Civil Engineering Reference
In-Depth Information
Balanced Load (Lower Roof Level)
The site is considered Terrain Category C. The lower roof, however, is shel-
tered by the presence of the upper roof. Therefore, the lower roof is classifi ed
as partially exposed, and
C
e
is 1.0 from Table 7-2. For an unheated space,
C
t
is 1.2 from Table 7-3. Although this is a storage space, it is not considered
prudent to classify this building as a “minor storage facility,” as described in
Category I in Table 1-1, because of its large footprint. Therefore, the build-
ing structure is classifi ed as Category II, and
I
s
is 1.0 per Table 1-2. Hence,
the balanced load on the lower level roof becomes
0.7(1.0)(1.2)(1.0)(1.0)(40 lb/ft
2
)
34 lb/ft
2
p
s
=
0.7
C
e
C
t
C
s
I
s
p
g
=
=
Drift Loads
The snow density is determined from
p
g
using Equation 7-3 below:
γ =
0.13
p
g
+
14
=
0.13(40 lb/ft
2
)
+
14
=
19 lb/ft
2
The balanced snow depth on the lower level roof is
34 lb/ft
2
19 lb/ft
3
p
s
h
b
=
=
=
1.8 ft
Therefore, the clear height above the balanced snow is
h
c
=
10
−
h
b
=
10
−
1.8
=
8.2 ft
By inspection,
h
c
/h
b
0.2; therefore, enough space is available for drift for-
mation, and drift loads must be evaluated.
>
Leeward Drift
For a wind out of the north, the upwind fetch for the resulting leeward drift is
the length of the upper roof (C
u
=
100 ft). Hence, the surcharge drift height is
0.43(100 ft)
1/3
(40 lb/ft
2
10)
1/4
h
d
=
0.43
3
p
g
+
10
1.5
=
+
1.5
=
3.8 ft
4
u
Windward Drift
For a wind out of the south, the upwind fetch for the resulting windward
drift is 170 ft. Hence, the surcharge drift height is
h
d
=
0.75[0.43
3
p
g
+
10
1.5]
4
u
0.75[0.43(170)
1/3
(40 lb/ft
2
10)
1/4
=
+
1.5]
=
3.6 ft
Thus, the leeward drift controls, and
h
d
is 3.8 ft. Since the drift is not full
(
h
c
>
h
d
), the drift width is four times the drift height:
w
=
4
h
d
=
4(3.8 ft)
=
15 ft
