Unbalanced Snow Loads on Slippery Roofs

[Irontrussman]

In ASCE 7-05 the Roof Slope Factor C[sub]s[/sub] provides for a slippery roof surface (fig. 7-2), which can greatly reduce the snow load. But, when calculating unbalanced roof loads the roof surface does not come into play, and because the worst case is used to design structure, this effectively negates the effect of having a slippery roof. Is this correct or did I miss something?

Another thing I find odd is that the unbalanced snow load is calculated differently whether the roof is trussed or raftered (7.6.1). Why should the structure under a roof make a difference as to how the snow is distributed on the roof? Anyone care to comment?

 

[Lion06]

Re: the first question - When determining the unbalanced loading ASCE 7 allows you to use ps (which takes Cs into account), so you're not neglecting the sliding factor.

Re: the scond question - I'm not seeing (in 7.6.1 anyway), anything that says the unbalanced load is calc'd differently for rafters compared to trusses. Maybe I'm missing it - where in that paragraph does it talk about it?

 

[Lion06]

I re-read it and I can see how it could be construed to be rafters or trusses, but it says "simply supported from ridge to eave" or others. I guess others would include trusses since they wouldn't be simply supported from ridge to eave. I can't say I have a clue to even guess at the answer to number 2.

ammendment to question 1. The sliding snow will benefit the ridge beam, but not the rafters, because, as you noted, the rafters have to be designed for the unbalanced case of Ipg. This is essentially just drift.

 

[Irontrussman]

I'm interpreting, "simply supported prismatic member," to mean rafters. For these the unbalanced snow load is Ip[sub]g[/sub]; no p[sub]s[/sub] or C[sub]s[/sub] involved. For all others you can use a reduced snowload on the windward and leeward sides of the roof as each side includes p[sub]s[/sub] in its calculation, but then the surcharge is calculated without the slippery roof consideration.

I'm working in New Hampshire, where the ground snow load in some locations reaches 120 psf. As I've interpreted it, with a raftered roof, you're going to see all 120 psf on the rafters at some point, slippery or not.

The second part of my original post was simply musing over the whys and wherefors of how the ASCE decided that snow drifting over the ridge of a 12 pitch raftered roof would be different than snow drifting over the ridge of a 12 pitch trussed roof.