Civil Engineering Reference
In-Depth Information
line in
Figure G7-8
. The Tobiasson and Greatorex relation suggests lower
loads for the same depth of snow. The differences are due in large part to the
nature of the two sets of relations. The ASCE 7-10 and Tabler relations are
based on simultaneous measurements of load and depth. On the other hand,
the Tobiasson and Greatorex formula relates a maximum annual snow depth
to a maximum annual snow load (50-yr ground snow depth to 50-yr ground
snow load). For a common scenario when the maximum depth occurs earlier
in the winter than the maximum load, the Tobiasson and Greatorex conver-
sion density for this maximum depth would be less than the actual density
when the load reached maximum.
Although the two sets of density relations provide different answers,
both are arguably appropriate for their intended purposes. ASCE 7-10 (Equa-
tion 7-3) and Tabler convert a snow depth at a point in time into a snow load
at the same point in time. Tobiasson and Greatorex (
Equation G2-1
) relate
a 50-year snow depth at a point in time to a 50-year snow load possibly at
another point in time.
All the above density relations are for naturally occurring snow. Occa-
sionally, naturally occurring snow is further compacted by mechanical opera-
tions such as plowing (for example, at the top level of a parking garage).
Limited case histories drawn primarily from the northeastern United States
indicate a density of 'piled' snow (using a truck with a plow) ranging from 25
to 46 lb/ft
3
. The prescribed density for 'piled' snow in Section 4 of the ASCE
7 load standard is 40 lb/ft
3
.
Equations G7-1
through
G7-3
indicate that the drift size is an increas-
ing function of both the ground snow load and the upwind fetch. In other
words, the bigger the snow source, the bigger the drift. However, the increase
is not linear. For example, doubling either the upwind fetch or the ground
snow load results in less than a doubling of the drift size. This is illustrated
in
Figure G7-9,
which is a plot of the ratio of the cross-sectional drift area
LIVE GRAPH
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Figure G7-9
Ratio of drift area to
source area versus
50-year ground snow
load.
25
20
u
= 50 ft
u
= 100 ft
15
10
u
= 200 ft
u
= 400 ft
5
0
0
10
20
30
40
50
60
70
80
90
100
2
50-Year Ground Snow Load,
p
g
lb/ft
