Ground Snow Loads - Snow Loads - page 7

latitude. As one might expect, Louisiana has relatively small ground snow loads
(0 or 5 lb/ft 2 ) whereas Wisconsin has relatively large values (25 to 70 lb/ft 2 ).
In the eastern United States, p g generally increases with latitude, but
two additional variables infl uence p g : site elevation and the distance from the
shoreline. Elevation is a factor in the East because of the string of mountains
along the Appalachian Trail. In some locations, such as eastern Tennessee or
Rochester, New York, the mapped ground snow load value in Figure 7-1 (10
and 40 lb/ft 2 , respectively) applies to sites with elevations less than the given
upper elevation limit (1,800 and 1,000 ft, respectively). Designers are pro-
vided with ground snow load information at lower elevations, where most
of the buildings are located. At elevations greater than the upper limits, a
site-specifi c case study (discussed shortly) is required.
Locations downwind of the Great Lakes get what is known as “lake
effect” snow. Low-pressure cells traveling over the Great Lakes pick up mois-
ture from the lakes and return it as snow upon landfall. As a result, regions to
the lee of the lake are particularly snowy. The Case Study (CS) areas of north-
western Indiana, western Michigan, northeastern Pennsylvania, and western
New York are so designated because of these lake effect snows.
Latitude and elevation also infl uence ground snow load values in the
West. For instance, the 50-year ground snow load at a given elevation in New
Mexico is typically less than that for the same elevation in Montana. How-
ever, because of the more rugged and variable terrain, the overall pattern in
the West is more complex. Unlike most regions in the Midwest and some in
the East, where the 50-year ground snow load is strictly a function of latitude,
all the design ground snow loads in the West are a function of site elevation.
For some locations, such as southeast Arizona, ground snow loads are speci-
fi ed for a range of elevations: zero for elevations of 3,500 ft or less, 5 lb/ft 2 for
elevations between 3,500 and 4,600 ft, etc. Other locales, such as the major-
ity of western Colorado, require site-specifi c case studies.

2.2 Site-Specii c Case Studies

All locations represented with a “CS” on Figure 7-1 require a site-specifi c case
study in order to establish the design ground snow load. As noted on the map
in relation to CS areas, “the extreme local variations in ground snow loads in
these areas preclude mapping at this scale.” Also, at all sites that have a higher
elevation than that designated in parentheses on the map, the ground snow
load must be established by a case study. For example, a case study is required
for all areas in eastern Tennessee that have an elevation higher than 1,800 ft.
As described in more detail by Tobiasson and Greatorex (1996), a case study
involves regressing 50-year ground snow load values versus elevation for a
number of sites in close proximity to the site of interest. The least squares
straight line then establishes the local “reverse lapse” rate, which in turn can
be used to establish the 50-year ground snow load for the site of interest. The
lapse rate is the decrease in temperature for a unit increase in elevation. As
used herein, a “reverse lapse” rate is the increase in ground snow load for a
unit increase in elevation.