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Unbalanced Snow Load IBC 2000
2

Unbalanced Snow Load IBC 2000

Unbalanced Snow Load IBC 2000

(OP)
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!

RE: Unbalanced Snow Load IBC 2000

So why dont you design with an unbalanced snow load.

RE: Unbalanced Snow Load IBC 2000

(OP)
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.

RE: Unbalanced Snow Load IBC 2000

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.   

RE: Unbalanced Snow Load IBC 2000

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.

RE: Unbalanced Snow Load IBC 2000

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
www.phoenix-engineer.com

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