unbalanced snow: ASCE 7-98 vs ASCE 7-02
unbalanced snow: ASCE 7-98 vs ASCE 7-02
(OP)
In my state, we will be going to the 2003 IBC sometime later this year, and it's my understanding that 2003 IBC references ASCE 7-02.
In ASCE 7-98, which I am currently required to use, there is a case for the unbalanced snow load where it varies linearly from 1.2pf/Ce at the ridge to 1.2(1+beta)pf/Ce. This can give you huge unbalanced snow loads, far in excess of what we used to be designing for.
In ASCE 7-02, I don't see this case, and the loads don't appear to be a function of the roof slope anymore.
Does anyone know the reasons for this change? I don't find the commentary very helpful in either case. It seems that with every edition of ASCE-7 they change the way you calculate unbalanced snow. The provisions of ASCE 7-98 are especially onerous in my opinion.
In ASCE 7-98, which I am currently required to use, there is a case for the unbalanced snow load where it varies linearly from 1.2pf/Ce at the ridge to 1.2(1+beta)pf/Ce. This can give you huge unbalanced snow loads, far in excess of what we used to be designing for.
In ASCE 7-02, I don't see this case, and the loads don't appear to be a function of the roof slope anymore.
Does anyone know the reasons for this change? I don't find the commentary very helpful in either case. It seems that with every edition of ASCE-7 they change the way you calculate unbalanced snow. The provisions of ASCE 7-98 are especially onerous in my opinion.






RE: unbalanced snow: ASCE 7-98 vs ASCE 7-02
Could it be that the new ASCE method follows this Canadian Code method?
DaveAtkins
RE: unbalanced snow: ASCE 7-98 vs ASCE 7-02
The problem in NY is that we have our own unique snow map, which has grossly conservative ground snow loads for many areas of the state, especially in the area where I live. When we go to the 2003 IBC there is talk the ground snow loads will be adjusted downwards to reflect case studies that have been done.
Because ASCE keeps significantly changing the way unbalanced snow loads are calculated, I wonder what basis in reality the unbalanced provisions actually have?
I am designing a wood roof with a 4:12 pitch theta=18.4 degrees. My ground snow load is 65 psf, W is 24 feet, beta=1, snow density is 22.45 pcf, and pf is 46 psf. These parameters put me into the bottom case in Figure 7-5. The resulting unbalanced snow load is about 109 psf at the eave, and 55 psf at the ridge. We used to use 1.5ps/Ce in our old code, which would have resulted in 69 psf.
With ASCE 7-02, the unbalanced snow load is only 69 psf, and is constant on the lee side. With ASCE 7-02, beta=.5 because pg>40. Ce=1 for all cases. Either I am missing something basic, or this is plain ridiculous. Maybe a 3 year code cycle is just too often. Maybe if they had more time between issuing revisions some of the more ridiculous code provisions could have been weeded out.
RE: unbalanced snow: ASCE 7-98 vs ASCE 7-02
RE: unbalanced snow: ASCE 7-98 vs ASCE 7-02
Your numbers look right and I agree that the unbalanced loads are now getting far higher than before. I try and visualize a foot or two of standing water at 62.4 pcf and can never really buy that that is a realistic load.
The complexity of analysis is ridiculous given the accuracy to which the snow heights and water content is known. Here in Colorado, where you can get tremendous roof snow loads depending on the elevation, I think the density numbers are way too high for our (typically) "powder" snow and the 1.15 duration factor that assumes that this load is present for two months is also far comservative given our high altitude UV.
What do you do at a roof valley in regards to unbalanced snow loads? The UBC had the Cv boogie factor, which is now gone in ASCE 7-02. The only nebulous guidance given is "Winds from all directions shall be accounted for when establishing unbalanced loads." Does this really mean that you need to consider the 100 year meteorlogical event and then assume the wind deposited two leeward drifts into your roof valley by changing 90 degrees?
RE: unbalanced snow: ASCE 7-98 vs ASCE 7-02
I agree that the codes are getting way to ridiculous. I am very dissatisfied with the IBC and ASCE-7, I wish they would simplify it. Most buildings built are one to two stories, and in my judgement there should be some simplified provisions regarding the loads for these buildings. The codes do have some simplified provisions, but I think they are too narrow in scope, or not very simple either.
RE: unbalanced snow: ASCE 7-98 vs ASCE 7-02
Is there really that much research going on that justifies these changes? Or, is it because the vast majority, not all, but the vast majority of the code committees are made up of engineering profs. and consultants that give seminars on all the updates? And, as noted, the complexity of the various codes is multipling. Just when we become familiar with a code, the next revision comes along!
The dollars generated by the sale of new codes and seminars needed to understand them is huge, not to mention the time consumed to go to them. Owners complain about the cost of construction, this is one place to look. (Maybe I've had too much coffee)
RE: unbalanced snow: ASCE 7-98 vs ASCE 7-02
RE: unbalanced snow: ASCE 7-98 vs ASCE 7-02
A while ago, I called ASCE and asked them how they came up with the ground snow load maps. They said they were hand drawn. Not very exact in my mind. Of course, now we have web sites that tell you what your load criteria are, such as seismic. So, codes are only as good as the people that develop them.
I think codes have evolved over time as we learn more about the field of engineering and building construction, however I'm with jheidt2543 - three year cycles are too close.