FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

Volume II DESIGNING THE FOUNDATION 10
EXAMPLE 10.5. PIER FOOTING UNDER UPLIFT AND LATERAL LOADS (concluded)
EQUATION A
where:
(see Figure 10-20)
e = eccentricity
Pt = total vertical load for the load combination being analyzed
M = applied moment Pl (H + x) (ft-lbs) where x and H are as defined
previously
P l is the lateral load applied at the top of the column. For equilibrium, R must be applied within the
“kern” of the footing (for a square footing, the kern is a square with dimension of L/3 centered about
the centroid of the footing). Mathematically, e cannot exceed L/6. Ensuring that the reaction R is
applied within the kern of the footing prevents tensile stresses from forming on the edge of the footing.
Calculating the minimum soils stress for various footing widths (using a recursive solution) shows
that the footing would need to be 11 ft 4 in. wide to prevent overturning. Increasing the footing
thickness to 2 feet would allow the footing size to be reduced to approximately 8 ft 9 in. Either design
is not practical to construct.
10.9.2 Pier Foundation Summary
These analyses indicate that piers with discrete footings are practical to construct when they are required to
resist gravity loads only but are not practical when they must resist uplift forces or lateral loads. Although
prescriptive designs for pier foundations are available in some codes and standards, users of the codes and
standards should ensure that the designs take into account all of the loads the foundations must resist.
Prescriptive designs should only be used to resist lateral and uplift loads after they have been confirmed to
be adequate.
Constructing piers on continuous footings makes pier foundations much more resistant to coastal hazards,
but prescriptive designs for piers on continuous footings are not present in widely adopted codes such as
the IRC and IBC. Until prescriptive designs using piers are developed, these styles of foundations should
be engineered. Continuous footings are discussed in Section 11.1.5 of FEMA 549, Hurricane Katrina in
the Gulf Coast (FEMA 2006), and continuous footing designs that can be used for the basis of engineered
foundations are contained in FEMA P-550.
COASTAL CONSTRUCTION MANUAL
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