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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 lateral constraint is not provided at the ground surface, such as by a rigid floor or rigid ground surface pavement, and where lateral constraint is not provided above the ground surface, such as by a structural diaphragm.
(Equation 18-1)
where:
A = 2.34P/(S1b).
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.
= 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.
(Equation 18-2)
or alternatively
(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 be not 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).

Related Code Sections

1807.3 Soils and Foundations, Embedded Posts and Poles
Designs to resist both axial and lateral loads employing posts or poles as columns embedded in earth or in concrete footings in earth shall ...
1807.3 Soils and Foundations, Embedded Posts and Poles
Designs to resist both axial and lateral loads employing posts or poles as columns embedded in earth or in concrete footings ...
1807.3.3 Soils and Foundations, Backfill
The backfill in the annular space around columns not embedded in poured footings shall be by one of the following methods: Backfill shall ...
1807.3.2.1 Soils and Foundations, Nonconstrained
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 ...