Civil Engineering Reference
In-Depth Information
in Section 7.9. The total sliding load per unit length of eave is 0.4
p
f
W
, where
W
is the horizontal distance from the eave to ridge for the sloped upper roof.
This sliding snow load is distributed uniformly on the lower roof over a dis-
tance of 15 ft starting from, and perpendicular to, the upper roof eave. If the
horizontal measurement of the lower roof from the eave of the upper roof
to the edge of the lower roof is less than 15 ft, the sliding load is reduced
proportionately.
Recognizing that the potential for sliding snow is an increasing function
of roof slope, ASCE 7-10 provides lower bounds where sliding loads do not
need to be considered. For instance, as shown in
Figure G9-2,
sliding only
occurs when the component of the gravity load parallel to the roof surface
(proportional to sin
θ
) is larger than the frictional resistance (proportional to
cos
). These lower limits for sliding snow are ¼ on 12 for slippery roof sur-
faces and 2 on 12 for nonslippery surfaces. These lower limits are approxi-
mately half the slope for some case histories where sliding snow was known
to have occurred; sliding has been recorded on a slippery ½-on-12 roof and
on a nonslippery 4-on-12 roof.
It is reasonable to assume that these limits and the sliding load are related
to the thermal factor,
C
t
, for the upper roof. With all other things being
equal, the potential for snow sliding off a warm roof is greater than for snow
sliding off a cold roof. Similarly, the potential for snow to slide off a south-
facing roof is greater than that for a north-facing roof. Such refi nement of
sliding snow loads requires additional case-history information.
Finally, the sliding snow load is superimposed on the lower roof's bal-
anced load. The sliding snow load may be reduced if a portion of the snow
from the upper roof is blocked by any combination of balanced and/or sliding
snow on the lower roof. As with partial loading and balanced loading below
a drift, the balanced load on the lower roof for the sliding load case is
p
s
,
as given in Equation 7-2. Therefore, the sliding load from the upper roof is
superimposed on 0.7
C
e
C
t
C
s
I
s
p
g
for the lower roof.
θ
Figure G9-2
Onset of sliding on a
sloped roof.
