Civil Engineering Reference
In-Depth Information
3.3
Exposure Factor (
C
e
)
The ASCE 7-10 exposure factors are shown in Table 7-2. They are a function
of the terrain or surface roughness category (ranging from urban areas to
shoreline) and the location of the structure with respect to various obstruc-
tions (ranging from fully exposed to sheltered) within the given terrain cat-
egory. The surface roughness or terrain categories are defi ned in Chapter 26
(Wind Loads) of ASCE 7-10. They are intended to capture the overall windi-
ness of the area surrounding the site and defi ne the variation of wind veloc-
ity with height within the atmospheric boundary layer. The variation of the
wind velocity with height is due to the friction at the earth's surface caused
by obstructions such as buildings and trees (the higher the obstruction, the
larger the friction). The wind slows as it gets closer to the ground and is often
modeled by the Power Law:
α
Vz
G
(
)
⎛
⎜
z
⎞
⎟
Equation G3-1
=
δ
where
V
(
z
)
=
wind velocity at elevation
z
G
=
geostropic wind speed at the top of the atmospheric boundary
layer
δ=
thickness of the atmospheric boundary layer
α=
Power Law coeffi cient
Values for
as suggested by Davenport (1965) are shown in
Table G3-3.
The variation of wind speed with height for Davenport's three terrain classes
is shown for a constant geostropic speed in
Figure G3-7.
As one might expect,
the thickest boundary layer is associated with the terrain that has the most
surface friction. Also, for the same geostropic speed, the wind velocity at a
given roof elevation would be largest for open terrain and smallest for city
centers. This is why the seashore is a great place to fl y a kite.
The decreased values for
C
e
in Table 7-2 as one moves in order from
Terrain Category B (city center) to Terrain Category D (shoreline) refl ect the
tendency for more snow to be blown off roofs with more roof level wind.
The same trend applies in terms of the local exposure classifi cation. Within
δ
and
α
Table G3-3
Power Law Coei cients
α
Power Law
Coei cient
δ
Boundary Layer
Thickness (ft)
Terrain
Open terrain
0.16
0,
900
Suburban terrain
0.28
1,300
Centers of large cities
0.40
1,700
Source: Davenport 1965.
