## 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?

I really appreciate your comments.

Thanks

Amy

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?

I really appreciate your comments.

Thanks

Amy

## RE: Unbraced length of beam-column

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## RE: Unbraced length of beam-column

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

## RE: Unbraced length of beam-column

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

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.