Methodology for establishing ground snow loads

Economical structural design is dependent upon accurate prediction of roof snow loads, which are obtained using ground snow loads. Various national, state and regional ground snow load studies are surveyed. Typical data bases consist of Soil Conservation Service (SCS) snow course records, and snow-load and snow-depth data from the National Weather Service (NWS) sites. Probabilistic models include Gumbel, lognormal and Log-Pearson Type III. A 50-year mean recurrence interval (mri) is generally used, but multipliers are adopted to shift to other mri's. Specific gravities of the snow must be estimated for converting NWS depths to loads. This is done using: a) a single value throughout the region; b) values dictated by forest type; c) conversion factor established from extreme depths and weights; or d) a depth-specific gravity relation given by seasonal maxima. Snow loads are spatially extrapolated plotting contours of either isolines, or normalized snow loads or by establishing snow load elevation relationships. The current study for the State of Idaho is described and a sample map created by computer plotting is shown.