ASCE 7-05 Unbalanced Snow Loads 1

[4thorns]

7.6.1 Says,
"Roofs with an eave to ridge distance, W, of 20' or less, having simply supported prismatic members spanning from ridge to eave shall be designed to resist an unbalanced uniform snow load on the leeward side equal to I x pg." (Importance Factor x Ground Snow Load)

Does this mean that regardless of slope, thermal and any other factors, the Unbalanced load will always control?

Importance factor will always be 1.
"W" will always be less than 20'
Slope will always be >2.38 and always < 70
Basically residential, gable roof homes.

I take the quote to mean that in this situation the roof flat out has to be designed to carry the Ground Snow Load.

Any thoughts?

 

[woodman88]

The maximum moment for the balanced condition is wll/8.
The maximum moment for the unbalanced is wll/14.2.
So when the roof snow load is greater than 8/14.2 (0.563) the ground snow load, the balanced condition member resists a greater moment.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[ToadJones]

Woodman-
Your signature is intriguing to me....what are you getting at?
I ask honestly.
Seems like an obvious thing, no?

 

[woodman88]

ToadJones, I thought so too.
But there seems to be a disconnect in some EOR's opinions (as I see it, in some of the answers given in these forums) concerning the responsibilities between EOR's and Wood Truss Engineers. I get a bad feeling that they have never even look into what they are required to do for the wood trusses (or even other components) they are allowing to be used in their plans. So I decided to make this statement a signature.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

As a matter of observation here, 7.6.1 appears to apply to the stick frame roof situation (with a ridge beam), not trusses, unless they span from a wall to a ridge beam also.

Mike McCann
MMC Engineering

 

[msquared48]

I get your point Woodman, and agree. Communication and coordination of design efforts among engineers, and the trades, is always essential.

I would mention though that I have seen some truss packages with high girder truss loads to the exterior walls where either insufficient studs and footings were provided, or the window/door headers were overstressed. They got past the local jurisdiction with no structural review other than the local plans checker.

I would suggest that, in order to mitigate this particular case, it would be prudent by the truss designer to red flag these reactions with a prominent note - something to the effect of needing the attention of a licensed structural engineer prior to any permit being issued. This could be noted on the roof truss framing plan.

Mike McCann
MMC Engineering

 

[woodman88]

Missed that point msquared48, though ASCE Section 7.6.1 does apply to all gable and hip roofs, the sentence in the OP applies only to gable and hip roofs with ridge beams (IMHO now). It was the prismatic members (prismatic I assumed meaning triangular shape as in a truss, not as in just one side of the triangle) that threw me off. So the unbalanced condition would always control the rafter and exterior walls/beams design. The balanced snow condition may apply to the ridge beam design.
As for the red flags, unless you get it in as a code requirement it will never happen. Trusses are sold to the contractor or sub contractor, usually after the job has been permitted. Give them a set of truss designs that red flags the job, they will scream and then go to another truss company that does not red flag the job. We (engineers) should be complaining to the building departments and code authorities about getting better reviews and engineering for these low end building designs. Not trying to pass it off onto other engineers.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[4thorns]

Thanks for the replies. This is a situation with rafters and a ridge beam. The conditions I set forth in the OP will 99% of the time hold true. One factor that may change is exposure. Drifting may or may not occur to such a point that it needs to be considered. In cases where this is an unknown, based on the ASCE, doesn't it make sense to design a roof system of this nature using the full ground snow load?

 

[4thorns]

Woodman,
Guess I need to slow down and pay attention to the replies. You've all but answered my question. If there is an unbalanced condition then the I*Pg controls correct? The key word here being if. Aren't unbalanced loads determined by Exposure?

 

[woodman88]

4thorns, Yes it does.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[4thorns]

Unless I miss my guess, you've answered 3 questions with 3 simple words. Very nicely done.

 

[woodman88]

4thorns, Sorry (you slipped a comment in on me) my above answer is to your second above statement and the above unbalanced question. For your question about exposure, I would say no. Because exposure only determines the amount of snow (see table 7-2).

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[woodman88]

Ahhhhh you did it again. LOL

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[4thorns]

The exposure factor is, from what I read, based on obstructions. Doesn't this refer to elements obstructing wind?

 

[woodman88]

The exposure factor is based upon the Terrain Category (see Section 6.5.6) and how exposed the building is during the life of the structure. Such as terrain, other buildings, trees, etc. It does not include building elements like mechanical equipment, parapets, etc. See the footnotes to Table 7-2.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[4thorns]

Agreed Woodman. I reread that earlier. In this situation there are no major building elements. I feel that the Exposure factor is going to determine whether or not there is going to be drifting of snow. If the building is sheltered by pines on all four sides (eg. a cabin in the woods) then it should have less drifting then a house (based on exposure definitions) that sits on a lake with it's eave side totally exposed to winds.

I'm just trying to understand the wording. Should I determine the roof system of the cabin with the same loads as the house on the lake?

 

[woodman88]

"I'm just trying to understand the wording. Should I determine the roof system of the cabin with the same loads as the house on the lake?"
I would always start with a conservative design and look at reducing it only if it becomes necessary. Keeping in mind that trees can be cut down during the life of the building.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.