Determining the Elastic Buckling Stress of a Section
Determining the Elastic Buckling Stress of a Section
(OP)
This is a very general question and I suspect I'll be lambasted here, but, no risk, no reward....
The commentary of section F.12 of AISC 360-05 mentions that a designer can determine the elastic buckling stress for an unsymmetrical shape by using finite element analysis.
Is there anyone out there that explain how one might go about doing this...even very generally?
The commentary of section F.12 of AISC 360-05 mentions that a designer can determine the elastic buckling stress for an unsymmetrical shape by using finite element analysis.
Is there anyone out there that explain how one might go about doing this...even very generally?






RE: Determining the Elastic Buckling Stress of a Section
BA
RE: Determining the Elastic Buckling Stress of a Section
With FEA software, you do a eigenvalue analysis with the stiffness matrix. It's like doing modal analysis without any mass. The mode shape will correspond to buckling shape and eigenvalue correspond to force factor !
RE: Determining the Elastic Buckling Stress of a Section
RE: Determining the Elastic Buckling Stress of a Section
thread507-267603: Newmark's Numerical Procedures
BA
RE: Determining the Elastic Buckling Stress of a Section
Toad, somewhere on these forums are Finite Element forums and perhaps also some devoted to different FEM software, you might check there.
RE: Determining the Elastic Buckling Stress of a Section
I think you have a good point. The Newmark Method would not be able to detect torsional buckling.
BA
RE: Determining the Elastic Buckling Stress of a Section
For LTB buckling, I don't know of any formulas for critical buckling strength of unsymmetric sections. Though I would think the SSRC Guide might have something....
RE: Determining the Elastic Buckling Stress of a Section
BA
RE: Determining the Elastic Buckling Stress of a Section
... never to forget that this is ELASTIC buckling load.
RE: Determining the Elastic Buckling Stress of a Section
Where are those formulas? I'm looking at CSA S16-01 and I can't find it. Keep in mind that I was referring to Lateral Torsional Buckling.... i.e. Bending of laterally unsupported members. So, I looked in clause 13.6. All I see there is a reference to the SSRC guide.
RE: Determining the Elastic Buckling Stress of a Section
BA
RE: Determining the Elastic Buckling Stress of a Section
Don't you just love how Lateral-Torsional buckling is a limit state related to FLEXURAL loading. But, FLEXURAL-Torsional Buckling is a limit state related to axial loading. How easy is it to get those two flipped around in your head?
RE: Determining the Elastic Buckling Stress of a Section
RE: Determining the Elastic Buckling Stress of a Section
The sixth edition of the Guide to Stability Design Criteria for Metal Structures, doesn't even try to talk about LTB of arbitrary sections. It has things for standard types of shapes, but that's about it. Buckling is a complicated process and coming up with arbitrary formulas isn't realistic, unfortunately. If it's an issue regarding member stability and it's not in that book then there probably isn't a proven consensus method of analysis.
I haven't done it, but I'd likely model the entire beam as plate elements, load it, place nominal horizontal loads or a deformation at the point of load application and then iterate a p delta analysis. Alternately, you could see if your software has a built in buckling analysis and whether it's appropriate for this situation.
I'd just avoid the situation as much as possible, personally. I'd go as far as ignoring portions of the cross sectional area if that's what's necessary to make the section at least singly symmetric.
RE: Determining the Elastic Buckling Stress of a Section
Sorry about that. I thought the OP was talking about column buckling. I am not familiar with Chapter F. I agree with you that the terminology is a bit confusing.
Toad,
Are you talking here about columns or beams? I would think pure torsional buckling is possible with a cruciform shaped column.
TLHS,
I agree with avoiding the situation as much as possible, but ignoring portions of the cross section may not always be a safe method.
For example, a laterally unsupported WF could have a long vertical plate welded on top. The WF may be adequate on its own but the plate attached to the compression flange can make it unstable.
BA
RE: Determining the Elastic Buckling Stress of a Section
RE: Determining the Elastic Buckling Stress of a Section
So yes, it's a buckling failure. Fair point. I believe it does, however, still maintain the ultimate strength of the original section.
I will amend my above comment to say that I would feel comfortable ignoring parts of the section for the purpose of lateral torsional buckling if I could satisfy myself that the elements would not individually buckle under the stresses they would see when loaded as a section.