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Compact Tubulars in Bending

Compact Tubulars in Bending

Compact Tubulars in Bending

(OP)
Hi
Got a quick one !
If I have a simply supported Tubular Member supported at the ends and simply loaded in the middle, no end compressive loads (CHS is totally compact with od 355 and wt 16) do I need to look at Buckling as well as Bending ?
My argument is that if its Compact then there is no need to analyse Buckling (as with I Beams) but am I correct ?

thanks

RE: Compact Tubulars in Bending

What manner of buckling are you referring to?

While it's unusual, high aspect ratio tubes can laterally torsionally buckle and some thought should be given to evaluating that.

At the supports, you've got web crippling and yielding to deal with as you would with a wide flange. Again, with two webs and thick-ish walls, it's unlikely to be a problem but compactness alone doesn't guarantee that. Compactness is about the sections ability to resist longitudinal compression stresses without buckling locally. The stresses here are really transverse.

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.

RE: Compact Tubulars in Bending

(OP)
Its just local Buckling and as the section is basically a pipe simply supported at the ends, with a D/t < 23 it is compact enough, I think, to say that Bending is the main criteria ?

RE: Compact Tubulars in Bending

Ah, I didn't catch the CHS bit. I suspect that bending probably is the main criteria but there's still potential for wall crippling. What does your support detail look like? I've not seen a case where a circular tube just sat there in plain bearing without a fabricated bearing assembly of some kind.

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.

RE: Compact Tubulars in Bending

(OP)
Yeah its basically a Tubular CHS 4m long sat on top of 2 RHS legs, essentially a Portal Frame but its the top member I'm more concerned about rather than the legs (which is a separate issue).
The Design is an 'A' Frame for deploying equipment over the side of a vessel which is just suspended in the middle of the CHS. The ends of the CHS are essentially free to rotate.

RE: Compact Tubulars in Bending

How wide are the RHS legs perpendicular to the CHS? Is the CHS welded into circular arc cutouts in the RHS?

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.

RE: Compact Tubulars in Bending

(OP)
the Tubular is 355 dia and the supporting vertical legs 400 (x 200) wide. and yes they are simply profiled to take the 355 dia.

RE: Compact Tubulars in Bending

You should be fine then as the end reaction will be distributed over a good chunk of the vertical sides of the CHS instead of being delivered as concentrated loads against the walls.

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.

RE: Compact Tubulars in Bending

(OP)
Thanks, much appreciated

thumbsup2

RE: Compact Tubulars in Bending

Anytime. Sometimes it's nice to have a straight forward thread that wraps up in short order.

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.

RE: Compact Tubulars in Bending

If you're looking for some code backup, AISC Spec F8.2(a) states that in flexure, local buckling does not apply to compact sections.

RE: Compact Tubulars in Bending

Can you share some details of how the equipment attaches that results in the tube being "simply loaded in the middle" ?

RE: Compact Tubulars in Bending

(OP)
Thanks for the Code reference, I see thats stated as for Limit State Design, I assume its still relevant for ASD design ?
Same principles ?

RE: Compact Tubulars in Bending

Same principles but I don't believe that they apply to your situation in the first place. Local buckling under concentrated loads is a different beast from local buckling under flexure. It's the same with a wide flange. Having a compact web doesn't get you out of checking web crippping or yielding.

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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