Unbalanced Snow Load IBC 2000 2

[cmrazik]

I am designing roof rafters for a single family residence. If I apply an unbalanced snow load, the rafters in the IBC Residential Code tables do not work. (My model includes a ceiling joist). I can only get the rafters in the table to work with balanced snow loads.

Does anybody know of a stipulation in the Code where you do not have to apply unbalanced snow loads to residential buildings?

Thanks!

 

[chicopee]

So why dont you design with an unbalanced snow load.

 

[cmrazik]

I don't want to design the roof for unbalanced snow load if I don't have to. I was already accused by another engineer of being overconservative with the roof design on a previous residential design project. I would like to know definatively either way.

 

[sundale]

The leeward drifts from the ASCE 7 hammer a rafter design with 150% Pf. As I understand it, this is what the IBC says to do irrespective of occupany (I=1.0).

You are governed by the IRC. The IBC is a code book whereas I view the IRC as more of a recipe book. I am doing good to stay up with the IBC changes every three years and thus do not profess to know that much about the IRC. Look in the IRC and I expect it says the span tables are OK w/o drifting. IRC R301.2.3.

For what it's worth, I design any rafter and its connections for the drifted snow condition.

 

[JAE]

Check whether IBC and ASCE 7 even govern this design. Usually, the IRC controls residential design and you may not have to even refer to the other two.

 

[zcp]

If it is single family with 3 stories or less, then IRC is your code. If your ground snow load is 70 psf or less, you use the tables in IRC (chap 5,6 and 8). I don't have the commentary handy, but I think your issue is taken into account in the table values.

ZCP

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