Round HSS design for torsion: Member length 'L' in section H3 of AISC 360-10 2

[SuG89]

Hello,

Please refer to the attached image which shows a round HSS spanning 20" with a nodal moment of 100 kip-in acting at beam center. As per section H3 of AISC 360-10, I have to use the length of member to calculate Fcr value. Does this 'length of member' mean only that portion of the beam which has an internal torsion due to the applied moment or does it mean the whole member length between supports irrespective of the internal forces?

Regards,
Su

 

[JAE]

Is your acting torsion force also fixed translationally? If so I'd say L = 10 inches.
If not I'd think 20 inches.

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[JoshPlumSE]

I'd think it would be 20". That equation, as I understand it, is meant to account for how local buckling issues can be affected by end conditions. So, unless there is some kind of significant rotational restraint provided at mid-point of the HSS, I'd have to believe L would be 20" for that equation.

 

[SuG89]

Thanks to both of you for replying!

I asked AISC and they confirmed that L is always the span length no matter where the torsion is applied.

Regards,
Su