Designs to resist both axial and lateral loads employing
posts or poles as columns embedded in earth or in concrete footings in earth
shall be in accordance with Sections 1807.3.1 through 1807.3.3.
1807.3.1
Limitations. The design
procedures outlined in this section are subject to the following limitations:
1. The frictional resistance for structural
walls and slabs on silts and clays shall be limited to one-half of the normal
force imposed on the soil by the weight of the footing or slab.
2. Posts embedded in earth shall not be used
to provide lateral support for structural or nonstructural materials such as
plaster, masonry or concrete unless bracing is provided that develops the
limited deflection required.
Wood poles shall be treated in accordance with AWPA U1 for
sawn timber posts (Commodity Specification A, Use Category 4B) and for round
timber posts (Commodity Specification B, Use Category 4B).
1807.3.2
Design criteria.
The depth to resist lateral loads shall be determined using the design
criteria established in Sections 1807.3.2.1 through 1807.3.2.3, or by other
methods
approved by the
building official.
1807.3.2.1
Nonconstrained. The
following formula shall be used in determining the depth of embedment required
to resist lateral loads where no lateral constraint is provided at the ground
surface, such as by a rigid floor or rigid ground surface pavement, and where
no lateral constraint is provided above the ground surface, such as by a
structural diaphragm.
|
d = 0.5A{1 + [1 +
(4.36h/A)]1/2} |
(Equation 18-1) |
where:
A =
2.34P/S1
b.
b = Diameter of round post or footing or
diagonal dimension of square post or footing, feet (m).
d = Depth of embedment in earth in feet (m)
but not over 12 feet (3658 mm) for purpose of computing lateral
pressure.
h = Distance in feet (m) from ground surface
to point of application of "P."
P = Applied lateral force in pounds
(kN).
S1 = Allowable
lateral soil-bearing pressure as set forth in Section 1806.2 based on a depth
of one-third the depth of embedment in pounds per square foot (psf)
(kPa).
1807.3.2.2
Constrained. The following formula shall be used to determine the depth
of embedment required to resist lateral loads where lateral constraint is
provided at the ground surface, such as by a rigid floor or pavement.
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image |
(Equation 18-2) |
or alternatively
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image |
(Equation 18-3) |
where:
Mg = Moment in the
post at grade, in foot-pounds (kN-m).
S3 = Allowable
lateral soil-bearing pressure as set forth in Section 1806.2 based on a depth
equal to the depth of embedment in pounds per square foot (kPa).
1807.3.2.3
Vertical load. The
resistance to vertical loads shall be determined using the vertical foundation
pressure set forth in Table 1806.2.
1807.3.3
Backfill. The backfill
in the annular space around columns not embedded in poured footings shall be by
one of the following methods:
1. Backfill
shall be of concrete with a specified compressive strength of not less than
2,000 psi (13.8 MPa). The hole shall not be less than 4 inches (102 mm) larger
than the diameter of the column at its bottom or 4 inches (102 mm) larger than
the diagonal dimension of a square or rectangular column.
2. Backfill shall be of clean sand. The sand
shall be thoroughly compacted by tamping in layers not more than 8 inches (203
mm) in depth.
3. Backfill shall be
of controlled low-strength material (CLSM).
