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RE: Snow Drift From Higher Structure

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The Canadian National Building Code has, in its commentary, the equations
for drift.  It has a statement that over 20m (65ft) snow on the lower plane
can be considered to the standard open flat roof load.  In between,
interpolate between your full calculated drift load to no drift.  The reason
is that, yes the snow diffuses as it drops so that by the time it reaches
the lower level it has diluted out to the open flat roof densities.

Thor A. Tandy P.Eng, C.Eng, Struct.Eng, MIStructE
Victoria, BC
Canada

-----Original Message-----
From: Jeremy White [mailto:admin(--nospam--at)structuralae.com]
Sent: Wednesday, November 19, 2008 9:06 AM
To: seaint(--nospam--at)seaint.org
Subject: Snow Drift From Higher Structure



ASCE 7 restricts the affects of drift from adjacent structures to within 20
feet of higher structures.  This seems to be only a horizontal limitation
and they place no restriction on vertical distance.  There must be a point
where drift from a higher structure will diffuse in the same wind event that
causes the drift before it reaches a low roof.  But I am not familiar with
any literature that addresses this topic.  Does anyone know of any reports
or data to confirm or deny my theory?  I am working on a building that is
lower than an adjacent building by approximately 10 stories so I think it
might be a judgement call if there is no definitive resources available.

Thanks,
Jeremy

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