FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

8 DETERMINING SITE-SPECIFIC LOADS Volume II
The guidance in ASCE 7-10, Sections 2.3 and 2.4 (Strength Design and Allowable Stress Design,
respectively) also indicates which load combinations the flood load should be applied to. In the portion of
Zone A landward of the LiMWA, the flood load F a could either be hydrostatic or hydrodynamic loads. Both
of these loads could be lateral loads; only hydrostatic will be a vertical load (buoyancy). When designing for
global forces that will create overturning, sliding or uplift reactions, the designer should use F a as the flood
load that creates the most restrictive condition. In sliding and overturning, F a should be determined by the
type of expected flooding. Hydrostatic forces govern if the flooding is primarily standing water possibly
saturating the ground surrounding a foundation; hydrodynamic forces govern if the flooding is primarily
from moving water.
When designing a building element such as a foundation, the designer should use F a as the greatest of the
flood forces that affect that element (F sta or F dyn ) + F i (impact loads on that element acting at the stillwater
level). The combination of these loads must be used to develop the required resistance that must be provided
by the building element.
The designer should assume that breaking waves will affect foundation elements in both Zone V and Zone
A. In determining total flood forces acting on the foundation at any given point during a flood event, it is
generally unrealistic to assume that impact loads occur on all piles at the same time as breaking wave loads.
Therefore, it is recommended that the load be calculated as a single wave impact load acting in combination
with other sources of flood loads.
For the design of foundations in Zone V or Coastal A Zone, load combination cases considered should
include breaking wave loads alone, hydrodynamic loads alone, and the greater of hydrodynamic loads and
breaking wave loads acting in combination with debris impact loads. The value of flood load, F a, used in
ASCE 7-10 load combinations, should be based on the greater of F brk or F dyn , as applicable for global forces
(see Table 8-5) or F i + (F brk or F dyn ), as applicable for an individual building element such as a pile.
Example 8-4 is a summary of the information regarding flood loads and the effects of flooding on an
example building.
EXAMPLE 8.4. FLOOD LOAD EXAMPLE PROBLEM
Given:
• Oceanfront building site on landward side of a primary frontal dune (see Example 8.1,
Illustration A)
• Topography along transect perpendicular to shoreline is shown in Example 8.1, Illustration B;
existing ground elevation at seaward row of pilings = 7.0 ft NGVD
• Soil is dense sand; no terminating stratum above –25 ft NGVD
• Data from FIRM are as follows: flood hazard zone at site is Zone VE, BFE = 14.0 ft NGVD
• Data from FIS are as follows: 100-year stillwater elevation = 10.1 ft NGVD, 10-year stillwater
elevation = 5.0 ft NGVD
8-38 COASTAL CONSTRUCTION MANUAL