FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

8 DETERMINING SITE-SPECIFIC LOADS Volume II
as “the average of the roof eave height and the height to the
highest point on the roof surface …” Mean roof height is not
the same as building height, which is the distance from the
ground to the highest point.
For calculating both MWFRS and C&C pressures, velocity
pressures (q) should be calculated in accordance with Equation
8.13. Velocity pressure varies depending on many factors
including mapped wind speed at the site, height of the
structure, local topographic effects, and surrounding terrain
that affects the exposure coefficient.
EQUATION 8.13. VELOCITY PRESSURE
NOTE
ASCE 7-10 Commentary states
that where a single component,
such as a roof truss, comprises
an assemblage of structural
elements, the elements of
that component should be
analyzed for loads based on
C&C coefficients, and the single
component should be analyzed
for loads as part of the MWFRS.
where:
qz = velocity pressure evaluated at height z (psf)
Kz = velocity pressure exposure coefficient evaluated at height z
Kzt = topographic factor
Kd = wind directionality factor
V = basic wind speed (mph) (3-sec gust speed at 33 ft above ground in Exposure
Category C)
EQUATION 8.14. DESIGN WIND PRESSURE FOR LOW-RISE BUILDINGS
(Eq. 8.13)
The design wind pressure is calculated from the combination of external and internal pressures acting on
a building element. This combination of pressures for both MWFRS and C&C loads in accordance with
provisions of ASCE 7-10 is represented by Equation 8.14.
(Eq. 8.14)
where:
p = design wind pressure
qh = velocity pressure evaluated at mean roof height (h) (see Figure 8-18 for an illustration
of mean roof height)
GCpf = external pressure coefficient for C&C loads or MWFRS loads per the low-rise
building provisions, as applicable
GCpi = internal pressure coefficients based on exposure classification as applicable; GCpi for
enclosed buildings is +/- 0.18
8-50 COASTAL CONSTRUCTION MANUAL