FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

Volume II DETERMINING SITE-SPECIFIC LOADS 8
EQUATION 8.8. HYDRODYNAMIC LOAD (FOR ALL FLOW VELOCITIES)
Flow around a building or building element also
creates flow-perpendicular forces (lift forces). When
a building element is rigid, lift forces can be assumed
to be small. When the element is not rigid, lift
forces can be greater than drag forces. The equation
for lift force is the same as that for hydrodynamic
force except that the drag coefficient (C d ) is replaced
with the lift coefficient (C l ). In this Manual, the
foundations of coastal residential buildings are
considered rigid, and hydrodynamic lift forces can
therefore be ignored.
Equation 8.8 provides the total force against a
building of a given surface area, A. Dividing the
total force by either length or width yields a force
per linear unit; dividing by surface area, A, yields
a force per unit area. Example 8.3 shows the
difference between the loads imposed on a vertical
pile by nonbreaking and breaking waves. As noted
in Section 8.5.8, nonbreaking wave forces on piles
can be calculated as hydrodynamic forces.
COASTAL CONSTRUCTION MANUAL
(Eq. 8.8)
where:
Fdyn = horizontal drag force (lb) acting at the stillwater mid-depth (half way between the
stillwater elevation and the eroded ground surface)
Cd = drag coefficient (recommended values are 2.0 for square or rectangular piles and 1.2
for round piles; for other obstructions, see Table 8-2)
= mass density of fluid (1.94 slugs/ft3 for fresh water and 1.99 slugs/ft3 for saltwater)
V = velocity of water (ft/sec); see Equation 8.2
A = surface area of obstruction normal to flow (ft2) = (w)(ds )(see Figure 8-13) or (w)(h) if
the object is completely immersed
Table 8-2. Drag Coefficients for Ratios of Width to
Depth (w/d s) and Width to Height (w/h)
Width-to-Depth Ratio
(w/d s or w/h)
Drag Coefficient
(C d )
1–12 1.25
13–20 1.3
21–32 1.4
33–40 1.5
41–80 1.75
81–120 1.8
>120 2.0
NOTE
Lift coefficients (C l ) are provided
in Introduction to Fluid Mechanics
(Fox and McDonald 1985) and
in many other fluid mechanics
textbooks.
8-29