FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

10 DESIGNING THE FOUNDATION Volume II
Pile foundations with grade beams must be constructed with adequate strength to resist all lateral and
vertical loads. Failures during Hurricane Katrina often resulted from inadequate connections between the
columns and footings or grade beams below (see Figure 10-7).
If grade beams are used with wood piles, the potential for rot must be considered when designing the
connection between the grade beam and the pile. The connection must not encourage water retention. The
maximum bending moment in the piles occurs at the grade beams, and decay caused by water retention at
critical points in the piles could induce failure under high-wind or flood forces.
While offering some advantages, grade beams can become exposed by moving floodwaters if they are not
placed deeply enough. Once exposed, the grade beams create large horizontal obstructions in the flood path
that significantly increase scour. Extensive scour was observed after Hurricane Ike in 2008 around scores of
homes constructed with grade beams (see Figure 10-8).
Although not possible for all piling materials, foundations should be constructed without grade beams
whenever possible. For treated timber piles, this can limit elevations to approximately 8 feet above grade.
The actual limit depends greatly on flood forces, number of piles, availability of piles long enough to be
driven to the required depth and extend above grade enough to adequately elevate the home, and wind
speed and geometry of the elevated structure. For steel and concrete piles, foundations without grade
beams are practical in many instances, even for taller foundations. Without grade beams to account for pile
placement, additional attention is needed for piling alignment, and soils test are needed for design because
pile performance depends on the soils present, and presumptive piling capacities may not adequately predict
pile performance.
Figure 10‑7.
Column connection
failure, Hurricane Katrina
(Belle Fontaine Point,
Jackson County, MS,
2005)
10-24 COASTAL CONSTRUCTION MANUAL