Civil Engineering Reference
In-Depth Information
slope of ¼ on 12,
C
s
is 1.0 irrespective of the roof's surface or thermal char-
acteristics. Hence, the balanced load is
0.7(1.0)(1.0)(1.0)(1.0)(30 lb/ft
2
)
21 lb/ft
2
p
s
=
0.7
C
e
C
t
C
s
I
s
p
g
=
=
Drift Load
The height of the parapet wall at the southeast and southwest corners is
h
=
250 ft (¼ in./ft)
=
62.5 in.
=
5.2 ft
0.13(30 lb/ft
2
)
18 lb/ft
3
(Equa-
The snow density is
γ=
0.13
p
g
+
14
=
+
14
=
tion 7-3), and the depth of the balanced snow is
21 lb/ft
2
18 lb/ft
3
p
s
h
b
=
=
=
1.17 ft
The space available for drift formation (the clear height above the balanced
snow,
h
c
, is 5.2 ft − 1.17 ft
=
4 ft) is large compared to the balanced snow
depth (
h
c
/
h
b
>
0.2). Therefore, the parapet wall drift must be considered. For
an upwind fetch of 220 ft and a ground snow load of 30 lb/ft
2
,
(
)
(
)
h
d
=
0.75 (0.43)
3
220
30
+
10
1.5
=
3.8 ft
4
The drift height is not limited by the space available for drift formation
because the drift height is less than
h
c
. The maximum drift surcharge load at
the parapet wall is
18 lb/ft
3
(3.8 ft)
68 lb/ft
2
p
d
=γ
h
d
=
=
The total maximum roof snow load (balanced plus drift) is
21 lb/ft
2
68 lb/ft
2
89 lb/ft
2
+
=
and the horizontal extent is
w
=
4
h
d
=
4(3.8 ft)
=
15.2 ft
The resulting parapet wall drift load at the southwest corner is shown in
Figure
G8-3
. The design drift at the southeast corner is similar. Since both drifts have
the same snow source area, it is unlikely that both design drifts would occur
simultaneously. The issue of the possible simultaneous occurrence of drifts on
all four sides of an RTU is discussed in Chapter 13 of this guide.
The ground snow load in this case is large enough that the minimum snow
load for low sloped roofs (
p
m
=
20 lb/ft
2
) is less than the balanced
load and thus does not govern. Similarly, the ground snow load is large enough
that the rain-on-snow surcharge does not apply (see Section 7.10).
I
s
(20 lb/ft
2
)
=
