Snow Engineering, Hjorth-Hansen, Holand, Lset & Norem (eds) 2000 Balkema, Rotterdam, ISBN 90 5809 148 1
Developing ground snow loads for New Hampshire
Wayne Tobiasson, James Buska & Alan Greatorex
Cold Regions Research and Engineering Laboratory (CRREL), Hanover, New Hampshire, USA
Jeff Tirey, Joel Fisher & Steve Johnson
Structural Engineers of New Hampshire, Inc. (SENH), Laconia, New Hampshire, USA
ABSTRACT: Because of New Hampshire's hilly landscape, mapped values of ground snow load are not avail-
able for much of its area. We conducted snow load case studies to establish ground snow loads for a specific
elevation in each of the 140 towns where no values are currently available. That work was done by three re-
searchers and three structural engineers practicing in New Hampshire. While our methods of analysis varied
somewhat, our results were comparable and the feedback we received from each other was quite valuable. We
then established an elevation correction factor to transfer our snow load answers to other elevations in each
town. We did not do case studies for the 102 towns in New Hampshire where mapped values are available. We
are now planning to do that, as we believe that case studies improve snow load design criteria. We suggest that
similar studies be conducted for other places in the United States.
1 INTRODUCTION
tions, ground snow loads "shall be based on an ex-
treme value statistical analysis of data available in the
vicinity of the site using the value with a 2% annual
The primary resource document for the design of struc-
probability of being exceeded (50-year mean recur-
tures in the United States is American Society of Civil
rence interval)".
Engineers (ASCE) Standard 7, "Minimum design loads
At CRREL a methodology has been developed to
for buildings and other structures" (ASCE 1996). It is
conduct snow load case studies. It and the data used
commonly referred to as ASCE 7-95. The first step in
are described in the paper, "Database and methodol-
determining design snow loads is to determine the
ogy for conducting site specific snow load case stud-
ground snow load at the place of interest. ASCE 7-95
ies for the United States," which was presented at the
contains a map of the United States overlaid with that
Third International Conference on Snow Engineering
information. That map was made by Tobiasson and
(Tobiasson & Greatorex 1997). That database also
Greatorex of CRREL using data from 226 "first or-
contains information from an additional 3300 locations
der" National Weather Service (NWS) stations, where
across the United States where ground snow loads are
snow depths and snow loads are measured frequently,
measured a few times each winter by other agencies
and data from about 11,000 other NWS "co-op" sta-
and companies.
tions, where only the depth of snow on the ground is
Figure 2 shows New Hampshire overlaid with town
measured frequently. In some areas, extreme local
boundaries and the location of each station in the da-
variations in ground snow loads preclude mapping at
tabase. There is 1 NWS "first order" station, and 89
a national scale. In those areas the national map con-
NWS "co-op" and 91 "non-NWS" stations in New
tains the designation "CS" instead of a value. CS indi-
Hampshire. First order stations in adjacent states within
cates that case studies are required to establish ground
50 miles (80 km) of the border and other stations within
snow loads in these areas. Figure 1 presents the infor-
25 miles (40 km) of the border were also used in our
mation from the ASCE 7-95 map on a larger map of
analysis. They are also shown in Figure 2. Shading in
New Hampshire, showing county and town boundaries.
that figure and its legend indicate towns we studied
The zoned values in Figure 1 are ground snow loads
and others we did not.
with a 2% annual probability of being exceeded (i.e.,
Structural Engineers of New Hampshire, Inc.
the 50-year mean recurrence interval value). As can
(SENH), is a non-profit professional association of
be seen in Figure 1, all of New Hampshire is either in
structural engineers. Their members expressed inter-
a "CS" area or the zoned values have elevation limits
est in using the CRREL database and methodology to
(the numbers in parentheses) above which case stud-
develop ground snow loads for each town in New
ies are needed. Thus, case studies are needed to deter-
Hampshire. Several volunteered their time to conduct
mine ground snow loads for many buildings in New
case studies. All prior case studies had been done by
Hampshire. ASCE 7-95 requires that, in these situa-
313
Previous Page
