Civil Engineering Reference
In-Depth Information
Figure G4-3
Uniform design roof
snow load and
elevation of a gable
roof for Example 4-3.
need to consider minimum roof snow loads. Hence, the uniform design roof
snow load is 16 lb/ft
2
.
Example 4-3
Uniform Roof Snow Load,
Wide Gable Roof
Problem
Determine the design uniform roof snow load for the unheated structure of
ordinary importance shown in
Figure G4-3.
The site is in a suburban area
(Terrain Category B) with a few nearby trees less than 10
h
o
from the struc-
ture that are tall enough to be considered obstructions. The ground snow
load for the area is 30 lb/ft
2
.
Solution
For a partially exposed roof in Terrain Category B,
C
e
is 1.0. Because the
structure is unheated,
C
t
is 1.2 (Table 7-3). The structure is of ordinary impor-
tance (Category II) so
I
s
is 1.0 from Table 1-2. Irrespective of how slippery the
surface for a ½-on-12 (2.39°) slope is,
C
s
is 1.0. Therefore, the sloped roof
snow load is
0.7(1.0)(1.2)(1.0)(1.0)(30 lb/ft
2
)
25.2 lb/ft
2
p
s
=
0.7
C
e
C
t
C
s
I
s
p
g
=
=
(round to 25 lb/ft
2
)
The rain-on-snow surcharge does not need to be considered because the
ground snow load is greater than 20 lb/ft
2
. Also, because the slope is less than
15º the minimum roof snow loading as per Section 7.3.4 needs to be con-
sidered. The ground snow load is greater than 20 lb/ft
2
and the importance
factor is 1.0, so the minimum load for this low-sloped roof is 20
I
s
or 20 lb/ft
2
which does not control. As a result, the design uniform snow load is 25 lb/ft
2
as sketched in
Figure G4-3
.
The unbalanced load for this structure is determined in Example 6-2.
