Civil Engineering Reference
In-Depth Information
Drift Loads
The balanced snow depth on the lower roof is determined from
p
s
and the
snow density (Equation 7-3):
p
s
13
0.13(15)
h
b
=
=
14
=
0.82 ft
+
The surcharge height for the leeward drift (wind from the north) is
0.43(100 ft)
1/3
(1 5 lb /ft
2
10)
1/4
h
d
=
0.43
p
g
+
10
1.5
=
+
1.5
3
4
u
=
2.96 ft
and the corresponding value for the windward drift (wind from the south) is
h
d
=
0.75[0.43
p
g
+
10
1.5]
3
4
u
0.75[0.43(170 ft)
1/3
(15 lb/ft
2
10)
1/4
=
+
1.5]
=
2.87 ft
As with Example 7-1, the leeward drift height is larger. Since the leeward
drift height is less than the clear height (
h
d
=
2.96 ft
<
h
c
=
10.0 ft
−
0.82 ft
=
9.18 ft), the width is equal to four times the surcharge height:
w
=
4
h
d
=
4(2.96 ft)
=
11.8 ft
and the drift surcharge load is
15 lb/ft
2
)
14 lb/ft
3
]
47.2 lb/ft
2
p
d
=
h
d
γ=
(2.96 ft)[(0.13
×
+
=
20 lb/ft
2
), mini-
mum roof snow loads (Section 7.3) need to be considered. Similarly, because
the ground snow load is small (
p
g
Because the ground snow load is comparatively small (
p
g
≤
20 lb/ft
2
), the roof slope is small (¼ on
12), and the eave-to-ridge distance is large (
W
≤
80 ft), the rain-on-snow sur-
charge must also be considered (see
Table G10-4
). Notice that the rain-on-
snow augmented design load and the minimum roof snow load correspond
to separate uniform load cases and need not be used in combination with
drift, sliding, unbalanced, or partial loads.
The minimum roof snow load for both the upper and lower roofs is
I
s
p
g
or 15 lb/ft
2
. The rain-on-snow surcharge of 5 lb/ft
2
is added to the sloped roof
snow loads. For the upper roof, the sloped roof snow load is 10 lb/ft
2
and the
rain-on-snow surcharge is 5 lb/ft
2
, for a total of 15 lb/ft
2
. For the lower roof,
the sloped roof snow load is 13 lb/ft
2
plus 5 lb/ft
2
rain-on-snow, for a total
of 18 lb/ft
2
. For both roof levels, the sum of the rain-on-snow surcharge and
the sloped roof snow load is greater than or equal to the minimum roof snow
load. Therefore, the rain-on-snow augmented load governs.
It is unclear which load case governs for the lower roof, so both the
uniform load case and the balanced-plus-drift load case require evaluation.
The resulting design load cases are shown in
Figure G7-13.
=
