Improved Snow Drift Relations

Abstract

Drift loads on roofs are arguably the most important snow load from a structural engineering perspective in the US, as they are the governing load for most cases of snow related structural collapse. Design relations for both leeward and windward roof step drifts are currently available. They were based upon an analysis of roughly 300 case histories from insurance company files. The relations have been in use in the US for roughly the past 30 years. When first introduced, they were considered an improvement over the prior relationship wherein the peak drift load was simply a multiple of the ground snow load. That is, the inclusion of the upwind fetch was intuitively appealing. Note however that neither wind speed nor wind duration were in the aforementioned original insurance company files. Hence winter windiness at the site could not be included as a variable in the original regression analysis, upon which the current relations are based. The overall purpose of this paper is to introduce a new drift relation which incorporates a winter windiness parameter. It is shown that this new relationship improves the associated drift prediction capability in comparison to the current drift relations.