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Re: Snow Drift Loads on Roofs with Slope Cha

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Roger:

This is one of those situation that the code wasn't written specifically to
address. This is where we are suppose to use our engineering judgement!

It is my opinion that Wb should be the width of that A-frame at the intersection
of the lower roof. Since I do not know what dimensions you are dealing with, I
would suggest that you also check to see if Wb is less than the width of the low
roof, then conservatively use the larger of the two values (1641.5 provision).
Note that hd is not very sensitve to the exact value of Wb. I would always
suggest being conservative, since the prediction of drift loads is rather
inexact.

The value of hr, in your case, is certainly not clear. The steeper the upper roof
is, the more it acts as a wall. What angle does it need to be at before it acts
as a wall? Who knows? Conservatively, you may take hr as the vertical dimension
from the peak to the intersection of the low roof. This would get you out of the
negative value situation and 4 hd most likely will control the drift length.

I hope this helps.

Jim Kestner, P.E.
Green Bay, Wi



Roger Turk wrote:

> This desert rat is looking for some advice from some of you in snow country
> 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 5(V):12(H)
> 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 A-frame
> roof.
>
> 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 meets
> 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.
>
> TIA
>
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona
>