Civil Engineering Reference
In-Depth Information
unbalanced loads are required, it is unlikely that minimum loads would
control the design.
Over the years, a fair amount of confusion has arisen concerning the
correct application of the minimum load in Section 7.3.4. In an attempt to
eliminate this confusion, ASCE 7-10 has added the following: “This mini-
mum roof snow load is a separate uniform load case. It need not be used
in determination of or in combination with drifting, sliding, unbalanced or
partial loads.”
One last consideration: for certain low-sloped roofs located in low
ground snow load areas, a 5 lb/ft
2
rain-on-snow surcharge must be added
to the fl at roof snow load. This rain-on-snow surcharge applies in regions
with
p
g
of 20 lb/ft
2
and less, and to wide, low-sloped roofs, specifi cally those
with slopes (in degrees) less than
W
/50. Chapter 10 provides examples and
further discussion about rain-on-snow surcharges.
Example 3-1
Roof Exposure
Problem
A new building is to be sited west of a large clearing in a forest. The mean
roof height is 18 ft. As shown in
Figure G3-8,
the distance from the edge of
the trees to the building's east wall is 80 ft and the east-west building dimen-
sion is 100 ft. Using an Abney level at the eastern edge of the building, the
treetops are located at 24° up from the horizontal. Do these trees serve as an
obstruction that provides shelter with respect to the center of the roof?
Solution
From trigonometry, the tops of the trees are 80 tan 24°
35.6 ft above eye
level, which is assumed as 6 ft. Hence, the tops of the trees are nominally
42 ft above ground level. Since the roof elevation is 18 ft, the elevation dif-
ference from the roof to the top of trees is 24 ft (
h
o
=
=
24 ft).
As per the problem statement, the point of reference is the center of the
roof, which is 130 ft from the edge of the clearing. Since 130 ft is less than
10
h
o
(240 ft), the trees serve as obstructions that provide shelter with respect
to the center of the roof. If all sides of the building are similarly sheltered,
then the building's roof would be categorized as sheltered. If not, the roof
would be categorized as partially exposed.
Figure G3-8
North elevation of pro-
posed building located
in a clearing for Exam-
ple 3-1.
