Unbalanced Loads - Snow Loads - page 45

height-to-width ratios (see Figure G7-6 ) for leeward roof step drifts. Both sets
suggest that the angle of repose of drifted snow is about 30º; hence drift loads
are not expected on roof slopes greater than about 30º.
Two unbalanced load distributions are prescribed for hip and gable
roofs. The general distribution applies to all wide-gable roof structures
(eave-to-ridge distance W 20 ft)

with roof framing other than simply supported prismatic members (SSPMs)
spanning from eave to ridge. A simplifi ed distribution applies to narrow
gables ( W

> 20 ft) as well as to narrow gables ( W ≤

20 ft) with SSPMs spanning from eave to ridge—for example,
wood or light gauge roof rafter systems with either a ridge board or a sup-
porting ridge beam.
In the general distribution, the uniform load on the windward side is
0.3 p s , while the leeward side has a rectangular surcharge on top of the bal-
anced load, p s . For a gable roof with a slope of 1 o n S , the inte nsity and hori-
zontal extent of the rectangular surcharge are h d γ

≤

/ S and 8 h d S /3, respec-
tively, where h d is the height of the leeward roof step drift with an upper-level
roof length u = W ,

h = 043 .

3

Wp +− 10 15 .

4

d g

is the unit weight of snow in lb/ft 3 given by Equation 7-3, and γ

30 lb/ft 3 γ= 0.13 p g + 14 ≤

As a fi rst approximation, the gable roof drift shape is a right triangle,
similar to the upper bound surcharge sketched in Figure G6-3 . However,
the right triangle shape requires a vertical downwind face that, in turn,
requires an unrealistic angle of repose of 90º. Hence, realistic gable roof
drifts have a somewhat larger horizontal extent and a somewhat smaller
peak height than the right triangular approximation. In ASCE 7-10, a more
user-friendly rectangular surcharge is specifi ed. The height of the rectan-
gular surcharge is specifi ed to be half the height of the triangle. The hori-
zontal extent of the rectangular surcharge is specifi ed to be a third larger
than the corresponding quantity for the triangle. This increase accounts
for the possibility that the top surface of the triangular drift may slope
downward towards the eaves (thereby increasing the horizontal extent of
the triangle) as well as the aforementioned angle of repose issue for the
downwind face.
For smaller ( W 20 ft) gable roof buildings with certain structural sys-

tems, a simpler unbalanced load distribution is specifi ed. The windward side
is taken as free of snow, while the leeward side has a uniform load of I s p g .
The expected triangular surcharge located close to the ridge is replaced with
a uniform load from eave to ridge. For the specifi ed structural systems—
SSPMs spanning from ridge to eave—the maximum moment and shear gen-
erated by the specifi ed uniform loading are larger than those generated by the
triangular surcharge. The requirement that the members be prismatic is due
to the fact that the locations of maximum moment and shear for the uniform
load are different than those for the triangular surcharge.

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