Unbalanced Snow Loads

[4thorns]

I'm designing a rafter framed residential roof. Rafter span is less than 20'. Based on ASCE 7-05 I have to use I*Pg for unbalanced snow load and it will control. See this thread (

http://www.eng-tips.com/viewthread.cfm?qid=308556).

The pitch of this roof is 12/12 (that's 45 degrees!) Do I seriously have to use an unfactorred Ground Snow Load to determine rafter size? I haven't found anything telling me otherwise. Am I missing something here?

 

[SteelPE]

I believe ASCE 7-10 requires unbalanced snow load cases to be considered for roofs with slopes between 1/2-12 to 7-12 otherwise unbalanced loads need not be considered. However, if your jurisdiction is using ASCE7-05 I don't believe this "out" is provided.

 

[focuseng]

That is what it says...
Alternative could just be a prescriptive code span table, of which there are many. How do they get away without that unbalanced loading? Maybe ASCE 7 will add a couple chapters next time to address this properly.

MAP

 

[4thorns]

Thanks for the replies guys. Figure 7-2 shows some pretty significant differences in slippery unobstructed surfaces and all others at a 12/12 pitch. I don't understand why this doesn't translate into reductions in the unbalanced load as well. 7-05 says that snow isn't expected to stick at slopes > or = 70 degrees. To me that means that someone got eg. 90lbs per square foot of snow to stick on a steel roof at a 12 pitch. I understand the need for a margin of safety but that seems like quite a reach. Bear in mind that I haven't tested it myself.