FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

10 DESIGNING THE FOUNDATION Volume II
to equalize hydrostatic pressures on either side of the wall. See FEMA Technical Bulletin 1, Openings in
Foundation Walls and Walls of Enclosures (FEMA 2008c). However, the flood vents do not significantly
reduce hydrodynamic loads or breaking wave loads, and even with flood vents, flood forces in Coastal A
Zones can damage or destroy these foundation styles.
Both closed/shallow foundations contained in FEMA P-550 are similar to foundations found in prescriptive
codes but contain the additional reinforcement requirement to resist moving floodwaters and short
(approximately 1.5-foot) breaking waves. Figure 10-16 shows the stem wall foundation design in FEMA P-550.
Figure 10‑16.
Stem wall foundation
design
SOURCE: ADAPTED FROM
FEMA P-550, CASE F
10.9 Pier Foundations
Properly designed pier foundations offer the following benefits: (1) their open nature reduces the loads they
must resist from moving floodwaters, (2) taller piers can often be constructed to provide additional protection
without requiring a lot more reinforcement, and (3) the piers can be constructed with reinforced concrete
and masonry materials commonly used in residential construction.
Pier foundations, however, can have drawbacks. If not properly designed and constructed, pier foundations
lack the required strength and stability to resist loads from flood, wind or seismic events. Many pier foundation
failures occurred when Hurricane Katrina struck the Gulf Coast in 2005.
The type of footing used in pier foundations greatly affects the foundation’s performance (see Figure 10-17).
When exposed to lateral loads, discrete footings can rotate so piers placed on discrete footings are suitable
10-36 COASTAL CONSTRUCTION MANUAL