Should lateral-distortional buckling be considered when designing cold form cee rafters?

[FLR84]

Hi everyone,

According to the AS4600 commentary, lateral-distortional buckling is more likely to occur in beams such as a hollow flange beams which have stiff flanges. ("The high torsional rigidity of the tubular compression flange prevents it from twisting during lateral displacement").

If when designing cold form lipped cee channel rafters I do the lateral-distortional buckling check, then the capacity is reduced by sometimes up to 30%. The results will also disagree with several of my structural design computer programs.

If I ignore the lateral-distortional buckling check and only check flange-distortional buckling, then the calculation returns the expected capacity, and agrees with my structural design programs.

Should I be skipping the lateral-distortional buckling check in these situations? And if so, why?

Thanks anyone in advance for your help.

 

[phamENG]

Where are you getting the "expected capacity"?

Just because your structural design program tells you something doesn't mean you didn't make an error on the inputs to produce an incorrect result.

I don't use AS4600 (Australian code, correct?), but I do check LTB for all open sections. Check your unbraced length - I bet your analysis program assumes continually braced at the top flange? That would preclude LTB for gravity loading.

 

[Lomarandil]

Just to clarify, distortional buckling (in open cross-sections with edge-stiffened flanges) and lateral torsional buckling are separate limit states.

And yes, distortional buckling does often control the flexural strength of stiffened (lipped) cee sections. 30% sounds about right.

I agree that you shouldn't skip the limit state check (not without further information). You're welcome to use the (admittedly rough) SMath sheet in the open source group as additional validation. It's set up for AISI, but the overall results should be comparable to AS4600.



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just call me Lo.

 

[FLR84]

phamENG:
Thank you for your reply. I am checking the results against two other structural analysis/design programs and also against the designs of some of my competitors. Both other structural analysis programs agree with each other but I am not sure if they check lateral distortional buckling. And by the way it is not lateral torsional buckling which is in question. I always check that, of course. I am talking about lateral distortional buckling.

Lomarandil:
Thank you very much for your insight. Does lateral-distortional buckling still control the flexural strength when the member is restrained at regular intervals, for example by purlins and fly bracing? In this case, I would think that the connection to the purlin on the top flange and the fly bracing on the bottom flange would prevent the flange from being able to displace to the side, negating the possibility of lateral-distortional buckling. Am I wrong?

 

[Lomarandil]

AISI doesn't distinguish between lateral-distortional buckling and flexural-distortional buckling, so I'm not familiar with the distinction. Most distortional buckling modes with which I have seen illustrated do not have a significant lateral displacement component, primarily rotation of the compression flange. So I wouldn't consider a purlin to restrain that mode, but a purlin together with fly bracing might (depending on the details).



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just call me Lo.

 

[phamENG]

Oops. That's what I get for reading posts before my coffee. Sorry.

 

[FLR84]

 

[Lomarandil]

Ah, perfect illustration, thanks!

Yes, it's pretty clear that the lateral distortional mode is a higher-energy mode than flange distortion. Does AS4600 not have any clauses that allow the lateral distortional check to be bypassed (e.g. when the flange elements are open and not closed)?

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just call me Lo.

 

[FLR84]

AS4600 has you check the lateral buckling (lateral torsional buckling) member capacity before leading into the section on "Members subject to distortional buckling". It then states "The following cases, as appropriate, shall be considered" and presents the checks for flange rotational distortional buckling and lateral distortional.

So I guess it's up to the engineer's judgement whether or not to apply them.

 

[Lomarandil]

Yes, then I'd think you have a case to neglect it by judgement that the purlin will restrain lateral movement.

In that case, I'd think that your f[sub]od[/sub] distortional buckling stress value would be so high that even should the check be performed, it wouldn't control. But, I've never run that calculation myself.

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just call me Lo.