LTB of cantilevers to AISC codes 1

[Ussuri]

We are currently looking at a design in our office which has been completed using AISC WSD codes (March 2005). We have a query which I was hoping some of the US based 'tippers' might be able to help with. The topic was discussed briefly in 2007 but the links provided no longer work.

thread507-180478

We have a single cantilever effectively built in at one end providing both lateral and torsional restraint. The section is a 305UC137 (section classification plastic UK/compact US). The other end is completely free. The load is applied to the tension flange (destabilizing).

The UK codes provide effective length factors for use when calculating LTB resistance, depending on end fixity. These range from 1.4 up to 7.5. In our case the effective length Le = 1.4L.

We were looking for something similar in the AISC code but couldn't see anything. LTB capacity seems to be based on Lb, the distance between two braced points, so not applicable for a completely free cantilever (Section F2). The only other reference seems to be Section J10 part 7 which dicusses unframed ends.

How does the AISC address lateral torsional buckling of free cantilevers? What are we missing in the code?

 

[JJOO]

I actually do not agree with the reasoning behind using Cb=1.0 and Lb=cant. length for LTB of cantilever in the reference WillisV provided. Dr. Zoruba says Cb=2.0 approx. for cant. and since K=2.0, the two factors would cancel out. That doesn't make sense..

AISC 13th Ed. mentions that "for cantilevers or overhangs where the free end is unbraced, Cb=1.0" I'd use Cb=1.0 in any case for cantilevers.

Salmon and Johnson in Steel Structures (4th Ed.) also explains LTB effective unbraced length. According to them, "LTB of a cantilever beam is not even as severe as the unbraced segment under uniform moment... Since the momemt at the free end of the cantilever is zero, the compression force in the flange decreases from a maximum at one end to zero at the free end; thus, the loading is less severe than if the compression force were constant over the entire length." They do recommend using the actual cantilever length as the effective laterally unbraced length.

 

[BAretired]

I'm not sure I can think of a practical example, but if a cantilever is loaded with a moment at its free end, the moment is constant throughout its length. In that case, it would be unsafe to consider the effective length Lb unless the free end is laterally braced.



Best regards,

BA