FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

Volume II DESIGNING THE FOUNDATION 10
Although connections play an integral role in the design of structures, they are typically regarded as the
weakest link. Guidance for typical wood-pile to wood-girder connections can be found in Fact Sheet 3.3,
Wood Pile to Beam Connections, in FEMA P-499.
10.6.1.2 Pile Bracing
When timber piles with a sufficiently large cross section are not available, timber piles may require bracing
to resist lateral loads. Bracing increases the lateral stiffness of a pile foundation system so that less sway is felt
under normal service loads. Bracing also lowers the location where lateral forces are applied to individual
piles and reduces bending stresses in the pile. When bracing is used, the forces from moving floodwaters and
from flood-borne debris that impacts the braces should be considered.
Diagonal Bracing
COASTAL CONSTRUCTION MANUAL
NOTE
Fact Sheet 3.2, Pile Installation, in FEMA
P-499 recommends that pile bracing be
used only for reducing the structure’s
sway and vibration for comfort. In other
words, bracing should be used to address
serviceability issues and not strength is sues.
The foundation design should consider the
piles as being unbraced as the condition that
may occur when floating debris removes or
damages the bracing. If the pile foundation
is not able to provide the desired strength
performance without bracing, the designer
should consider increasing the pile size.
Bracing is typically provided by diagonal bracing
or knee bracing. Diagonal bracing is more effective
from a structural standpoint, but because diagonal
bracing extends lower into floodwaters, it is more
likely to be damaged by flood-borne debris. It
can also trap flood-borne debris, and trapped
flood-borne debris increases flood forces on the
foundation.
Knee bracing does not extend as deeply into
floodwaters as cross bracing and is less likely to be
affected by flood-borne debris but is less effective
at reducing stresses in the pile and also typically
requires much stronger connections to achieve
similar structural performance as full-length cross
bracing.
Diagonal bracing often consists of dimensional lumber that is nailed or bolted to the wood piles. Steel
rod bracing and wire rope (cable) bracing can also be used. Steel rod bracing and cable bracing have the
benefit of being able to use tensioning devices, such as turnbuckles, which allow the tension of the bracing
to be maintained. Cable bracing has an additional benefit in that the cables can be wrapped around pilings
without having to rely on bolted connections, and wrapped connections can transfer greater loads than
bolted connections. Figure 10-10 shows an example of diagonal bracing using dimensional lumber.
Diagonal braces tend to be slender, and slender braces are vulnerable to compression buckling. Most bracing
is therefore considered tension-only bracing. Because wind and flood loads can act in opposite directions,
tension-only bracing must be installed in pairs. One set of braces resists loads from one direction, and the
second set resists loads from the opposite direction. Figure 10-11 shows how tension-only bracing pairs resist
lateral loads on a home.
The placement of the lower bolted connection of the diagonal brace to the pile requires some judgment. If
the connection is too far above grade, the pile length below the connection is not braced and the overall
foundation system is less strong and stiff.
10-27