sliding snow

huangyhg

sliding snow
asce 7, section 7.9 "sliding snow" requires the designer to apply all the snow that accumulates on the upper roof to the lower roof.  questions:
1.  how is the sliding snow applied to the lower roof?  over what width or area?  in what shape?  we typically design from the ubc which simply applies a 1.4 factor to the drift height when the roof is pitched over 2:12.
2.  at what pitch do you not bother with sliding snow.  again, the ubc has a 2:12 limit.  asce 7 does not.  the commentary in asce 7-98 indicates that even small slopes can cause sliding, but doesn't provide any guidelines.
one thought was to apply the sliding snow over the low roof snow across the same width as the high roof, but this doesn't seem to be realistic if the high roof is very wide; there would have to be some piling similar to the ubc drift.
just curious as to what others may be doing.
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check the commentary on 7.9. all snow is assumed to slide (not a factor of the drift) and uniformly is stated as the shape of the distribution (pile) onto the lower roof whose width might vary from 5ft to 20ft depending on 1)size, position, and orientation of each roof 2)width of the lower roof drift 3)vertical distance between roofs, and 4)upper roof angle (so basically use your judgement in your situation for sliding width, since it doesn't give a formula). the angle of the higher roof pitch determines the high roof balanced snow load according to solid line in fig 7-2 (asce7-95), (if there is balanced snow on upper roof it will slide no matter what pitch).this is how i would interpret asce-7 but i haven't used it for sliding before.
thanks for the response.  we also noted that it appears the sliding snow is added to the "balanced" snow of the lower roof and is not added to any drifting on the lower roof.  this compares much better with ubc where they just factor up the drift.