Analytical Simulation of Snow Drift Loading

An analytical procedure for simulating roof snow drift loads is presented. The procedure is based upon the physics of drift formation and consists of two elements. The first element is the transport rate which quantifies the snow flux from the snow source area. The second element is the trapping efficiency which quantifies the percentage of transported snow from the source area which is captured at the drift. Knowing wind speed and snowfall information for a given winter, the procedure can be used to simulate the annual maximum drift for a given snow source area. The method is applied to over 40  years of weather data at each of over 50 NWS first-order sites across the United States. The calculated annual maxima are fit with an extreme value distribution to determine 50-year drift loads for both roof step and gable roof geometries. It is shown that current leeward roof step drift provisions in ASCE 7-02 as well as gable roof drift provisions proposed for ASCE 7-05 yield loads with something close to a 50  year mean recurrence interval.