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RE: Snow Drift Loads on Roofs with Slope Cha

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My $.02 worth:

Based on Fig. A-16-6, I would take Wb as the width of the A-frame at the
level of the bump-out (slightly more conservative to take it as total width
of the A-frame at eave level).  Hr would be height of the A-frame above the
bump-out, and Hb the snow depth on the bump-out without drift from the upper
roof.  Note the surcharge for sliding snow per Fig. A-16-6 and Para. 1641.4
A rough check would be to calculate the weight of all the snow  falling on
the bump-out side of the A-frame ridge, and assign it to the bump-out roof
as a triangular or trapezoidal load.  The logic would be that you can't
accumulate more snow than a flat roof with the same vertical projection
would catch, and the worst case would have all the snow sliding onto the

	-----Original Message-----
	From:	Roger Turk [SMTP:73527.1356(--nospam--at)]
	Sent:	Wednesday, October 28, 1998 10:35 AM
	To:	SEAOC Listservice
	Subject:	Snow Drift Loads on Roofs with Slope Cha

	This desert rat is looking for some advice from some of you in snow
	or experienced in designing for snow.

	I'm designing an A-Frame mountain cabin which has a roof pitch of 12

	(vertical) in 6 (horizontal).  A bump-out with a roof pitch of
	intersects with the A-Frame roof.  Intuition tells me that this
bump-out roof 
	will be subjected to drift and sliding snow loads from the main

	With regards to 1994 UBC Appendix Chapter 16, Section 1641:

	What do you take for W(b) in equation 41-1?  The width of the
A-frame at the 
	base or the width of the A-frame at the level that the bump-out roof
	the A-frame, or ???  ?

	How do you calculate the width of the drift load?  4 * h(d) is easy
enough to 
	calculate [providing the appropriate W(b) is used in equation 41-1].

	However, 4[h(r)-h(b)] would come out as a negative number as h(r) is
(as I 
	understand it) the height of the vertical wall between the low roof
and the 
	high roof and the drift width is to be the smaller of the two.


	A. Roger Turk, P.E.(Structural)
	Tucson, Arizona