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# Unbraced length of beam-column

## Unbraced length of beam-column

(OP)
Hi All,
I am designing an industrial building. The perimeter beam has axial force as well as moments. The beam is simply supported at two ends and laterally braced at the top compression flange at every L/4 points. The total length of the beam is 16m. I know for lateral torsional buckling I can use braced length= L/4=4m. My question is when I am calculating the axial capacity of the member what will be the length? Is it the total length of the member (16m) or the L/4(4m). For compression member kL/r <=200. In this case what will be the value of L?
Thanks
Amy

### RE: Unbraced length of beam-column

For KL/r we would usually assume your unbraced length is 4m.

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### RE: Unbraced length of beam-column

It depends on how the member is braced.... Even the strength and stiffness of the bracing. The AISC appendix on bracing should give you some ideas (at least about strength and stiffness). I have a hard time justifying steel decking and such as bracing against weak axis buckling under axial loads. It can be done, I suppose. But, it would (in my opinion) require some extra justification.

For whether the top flange bracing can brace weak axis buckling of the beam under axial load, it's usually a reasonable engineering judgment call. What's bracing the top flange, does that prevent lateral translation of the whole beam or, only of the top compression flange.

### RE: Unbraced length of beam-column

If this is a member which has virtually no axial force (like many beams) and you are really just worried about the KL/r ratio, then you have some more leeway. I usually look at the buckling strength equations and determine what unbraced length would be required to satisfy the axial compression demand (ignoring KL/r restrictions) and make sure that this is provided in some way. If the demand (when considering the full member unbraced) is greater than some reasonable percentage of my axial capacity, then I would want to provide some lateral bracing somewhere rather than relying on top flange bracing exclusively.

### RE: Unbraced length of beam-column

From your description, there are three buckling modes for axial compression: major-axis flexural buckling, minor-axis flexural buckling, and constrained axis torsional buckling (CATB).

The unbraced length for major-axis FB is 16 m.

The unbraced length for minor-axis FB is 4 m.

With CATB, the bottom flange can move laterally, but the entire section is constrained to buckle about the top flange. The unbraced length is 16 m. Information on this mode is in the 15th ed. AISC Commentary to Chapter E, in the AISC Design Guide 25, and in the AISC Engineering Journal by Liu et al. (2013).

### RE: Unbraced length of beam-column

X2 for 271828's recommendation. That's exactly how i understand it. CATB is a bit of a pain to calculate, particularly as I don't know of any commercial software package set up for it. The constrained axis part does a lot to improve capacity though.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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