FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

Volume II DESIGNING THE FOUNDATION 10
COASTAL CONSTRUCTION MANUAL
Figure 10‑17.
Performance comparison
of pier foundations: piers
on discrete footings
(foreground) failed by
rotating and overturning
while piers on more
substantial footings (in
this case a concrete
mat) survived Hurricane
Katrina (Pass Christian,
MS, 2005)
only when wind and flood loads are relatively low. Piers placed on continuous concrete grade beams or
concrete footings provide much greater resistance to lateral loads and are much less prone to failure. Footings
and grade beams must be reinforced to resist the moment forces that develop at the base of the piers from the
lateral loads on the foundation and the elevated home.
Like other open/shallow foundations, pier foundations are appropriate only where there is limited potential
for erosion or scour. The maximum estimated depth for long- and short-term erosion and localized scour
should not extend below the bottom of the footing or grade beam. In addition, adequate resistance to lateral
loads is often difficult to achieve for common pier sizes on continuous footings. Even for relatively small
lateral loads, larger piers designed as shear walls are often necessary to provide adequate resistance.
The following section provides an analysis of a pier foundation on discrete concrete footings. The analysis
shows that discrete pier footings that must resist lateral loads are typically not practical.
10.9.1 Pier Foundation Design Examples
The following three examples discuss pier foundation design. Example 10.3 provides an analysis of the
pier footing under gravity loads only (see Figure 10-18) and the footing size required to ensure that the
allowable soil bearing pressure is not exceeded. Example 10.4 provides a consideration of uplift forces that
many footings (see Figure 10-19) must resist to prevent failure during a design wind event. The analysis
in Example 10.4 assumes that other foundation elements are in place to resist the lateral loads that must
accompany uplift forces. Example 10.5 adds lateral loads to the pier and footing (see Figure 10-20) to model
buildings that lack continuous foundation walls or other lateral load resisting features. The lateral loads can
result from wind, seismic or moving floodwaters.
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