Civil Engineering Reference
In-Depth Information
to the upwind snow source area versus the 50-year ground snow load. The
drift area is
2(
h
d
)
2
Drift Area
=
1
2
h
d
w
=
Equation G7-4
and the upwind snow source area is
p
g
u
p
g
0.13
p
g
Source Area
=
=
Equation G7-5
u
+
14
As shown in
Figure G7-9,
the “design” leeward drift is 10% to 25% of the
“design” snow source area. The percentage is a decreasing function of the
ground snow load,
p
g
, and the upwind fetch, C
u
, although less so for C
u
. Both
of these trends are sensible. If the upwind fetch is small or the snowpack depth
is shallow, then a typical wind event could easily remove or transport almost
all of the snow from the small source area. Hence, it is likely that a signifi cant
fraction of a small snow source area could end up in the drift. Conversely, for
larger fetch areas and/or deep snowpacks subject to the same typical wind
event, a smaller percentage of snow is transported. Note that the range of per-
centages (10% to 25%) in
Figure G7-9
is based on the 50-year ground snow
load, as are those in ASCE 7-10 (see
Equation G7-3
and Figure 7-9). When
the ratio of drift area to source area is compared with observed ground snow
loads from case studies (
Equation G7-1
) instead of the 50-year ground snow
load, the percentages double to roughly 20% to 50%. This occurs because the
0.7 modifi cation factor used in ASCE 7-10 is applied to both the surcharge
height,
h
d
, and the width,
w
4
h
d
, for a given source area [(1/0.7)
2
2.0].
In other words, 20% to 50% of the upwind snow source typically ended up
in the case-history drifts. The larger of these percentages (50%) is consis-
tent with water fl ume studies. They suggest that the trapping effi ciency of a
leeward roof step (percentage of transported snow impinging upon the roof
step that remains or settles into the drift) is about 50%. Using the ASCE 7-10
relationships (in which the snow is characterized by the 50-yr value), 10% to
25% of the “design” upwind source area ended up in the “design” drift.
=
∼
7.2
Windward Drift
Equation G7-3
and Figure 7-9 can be used to determine the windward drift
height as well, with some modifi cations. In
Equat
i
on G7-3
and Figure 7-9, C
u
is
replaced with C
C
and then the calculated height is multiplied by 0.75. Case his-
tories suggest that windward steps trap snow less effi ciently than leeward steps,
resulting in a reduced drift height. More detailed justifi cation for the three-
quarters factor is provided in Chapter 8 of this guide. In all cases, the triangular
drift surcharge is superimposed on the balanced load
p
s
for the lower roof.
For most roof steps, a leeward drift or a windward drift or some combi-
nation of the two are possible. However in design, the windward and leeward
drift heights are calculated separately, and the larger value is used to estab-
lish the design drift loading. This approach (i.e., using the
larger
of the drift
