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RE: Snow drift

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Snow fencing, parapets, and any obstructions create snow drifts on both the windward and leeward side of the obstruction.  The snow flakes are carried in suspension by the wind.  As the wind remains in a more or less laminar flow, the snow flakes are held in suspension.  When the wind flow becomes turbulent, the snow flakes precipitate out and form drifts.  Some of the illustrations in chapter 7 of the ASCE 7 illustrate this.
Snow from a high roof will be carried by wind and precipitate on the low roof as the turbulence forms on the leeward side of the high roof.  They form in the stagnation zone of the windward side as well.  A parapet will alleviate the drifting. 

Harold Sprague

> Wesley Werner wrote:
> >
> > I am doing a preliminary design for an open structure next to an
> > existing building. The new structure will be higher than part of the
> > existing building. I don't want to surcharge the building with snow
> > drift from the new roof. If I put a parapet around the new roof that
> > is higher than the calculated windward drift height, do I have to
> > consider snow drift on the low roof? There won't be a windward drift
> > from the length of the low roof because the new structure doesn't have
> > any sides for the snow to pile up against. My only concern is with
> > snow blowing off the high roof.
> > /
> >
> > /Wesley C. Werner, P.E./
> > wwerner(--nospam--at)
> > Phone (717) 632-7722
> > Fax (717) 632-8665
> >
> > /
> >

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