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Timothy,
 
The problem of falling snow onto a lower roof is complicated. I have seen
the results of snow sliding from an upper roof onto a lower 2X deck roof in
Fairbanks, Alaska. The impact fractured the timber decking and cracked the
glulam joist. The design should consider snow density, depth of snow,
quantify of snow, and height of fall. This is a physics problem with several
assumed parameters. You can estimate the impact by following Roark's Section
15.3 (5th Ed). The impact factors will be high, but may be reduced by
considering energy absorption by snow compaction and ice crushing.
 
The safest solution is to prevent the snow from sliding by installing ice
clips.
 
regards,
 
Bill
 

-----Original Message-----
From: Timothy Allison [mailto:t_allison(--nospam--at)illinoisalumni.org]
Sent: Friday, April 09, 2004 5:28 AM
To: seaint(--nospam--at)seaint.org
Subject: (no subject)


Sorry if this is duplicate - I haven't seen it post to the listserv...



In Section 7.9 of ASCE 7, the designer is directed to consider all of the
snow from a sloped roof above to have fallen onto the lower roof. I have two
questions regarding this:

1) Is the superimposed load from sliding snow to be applied uniformly over
the lower roof? If it is tapered, where are the equations explaining how far
to project the sliding snow load?
2) Is there some point that the sliding load can be neglected, based on
relative elevations? If the eave of the upper roof is at a height of 12',
and the lower roof at this location is only a few inches below, it seems
ludicrous to think that the entire upper roof could/would actually slide
down.

Thanks 



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