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steel tube brace buckling

steel tube brace buckling

steel tube brace buckling

I am developing options for strengthening a steel tube brace that cannot be upsized due to architectural constraints.  One option I have considered in order to increase the buckling strength is filling the tube with concrete.  Does anyone know of a good reference (preferably based on U.S. codes) for calculating the buckling strenth of a concrete-filled tube?  This will not be a composite member, the concrete would be for stability only (i.e. increasing the radius of gyration).

RE: steel tube brace buckling

If you look in the silver AISC LRFD Manual, on page 5-67 thru 5-142, there are tables for tubes and pipes filled with concrete.  Now this does utilize composite action (see the discussion on page 5-67) per AISC Spec section I2.2.

By filling the tube, you will essentially create a composite action whether you utilize it or not.

RE: steel tube brace buckling

However, spec section I2.4 says that the force transfer mechanism to get the load into the concrete must be direct bearing.  I will not have this condition.  Is there any other reference available that discusses the stiffening effect of the concrete fill in resisting buckling?  I would like to be able to cite an authoritative reference in case the permit reviewer demands justification for this system beyond my say so.

RE: steel tube brace buckling

Try to go from here:

H.D. Wright, Local Stability of Filled and Encased Steel Sections, J.of Struct.Eng., Oct., 1995.

RE: steel tube brace buckling

Thanks for the tip chandr, I will try to track down that reference.  The title of the article seems to indicate that it deals with LOCAL buckling, though.  Is this correct?  The issue I'm trying to address is more global flexural buckling (KL/r) rather than local tube wall buckling (b/t).  Does the article also contain information on global buckling?

RE: steel tube brace buckling

Yes, the paper I quoted and its refs. talk mainly with local buckling.  If your problem is member bckl., it can't help.  In your case, if the filling concrete possesses very low shrinkage or is even expansive and the tube is end-closed, the concrete will be in more-or-less confined state.  Even though your framing is in steel and the force is carried to the brace by interaction among just steel elements, I believe the composite action is achieved.  Sorry that I can't give you any specific refs.  What I can recall is just one test emphasising the importance of end-closure plate in increasing the strength of  concrete-filled steel box beam.  If the flexure behavior is changed by end plates, the buckling strength will be surely affected.  If you can't find a ref., you might try modeling with tube in contact with concrete solid elms w/o friction (1/4 model), to see if end plate alone can increase the critical load.

RE: steel tube brace buckling

TS Brace,
Don't know of any direct publications that deal with this other than what you might find through AISC.  

One thing to note, though.  If you're trying to add to your radius of gyration by adding concrete, you need to remember that the r is based on the square root of I / A.  To increase r, you need to add to I in a way that I increase faster than A.  To add to I, you can go non-composite with the concrete, but it would only amount to the transformed concrete by itself, not using the total shape of the steel and concrete.

So the new I would be Is plus Ic.

In addition, the Ic by itself may not do you a whole lot of good as a singular piece of concrete within the tube as it will likely crack at a very low flexural stress.  Thus, composite action is really required to be effective in adding to your "r".

I didn't notice the I2.4 section...thanks for pointing that out.  Is there a way you can use a cap plate (per chandr) to create a bearing reaction at each end?

RE: steel tube brace buckling

I think you would be justified in looking at the thing as a fully composite member. You can justify the local compressive strength without composite action anyway and then design as a composite member for buckling. But as JAE pointed out, the concrete alone won't you give you much extra radius of gyration. What you could do is put a small circular steel reinforcing cage inside the tube in the concrete. If you wanted to ensure composite action you could drill trough the tube at intervals and insert a steel dowel plug welded in place and ground flush.

Another possible solution would be to put a cruciform piece of steel down the centre of the tube. Then cut holes in the tube at intervals so you can weld the tube to the internal cruciform. A bit messy perhaps but if it was the only solution it might work.

Carl Bauer

RE: steel tube brace buckling

Try "Guide to Stability Design Criteria for Metal Structures", 4th or 5th Editions,Ed: Theodore V Galambos, John Wiley.

Frank Hartzell
Jacobs Engineering
Conshohocken, PA

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