FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

8 DETERMINING SITE-SPECIFIC LOADS Volume II
8.5.9 Hydrodynamic Loads
As shown in Figure 8-13, water flowing around a building (or a structural element or other object) imposes
loads on the building. In the figure, note that the lowest floor of the building is above the flood level and the
loads imposed by flowing water affect only the foundation walls. However, open foundation systems, unlike
that shown in Figure 8-13, can greatly reduce hydrodynamic loading. Hydrodynamic loads, which are a
function of flow velocity and structural geometry, include frontal impact on the upstream face, drag along
the sides, and suction on the downstream side. One of the most difficult steps in quantifying loads imposed
by moving water is determining the expected flood velocity (see Section 8.5.6 for guidance on design flood
velocities). In this Manual, the velocity of floodwater is assumed to be constant (i.e., steady-state flow).
Hydrodynamic loads can be calculated using Equation 8.8.
Elevating above the DFE provides additional protection from hydrodynamic loads for elevated enclosed areas.
The drag coefficient used in Equation 8.8 is taken from the Shore Protection Manual, Volume 2 (USACE
1984). Additional guidance is provided in Section 5.4.3 of ASCE 7-10 and in FEMA 259, Engineering
Principles and Practices for Retrofitting Floodprone Residential Buildings (FEMA 2001). The drag coefficient
is a function of the shape of the object around which flow is directed. When an object is something other
than a round, square, or rectangular pile, the coefficient is determined by one of the following ratios (see
Table 8-2):
1. The ratio of the width of the object (w) to the height of the object (h) if the object is completely
immersed in water
2. The ratio of the width of the object (w) to the stillwater flood depth of the water (d s ) if the object is not
fully immersed
Figure 8-13.
Hydrodynamic loads on a
building
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