Civil Engineering Reference
In-Depth Information
present 50-year ground snow loads; some also present snow provisions. For
New Hampshire, Tobiasson et al. (2002) present a town-by-town listing of
ground snow loads for a specifi c elevation in each town.
Note that for some locations, the 50-year ground snow load value using
a state reference is different from that using the ASCE 7-10 map. For example,
the most recent state map for Washington (Structural Engineers Association
of Washington (SEAW) 1995) contains zones that, when multiplied by the
site elevation, give the 50-year ground snow load. For a site in Bellingham,
Washington, with an elevation of 100 ft, the SEAW procedure gives a 50-year
ground snow load of 15 lb/ft
2
, while the ASCE 7-10 map gives 20 lb/ft
2
.
Unfortunately, the ASCE 7-10 text and Commentary are not clear about
which 50-year value is to be used: the ASCE 7-10 value, the state value, the
larger, the average, or other permutation. Structural engineers are advised to
contact the local building offi cial for guidance. It is anticipated that future
versions of ASCE 7 will recommend using the state or local ground snow load
map in such cases, as long as the state/local document meets certain criteria
(e.g., 50-yr values are based on an extreme value statistical analysis). Never-
theless, the ASCE 7-10 text is clear that a 50-year MRI ground snow load
is to be used in its provisions. Furthermore, irrespective of the exact source
of the ground snow load value, it also is clear that a design according to the
ASCE 7-10 provisions requires the use of the ASCE exposure and thermal
factors, drifting relations, etc., as opposed to alternate provisions that may be
part of the aforementioned state documents.
Example 2-1
Ground Snow Loads
Problem
Determine the 50-year ground snow load for (a) Chicago, Illinois, and
(b) Williamsport, Pennsylvania.
Solution
a)
Chicago, on the southwestern shore of Lake Michigan, is in a 25 lb/
ft
2
ground snow load zone, as shown in
Figure G2-3.
Figure G2-3
Portion of ASCE 7-10
ground snow load map
showing Chicago, Illi-
nois, and Williamsport,
Pennsylvania (zone for
Williamsport is cross-
hatched) for use with
Example 2-1.
