Revised ground snow loads for the 1990 National Building Code of Canada

Abstract

The last systematic recalculation of ground snow loads in the Supplement to the National Building Code of Canada was made in 1977 and used data up to 1975. Data from three times as many stations are now available and there is also an additional 10 years of record. Using this expanded data base, ground snow loads have been recalculated for the 1990 Supplement.Several changes in methods have been utilized, the most significant of which is the use of an objective technique to estimate ground snow loads at Code (or other) locations. It explicitly incorporates an assumed dependence of the snow load on topographical elevation, and accounts for the magnitude of errors at snow depth observation sites. Other differences include (a) the use of the method of moments to fit the Gumbel extreme value distribution for the purpose of estimating the 30-year return period snow depth; (b) the use of geographically varying snow pack densities; and (c) using probabilistic rain components of the total snow load and estimating this component by use of a snow pack model.Results show an average national decrease of 6.6% in the 1990 loads compared with those in the 1985 Supplement. A regional exception is in the Northwest Territories where the use of a greater snow density has led to an average increase of about 16% in the loads. A reduction in the standard deviation about the mean load suggests a more spatially consistent set of values for the 1990 Supplement. Key words: snow, loads, building, code.