Topic: U.S. Snow Drift Loading

Michael O’Rourke, Ph.D., P.E., received his B.S. in civil engineering from Illinois Institute of Technology and his M.S. and Ph.D. from Northwestern University. During most of his 47 years on the faculty in civil engineering at Rensselaer Polytechnic Institute, he has been involved in snow load research sponsored by the U.S. Army Cold Regions Research and Engineering lab, the National Bureau of Standards, the National Science Foundation, and the Metal Building Manufacturers Association, among others. This research work has resulted in publication of thirty snow loading papers in refereed journals and a comparable number of conference proceedings papers. He is the author of six ASCE Press books and three ASCE Press E-books.  Dr. O’Rourke has been a member of the ASCE 7 Snow and Rain Loads Committee since 1978 and served as chair from 1997 through 2017.

Abstract: 

The keynote lecture will be divided into three sections. The first section is the shortest, but arguably the most important. It will establish, through a review of the author’s forensic consulting cases, that snow drifts are the most important snow load in the U.S. in terms of building collapse.

The second section will describe the current U.S. approach to snowdrift loading. Roof step drifts, both windward and leeward, will be discussed as well as parapet wall drifts, drifts at roof top projections, and finally unbalanced loads (across-the-ridge) gable roof drifts. The basis for the provisions – purely theory, purely observation or a mix of theory and observation – will be provided.

The third and final section will present expected changes to current U.S. snow drift provisions. The first expected changes is inclusion of a winter wind parameter. Currently drift size (cross-sectional area) is a function of the potential snow source, specifically the ground snow load and the upward fetch. These procedures are expected to be enhanced by inclusion of a winter wind parameter.

The second expected improvement is a new specification for the height of a wall which shields a downwind area from drift formation. This is expected to be particularly useful as a potential mitigation measure for a taller building addiction adjacent to a lower level roof initially not designed for the now expected leeward roof step drift.

The third expected change is recommended procedure for roof geometries where a 90° change in wind direction results in two 2-D drifts at the same location.

The final expected change envisions, documentation in the commentary of an unusual drift observed downward of elevated roof top piping.