FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

Volume II DESIGNING THE FOUNDATION 10
COASTAL CONSTRUCTION MANUAL
Figure 10‑20.
Pier foundation and
spread footing exposed to
uplift and lateral forces
Several equations exist for designing discrete footings exposed to gravity loads only. Equation 10.5, which
models the weight of the footing by reducing the allowable bearing capacity of the soils by the weight of the
footing, is used for Example 10-3.
Equation 10.5 considers the weight of the pier and footing, the gravity load imposed on the top of the
pier, and the allowable soil bearing capacity of the soils to determine footing dimensions. The equation
provides the length
(L) of a square footing. The equation can be modified for rectangular footings of a given
aspect ratio (ratio of width to length) and including in the denominator of the term to the right of the
equals sign.
Equation 10.5 assumes that the gravity load is equally distributed across the bottom surface of the footing
and the soil stresses are constant. This condition is appropriate when the gravity loads are applied at the
center of the pier (and the pier is centered on the footing) and when no lateral loads are applied.
The foundation system must have sufficient weight to prevent failure when uplift loads are applied. ASCE 7-10
requires the designer to consider only 60 percent of the dead load when designing for uplift (ASD load
combination #7). If the foundation is located in an SFHA, portions of it will be located below the stillwater
elevation and will be submerged during a design event. The dead load of a material is less when submerged
so the submerged weight must be considered (see Section 8.5.7). In Example 10.4, it is assumed that the
stillwater depth at the site is 2 feet.
10-39