The American National Standards Institute and subsequently, the American Society of Civil Engineers mapped the ground snow loads for the nation, but deferred to local knowledge and case studies in the western U.S As a result many of the western states have written reports to describe the ground snow loads produced by the varied terrain and complex weather patterns. In 1976 we published the first comprehensive report of ground snow loads for Idaho, which was updated in 1986 with an expanded data base and loads associated with a two percent chance that the value will be exceeded in a given year (also known as a 50-year mean recurrence interval); we used a three percent chance in the 1976 study.
The database for this 2015 study contains 31 additional years of record from the National Weather Service (NWS) and the National Resources Conservation Service (NRCS), formerly the Soil Conservation Service (SCS). We used a total of 408 stations within Idaho and 260 additional stations located in bordering states. As in the 1986 study we used the Rocky Mountain Conversion Density (RMCD) to obtain loads from the snow depths recorded by the NWS Coop stations, and mapped the Normalized Ground Snow Loads (NGSL). Appendix 1 lists the ground snow loads for Idaho towns and cities by county. Several areas exhibited exceptions to the mapped values; consequently we have provided Appendix 2 that compares the 1976, 1986 and 2015 loads, and it includes notes to aid in interpreting any mapping exceptions and/or values that have changed significantly from previous studies. We envision that the notes will be useful to the user in interpreting loads for sites that are not specifically documented.
The 31 years of additional data, plus new Idaho stations and also stations from bordering states produced results slightly different from those of our previous studies as shown in Appendix 2. For this study, the counties of Ada, Bannock, Benewah, Bingham, Butte, Clark, Clearwater, Jefferson, Jerome, Minidoka, and Payette did not show significant changes from the 1986 report. The Washington-Idaho State line shows improved agreement for towns near the state border. Also, there are differences in the studies at the Idaho-Oregon border and at the Idaho-Montana border. Since this study focuses only on the ground snow loads, we have not included information on the ground-to-roof conversion factors, which can be found in the ASCE/SEI 7 Standard.
We urge our readers to use the information with care. The uncertainties associated with snow should suggest that the scatter in the data could be large in spite of our many efforts. For unusual structures or siting we suggest that all available information be considered in determining the snow loads. Finally, we remind the user that the design snow loads are the ultimate responsibility of the person in charge of the project.
For detailed information see Ground Snow Loads for Idaho - 2015 Edition: http://www.lib.uidaho.edu/digital/idahosnow/GroundSnowLoadsforIdaho2015.pdf.