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# Why do G-Factors for effective length factors use I about the strong axis?

## Why do G-Factors for effective length factors use I about the strong axis?

(OP)
Hello,

This is likely to be a very basic question but as most structural engineers know, AISC calculates the effective length of frames in section C7.2 utilizing stiffness ratios (G). My question is this: why are the stiffness ratios calculated using the beam/girder's moment of inertia about their strong axis? Both the column and the beam are in compression so they would buckle about their weak axis prior to buckling about their strong axis which makes me think I'm missing something simple that I'm not thinking of.

Thanks for any help in advance!

### RE: Why do G-Factors for effective length factors use I about the strong axis?

It's totally dependent on the direction of bending because it's all about stiffness.

I don't think that commentary ever says anything about the strong axis. Rather, it's based on the direction of bending and the relative stiffness of the members that are bending.

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