Mitered Tube Steel- Small Heel Angle Weld

[Gumpmaster]

I have a mitered tube steel connection with a small angle between the chord and the branch, 16 degrees. AWS D1.1 Figure 3.5 (AISC 13th edition manual Table 8-2) limits you to a 30degree mininum heel angle (upsilon). The chord is a 10 inch tube and the branch is a 6 inch tube.

Does anyone know of a pre-qualified joint that would allow such a small angle? If there's not a pre-qualified joint, is it possible, and if so how what would the fabricator need to do to ensure compliance with D1.1? Would the fabricator need to use a certain process?

I think there's joints like this fairly often in tube steel car space frames, but those are much smaller diameter tubes.

Thanks for any advice you can give.

 

[MikeHalloran]

In a former life, I worked in a marine exhaust shop, where those would be small to medium size tubes, and the branch angle not extraordinarily small.

We didn't work to AWS or any other standard except "weld the sh!t out of it".

The only slightly unusual part of the process for that part would be to extend the tungsten as necessary to reach the weld area on the acute angle part of the joint.

Sorry that doesn't answer your actual question.

Mike Halloran
Pembroke Pines, FL, USA

 

[racookpe1978]

How thick? The effective weld may extend through the tube wall anyway, almost regardless of the prep angle.

Are you back purging?

 

[Gumpmaster]

The wall thickenss of the branch is 0.280 inches. I'm not really sure what back purging is. This is the first time I've specified a mitered tube joint, is it something specific to that? Back gouging?

I was thinking more of a fillet than a PJP or CJP weld, although if PJP or CJP are all that would work with this geometry, that would be OK.

 

[berkshire]

gumpmaster
Back purging is the practice of preventing the back side of a weld from oxidizing. Usually by adding an inert gas to the inside of the tubes you are trying to weld together.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[racookpe1978]

With that small of a miter joint angle, the acute angle will be a fillet - regardless of what is specified by the drawing! - simply because the weld material as it solidifies from the molten steel will build up that way. It is almost impossible to insert melted filler metal "down" into the deep "V" of the acute angle side of the miter angle small, so the final weld fills the gap but doesn't really act like a perfect joint.

The obtuse side of the miter joint will actually behave like a butt weld under stress - and actually look much more like a butt weld joint than a fillet - again, regardless of what is on the drawing.

Calculate your weld bead as if they were fillets, weld them in the shop so the weld is never smaller than the pipe walls being welded as if they were butt welds. Keep the joint clean with a typical 37 degree taper through the pipe wall and a 1/16 flat.

This is why pre-fabbed fittings for Y's and Tee's and weld-o-lets were invented.

 

[berkshire]

One of the things to watch out for, is that the weld joint will climb up the tubes, unless the operator takes great care. So the weld profile is not even around the joint, and the result is like a bishops mitre.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[Mr168]

To answer the latter part of the original question, once you're outside of 30 degrees acute in D1.1 for tubular connections, you're out of prequalified space. As such, the fabricator will need to run separate qualifications per Clause 4 for tubular T/Y/K connections, the requirements of which are based on the joint detail you've specified.

 

[Gumpmaster]

Good information everyone. Thank you. We'll see how it turns out.