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Joint design for structural tubing

Joint design for structural tubing

Joint design for structural tubing

I have come across a joint with an approved design for its application but I would like to learn more about my doubts.

The joint is a mitered joint between two pieces of structural tubing. Instead of butt welding the ends together a 1/4" filler plate is placed between the two ends and a fillet weld is used to join each piece of tubing to the 1/4" filler plate. A picture is available at http://weldinginspectionsvcs.com/TubingFitupQuestion.htm

My questions are

1) Is the filler plate tensile strength lower in the through thickness direction than the nominal strength of the plate.

2) Can the residual stresses from welding cause residual stress in the plate greater than would normally be encountered in a "typical joint".

3) Has anyone encountered this type of joint before. And wht type of service.

Any refernces to papers or publications for supporting documentation/research would be appreciated.

G Austin

RE: Joint design for structural tubing

The filler plate would not be as much of a concern as the effecience of the fillet welds vs typical full penetration V joints.  

The stuctural integity question requires more information like tubing thickness, material, weld size, material ect.


RE: Joint design for structural tubing

G. Austin---Yes I have used this type of 'joint' once at the ARCO refinery in Wilmington, CA.  It was used where repairs were needed on a damaged pipe rack.  The problem was access to achieve a decent FP weld joint. the bent tubing was replaced by a short section of equal(?) tube and welded at both ends as you describe.  That was in 1983 I think and it's still there as far as I know.  It is not a very pretty splice and it was actually MORE difficult to implement than a standard FP butt weld.  It was deffinately NOT my first choice.


RE: Joint design for structural tubing

(1) Filler plate tensile strength will be lower in the through-thickness direction, but not appreciably so with modern plates. It usually doesn't matter because the cross-sectional area is increased by the spread of the fillet weld. For instance, with a 0.25" wall you would probably use a 5/16" leg-length fillet weld, giving a 25% increase in the cross-sectional area.

(2) Residual stresses will result from welds both sides, but should be no greater than normal. Look at the joint from the other way, and you have a pair of 5/16" fillet welds onto a 1/4" web. Nothing strange about that. Residual stresses don't affect the ultimate strength of "good" steel. If your plate was 1.5" thick, then I would be cautious about through thickness properties and residual stresses. Remember it all comes from billets 10" thick. Rolling down to 1.5" doesn't dispurse the defects nearly as much.

(3) A plate through the common plane of two sections with mitre cuts is one of the standard methods of making the moment joint where a building rafter meets the column. I use it regularly for such joints with RHS sections. I usually make the plate thicker - say 3/8" for 1/4" wall.

Reference Material: Being Australian, I have access to the Australian Institute of Steel Construction publications. I recommend their "Design of Structural Steel Hollow Section Connections", or "Design of Structural Connections". Try www.aisc.com.au - they might airmail them to you.

Russell Keays

RE: Joint design for structural tubing

CORUS (formerly British Steel) designate this type of joint as a "Reinforced Knee Joint".  The name gives you a clue that this type of joint is considered to be structurally stronger than a butt-welded mitred joint.

The short - hand advice in their pamphlet "Design of SHS Welded Joints" is that provided that the thickness of the plate is 1.5 times thicker than the tube wall thickness, then the joint will be 100% efficient.


Andy Machon


RE: Joint design for structural tubing

Thanks to all who responded. All of the information provided was useful and re-inforced my feeling that the enginneers and designers are usually correct in their decisions but the un-trained (myself) don't understand why.

Gerald Austin

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