post sizes
<br />and depths of embedment for various wind and
<br />roof load zones and heights above grade are
<br />presented in Chapter IV, "Design of Elevated
<br />Foundations. "
<br />Post foundations can be of wood, concrete, or
<br />steel with members set in predug holes or con-
<br />nected to poured concrete piers, Posts can be
<br />round, square, or rectangular and may range from
<br />four to 12 inches in cross section, Rectangular
<br />posts are easier to frame into and are generally
<br />stronger for a given cross sectional area,
<br />Post foundation holes may be excavated by
<br />hand or machine; however, holes deeper than six
<br />feet generally require machine assistance, The
<br />post design, method of installation, and depth to
<br />which posts should be embedded are addressed
<br />in Chapter IV and depend on many conditions in,
<br />eluding: the type of soil; the depth of the frostline;
<br />anticipated iateral loads from floodwater; debris
<br />impact; wir)d forces; anticipated erosion from
<br />water flow; and the size of the posts, Post holes
<br />should be a minimum of eight inches larger in
<br />diameter than the greatest dimension of the post
<br />section, thereby allowing for proper alignment and
<br />backfilling,
<br />
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