FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

Volume II DETERMINING SITE-SPECIFIC LOADS 8
Table 8-3. Depth Coefficient (C D ) by Flood Hazard Zone and Water Depth
Flood Hazard Zone and Water Depth C D
Floodway (a) or Zone V 1.0
Zone A, stillwater flood depth > 5 ft 1.0
Zone A, stillwater flood depth = 4 ft 0.75
Zone A, stillwater flood depth = 2.5 ft 0.375
Zone A, stillwater flood depth < 1 ft 0.00
(a) Per ASCE 24-05, a “floodway” is a “channel and that portion of the floodplain reserved to convey the
base flood without cumulatively increasing the water surface elevation more than a designated height.”
Blockage coefficient. The blockage coefficient (C B ) is used to account for the reduction in debris
velocity expected to occur because of the screening provided by trees and other structures upstream
from the structure or building on which the impact load is being calculated. The blockage coefficient
varies, as shown in Table 8-4.
Table 8-4. Values of Blockage Coefficient C B
Degree of Screening or Sheltering within 100 Ft Upstream CB No upstream screening, flow path wider than 30 ft 1.0
Limited upstream screening, flow path 20-ft wide 0.6
Moderate upstream screening, flow path 10-ft wide 0.2
Dense upstream screening, flow path less than 5-ft wide 0.0
Building structure coefficient. The building structure coefficient, C str , is derived from Equation C5-3,
Chapter C5, ASCE 7-10. Coefficient values for C str , (0.2, 0.4, and 0.8 as defined above for Equation 8.9)
were generated by selecting input values recommended in ASCE 7-10, Chapter C5, with appropriate
assumptions made to model typical coastal residential structures. The derived building structure
coefficient formula with inputs is defined as follows:
where:
CI = importance coefficient = 1.0
CO = orientation coefficient = 0.80
t = duration of impact = 0.03 sec
g = gravitational constant (32.2 ft/sec2) Rmax = maximum response ratio assuming approximate natural period, T, of building types as
follows: for timber pile and masonry column, T = 0.75 sec; for concrete pile or concrete or
steel moment resisting frames, T = 0.35 sec; and for reinforced concrete foundation walls,
T = 0.2 sec. The ratio of impact duration (0.03 sec) to approximate natural period (T) is
entered into Table C5-4 of ASCE 7-10 to yield the Rmax value.
COASTAL CONSTRUCTION MANUAL
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