Civil Engineering Reference
In-Depth Information
Chapter 6
Unbalanced
Loads
Unlike the partial loads discussed in Chapter 5 of this guide, the unbalanced
loads in Section 7.6 of ASCE 7-10 are in most cases actually drift loads.
Except for sawtooth-type roofs, the windward portion of the roof is the
snow source and the leeward portion accumulates a percentage of the drifted
snow. This is termed an
unbalanced
condition because the leeward portion
has more snow than the windward portion.
The unbalanced snow loading on hip and gable roofs (Section 7.6.1),
curved roofs (Section 7.6.2), sawtooth roofs (Section 7.6.3), and dome roofs
(Section 7.6.4) are discussed in the following sections.
6.1
Hip and Gable Roofs
For hip and gable roofs, the unbalanced loading condition results from
a drifting process in which wind removes snow from the windward side
of the ridge line. Some of this snow fl ux from the source area eventually
settles in the aerodynamic shade region on the leeward side, downwind of
the ridge.
Figure G6-1
is a general sketch of this process for wind blowing
from left to right, while
Figure G6-2
shows a gable roof drift for wind from
right to left.
Model studies in a water fl ume suggest that the drift surcharge is more
or less triangular in shape with a nominally fl at top surface as shown in
Figure
G6-1
(Kuskowski 2003). Given suffi cient wind and roof snow originally on
the windward side of the ridge, the drift will grow until the leeward-side
aerodynamic shade area is fi lled with drifted snow. This full-drift condition
is sketched in
Figure G6-3
for wind from left to right;
Figure G6-4
shows
a full drift on a small arch for wind from right to left. As a matter of fact,
Commentary Section C7.6.1 identifi es the loading sketched in
Figure G6-3
as an upper bound unbalanced load for gable roofs.
Water fl ume studies by O'Rourke et al. (2005) indicate that the trap-
ping effi ciency (i.e., the percentage of the snow fl ux that remains at the drift)
