Drifted Snow on Roofsby Michael O'Rourke, Rensselaer Polytechnic Inst, Troy, United States,
Abstract: Drift snow loads on multilevel roofs account for about 75 percent of all snow load related roof collapses. A case history of snow drifts provides the foundation for a discussion of the two drift types which form on multilevel roofs. These are a triangular drift which typically forms at a backward facing step and a quadrilateral drift which typically forms at a forward facing step. Empirical relations for estimating the size and structural load resulting from both triangular and quadrilateral drifts are presented. These empirical relations were developed from an analysis of a number of case histories and form the basis for current building code provisions for drift loads on multilevel roofs. It is shown that for similar snow source areas, triangular drifts tend to be larger than quadrilateral drifts. Recent research results which include wind speed in the estimation of triangular drift size are briefly reviewed. Finally a physical model for estimating drift size is outlined.
Subject Headings: Snow loads | Roofs | Drift (structural) | Case studies | Failure loads | Load factors | Empirical equations
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