Civil Engineering Reference
In-Depth Information
Figure G3-7
Variation of wind speed
with elevation for three
terrain categories and
constant geostropic
speed.
LIVE GRAPH
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a specifi c terrain category, the wind experienced at the roof level decreases
as the exposure changes from fully exposed to sheltered, and the tabulated
values for
C
e
increase correspondingly.
The exposure factors for design purposes range from 0.7 to 1.2, as
shown in Table 7-2. This range is broader than the 0.9 to 1.3 range for the
“normalized” conversion factors from
Figure G3-5.
Note, however, that the
C
e
=
0.7 value in Table 7-2 is for fully exposed structures at treeless or “above
the tree line in windswept mountainous” terrain. Such terrain was not rep-
resented in the CRREL study. Hence, one could argue that Table 7-2 is rea-
sonably consistent with the conversion factor measurements determined in
the CRREL study.
Footnotes at the bottom of Table 7-2 provide defi nitions of the various
roof exposures used in ASCE 7-10. At one extreme is “sheltered,” which cor-
responds to roofs tight against conifers that qualify as obstructions. The other
extreme is “fully exposed,” which corresponds to a roof with no obstruc-
tions, including large rooftop equipment and tall parapet walls (parapets that
extend above the height of the balanced snow). The middle class, “partially
exposed,” is arguably the most common roof exposure. It corresponds to all
roofs that are not sheltered or fully exposed. It should be noted that two roofs
on the same structure could have different roof exposures. A classic example
is a two-level roof in which the upper level roof is fully exposed while the
lower level is partially exposed due to the obstruction provided by the upper
level roof.
The footnotes for Table 7-2 also provide a defi nition for obstructions
that provide shelter. Specifi cally, if the top of an object—such as an adjacent
building or a group of trees—is
h
o
above the elevation of the roof in ques-
tion, then the object must be located within a distance 10
h
o
of the roof in
question for the object to be considered an obstruction. In relation to these
sorts of sheltering effects, Sachs (1972) presents useful data. At the transi-
