Civil Engineering Reference
In-Depth Information
Figure G8-1
Comparison of
observed windward
surcharge drift height
with values predicted
by ASCE 7.
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right-hand side. In another case history, the observed surcharge height was
generally characterized as ranging from 2.5 to 3.5 ft.
For three of the case histories, the provisions overpredict the observed
values, and for two others, the provisions underpredict the observed values.
The ratios of observed to predicted range from 0.53 to 1.23, with a mean
of 0.84. For the windward drifts considered, the overload for the
surcharge
was no more than 23%. As noted by O'Rourke and DeAngelis, the over-
load for the total snow load (balanced plus surcharge) is less, and it is
unlikely that a snow overload of 23% would result in signifi cant structural
performance problems given the safety factors commonly used in building
design.
As with drifts on lower roofs discussed in Chapter 7 of this guide, the
sloped roof snow load in Equation 7-2 of ASCE 7 is the balanced load below
the roof projection drift load. The sloped roof snow load,
p
s
,
0.7
C
e
C
t
I
s
C
s
p
g
,
where
p
g
is the 50-year Mean Recurrence Interval (MRI) ground snow load.
Minimum roof snow loading and rain-on-snow surcharge loads do not infl u-
ence this balanced load. In addition, if the cross-wind length of the roof pro-
jection is small (that is, the plan dimension perpendicular to the direction of
wind under consideration is less than 15 ft) then the drift load does not need
to be considered for that wind direction. Drifts will form at such roof projec-
tions, but the cross-wind plan dimension of the drift and the total drift load
(in lbs) is relatively small and can be neglected without affecting the overall
