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Hemi79

KL/r >200 in compression FOR A BEAM

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I wanted to bring this topic up even though I know there is stuff out there in google that talk about it. For those who feel confident in this topic and would like to share their knowledge and practice, very much appreciated. So here I go. 

If i have a beam (designed in flexure) yet my Kl/r is greater than 200. Isn't it as simple as bracing the member laterally? Therefore, no longer having Kl/r >200? Some have mentioned about checking that the compression stress in the member isn't greater than the critical stress from AISC equation E3-3 (Fcr), however, that would probably mean you barely have any load on the member. 

Anyways, the biggest question from me is some have mentioned Kl/r > 200, for members NOT designed in compression, should just be ignored. Thats scary to me. If in fact my compression stress, in a flexural member, is greater than the critical stress caclulated then that would mean the member would fail due to flexural buckling.  Am I missing something? 

Edited by Hemi79

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Please refer to AISC 360-10 E2 Commentary. There is a very interesting discussion on the slenderness ratio. It illustrates how the Slenderness ratio started and how it moved from being mandatory to just a recommendation for compression members. Furthermore, flexural members have no limitations in AISC for KL/r. AISC Table F1.1 has a list of all the possible limit states that a flexural member has to be designed for and "Flexural Buckling" is not one of them. You might mean "Lateral Torsional Buckling" or "Local Buckling" and these are all covered in chapter F.
Let me know if you need anything else.
 

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If your beam subjected to compression and bending, then you have to check the beam for both of them (by considering the KL/r ratio for compression only) and the beam have to be checked for combined stress per AISC 360 chapter H (for combined forces) while if your beam subjected to bending only then I'd say to ignore the KL/r ratio because you have to check Y, LTB, FLB, WLB ....etc per AISC 360 chapter F base on the the beam condition.

Edited by zaidfadhill
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I've actually used a wide flange beam in light compression with KL/r > 200. Before you freak out it was being used as a non-critical lifting strap brace to avoid loading up a large precast piece in compression from the straps (basically using the beam as a spreader bar). It was very interesting experience, the beam becomes very flexible and we had it laying sideways so there was a noticeable weak-axis sag in it from the self-weight. However, we were using only like 20% of the compression capacity (including the P-delta effects) so it was a non-issue.

The pieces lifted fine and the beam was used many times until someone ran over it with a heavy forklift and bent the flanges. Really goes to show that the KL/r > 200 is really just a good recommendation but that if there is justification and engineering judgement involved you can definitely exceed that "limit". As Kamba said, read the E2 commentary, it's really good info on why that recommendation is there.

For your specific question, a KL/r > 200 beam will work fine assuming it truly takes no compression. The erectors will likely need to brace the beam to get things connected without it flopping all over the place but once installed global buckling shouldn't matter as much. Some engineering judgement will be needed and I'd throw in a small brace just as cheap insurance but really it should be good without it.

Edited by TehMightyEngineer
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Thanks for the reply everyone! Much appreciated. I went ahead and used the members. I braced at 2 points, 3 equal spaces on the beam, and so therefore Kl/r is no longer greater than 200. However, It was freaking me out. 

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