Torsion in I-Beam and Channels

[palves]

Does anybody have a good method for finding the torsional strength and stresses in I-beams and channels? I looked through the AISC design guide, but their process seems too cumbersome and time consuming for easy use. If that is the only way to do it then so be it, but if there is a simpler way out there I'd love to know about it.

 

[KootK]

I hear ya'. Google "Torsion + Bimoment method". Basically, you divide the beam in half vertically and consider it as two separate beams resisting the twist via weak axis bending.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[JoshPlumSE]

I agree with KootK. But, would also point out that this is described in the AISC design guide. Not the main emphasis of the design guide, of course.

Refer to figure 4.4 and section 4.1.4 (approximate shear and normal stresses due to warping on I shapes).

What I really like about this method is that it gives the engineer a physical sense of what these warping stresses are and why they develop. You can also extend this concept out to more complex situations (unusual loading, partial restraint, et cetera) with relative ease. Whereas the rigorous procedure can really only handle perfectly pinned or perfectly fixed end restraints.

 

[MacGruber22]

I agree with Josh. The approximate method that uses a force couple due to the shear stresses in the flanges is the way to go when you don't have hours to analyze.

 

[GregLocock]

C channels are a whole different ballgame of course.

Cheers

Greg Locock


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[KootK]

How so Greg? I treat them as a pair of angles rather than a pair of WT sections.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[GregLocock]

I've never tried your method, what sort of accuracy do you get on a C section with equal length legs and flange?

Cheers

Greg Locock


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[KootK]

I've never thought to check Greg. I'm usually using bi-moment because I'm lazy and I want to avoid the ugly "exact" calculation. Rationally, I just don't see how it's different than the situation with a W-beam. In the case of a channel, there's no need to restrict ourselves to hypothetical angles with equal legs.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.