unbalanced snow load

[broncosfan]

For a large roof (1200'x500') with a peak down the middle and a 1/4" per foot roof slope, does anyone know if unbalanced snow loads really need to be considered. Some engineers might say no because it is really pretty close to a flat roof. However, if you go through the ASCE7 calculations, it does fall into an unbalanced load situation. So, even though the roof snow if 30psf, alot of the joists may need to be designed for 60psf plus or minus. I didn't know if somebody had run into this situation before. Thanks.

 

[rday]

broncosfan,

Well, I haven't done a monoslope like that without a parapet. But yes, I would consider unbalanced loads even with that shallow of a slope.

Rik

 

[RARWOOD]

The unbalanced snow loads load requirements in ASCE 7-02 raises a lot of questions. Following the code in most cases when you have a gable roof you have a pretty severve unbalanced load requirement. ASCE 7-02 requires the application of unbalanced snow loads to situations where in Minnesota, in the past unbalanced snow would not have been considered.

In the past I considered unbalanced snow loads in systems where unbalanced loads would produce stress reversals in members. I would not have considered unbalanced loads in a gable roof with rafters and a ridge beam.

In the past I would have designed a building in out state Minnesota for 30 p.s.f. roof snow load. The firm I work for, in the past designed a very large number of structures in out state Minnesota using 2" mominal wood deck, rafters at 8' o.c. supported by a ridge beam, with a design snow load of 30 p.s.f. No unbalanced loads were considered.

Under the current State Code which follows ASCE 7, almost of those structures would not comply with the code. Yet they are still standing and performing well.

Those same structures if designed today would have to be designed for a basic snow load o f 42 p.s.f. and unbalanced snow load. This load would apply to the 2" deck, all the rafters and the ridge beam.

My opinion is that the new codes eliminate sound engineering judgement.

 

[jike]

This is an area of the code that needs clarification.

I can see the need for it on a single span truss roof with a ridge. It require stress reversals in the web members.


I am not sure if it was the intention of the code to apply to other structures.

 

[broncosfan]

Thanks. The building roof is framed with joists and joist girders. The ridge is right down the middle, 1200' long. The unbalanced snow requirement seems kind of harsh to me for this situation. I've seen a few structural drawings by other firms for similar buildings and it doesn't appear that they accounted for unbalanced snow loads either.

 

[prblmfxr]

Undoubtably, unbalanced snow loads will occur for this roof because when wind blows across the roof, it will remove snow in one spot and deposit it in another. The more important question is what will the magnitude of this unbalanced snow load be? IMHO, code provisions are conservative, but if you're in the consulting field, so what--it's only protecting your liability.