Gap distance when welding RHS, is it required?

[Grindy]

Hello,

I've been tasked with designing and fabricating a steel structure to support a sand hopper. I have done a lot of MIG welding in the past which has been successful, but due to the weight of the sand hopper acting vertically on the steel framed structure I am seeking advice as to the gaps required (or not) between each section of RHS to be welded.

RHS measures 1200mm x 60mm x 6mm (Hybox 355)

The structure is 2mtrs tall with a footprint of approx 1mtr x 800 mm.

The sand hopper will be placed above the frame that I am to fabricate in order to support it. Approx weight 1500kg.

I was planning on mitreing 4 sections of RHS to make the top of the structure then having 4 legs extending down to the ground.

In the past (previous fabrications) I have simply ground back the edges of the RHS at about 45 degrees leaving a root of about 2mm. I then butt the two sections together and weld them.

However due to the weight of the sand hopper I have been trying to find out how best to weld the structure to ensure it is as safe and strong as possible. Various texts have said that a gap is required between the faces to be welded. E.g. 0-3mm, 3-7mm etc. However I have never done this with previous structures I have fabricated and they have all proved to be strong and supportive.

Do you need gaps between each RHS section or this only when thick plate is being welded together?

Is the gap required to ensure full penetration?

If I do need a gap between each piece of RHS to be welded together, do I simply slip spacers between each, tack it then pull out the spacers and fully weld?

Apologies for the long winded question, and thank you in advance for any advice.

Grindy

 

[mock1]

I've been a welder for 20 years and I've welded a lot of structures. Some with zero gaps and some with gaps big enough to stick your finger in. In my experience it is really the quality of the welder, and the type of weld your'e gonna do on the structure. You mention that you've done some MIG, is that how your'e gonna weld this out?
If so I would use a .045 s-70 flux-cored wire with a 75/25 carbon dioxide/ argon mix and you will have no prob with penetration.
Personally I like stick welding, it's a little slower but you can watch the metals tie in to each other and you know that you not only have penetration but a solid seal.

 

[Grindy]

Thank you Mock1 for the advice. I am going to be MIG welding the structure. I can stick weld but I'm not great at it so it can be a little hit and miss depending on the positioning, so I tend to use MIG. I'll get myself some of the wire you recommend.

 

[mock1]

That wire welds great in the uphill potsition, and I use the 1001-1002 count to weave it. it will look like or should look like a rattlesnake tail when the flux falls off.

 

[Ron]

Grindy...the gap is there to primarily give good root fusion. It does not guarantee full penetration. You're not likely to get full penetration without specific root treatment and, preferably, backing.

With something this heavy and with the dynamic loading involved, I would get some analytical design done for the welds and the structure. "When in doubt, make it stout" doesn't always work.

 

[Grindy]

Ron: I've never used backing. Am I correct in saying it is thin sheet steel placed behind the weld? Is backing only used when there is a gap between the parts being welded?

The sand hopper doesn't sit directly over each support leg as the structure features a small over hang on the front and sits within the support legs. Structural analysis (SolidWorks Cosmos) has shown that the horizontal mitred sections (which the hopper sits directly on top of) have a safety factor of 8. Analysis results for the over hang show a deflection of 0.08mm.

How would I go about doing analytical design done for the welds?

 

[Ron]

You have a very deep, narrow section. I would be concerned, if the loads are high, that you would buckle the walls of the tube, assuming I'm properly understanding your configuration.

With a 6mm thickness, I would prepare the joint with a single Vee groove, a 1.5mm land and a 1.5mm gap. Backing can be a 3mm thick steel sheet. This allows you to penetrate the root and get good fusion throughout.

Since your material is a high strength, cold formed material, I would be concerned about weldability and potential cracking in the thin section on cooling. Check with your consumables supplier for compatible wire for the material you are welding.

As for weld stresses, again if I'm understanding your configuration, most of the weld stress will be in shear, unless you have some lateral loading that would be pushing from the side onto your tube. If that's the case, you'll have both bending moment and shear on your welds, and possibly in two planes at once.

 

[Grindy]

According to Corus (manufacturers of Hybox 355)the RHS is weldable.

Ron: You are correct with regard to the welds being subject to sheer stress in the structure I have designed. There are no lateral loads acting on the side of the tube.

Can anyone recommend me a good book about welding, preferably one that contains the sort of helpful information that has been posted so far? E.g. Weld/root gaps, backing, required angles, stress' etc.

If I have a gap (1.5mm as suggested above) when doing a butt weld, do I concentrate the MIG torch/arc at the gap doing a slight weave to join the two sections and then go back over to fill. Or, do I do a wider weave and join and fill at the same time. At what point do you decide whether a joint needs multiple runs? Is it above 6mm, 10mm, 15mm?

 

[Ron]

I would do a 6mm weld without backing in three passes. Lean the first pass to one side of the root, lean the second pass to the opposite side of the root, and use the third pass as a cap pass. Putting the first pass deep into the gap will only cause blow-through. If you have a backing plate, you can weld it most any way you want, including the first pass directly into the gap. You can weave the pass to get it done in one pass with backing.

As for references, one of the better welding references I have and have seen, is "Metals and How to Weld Them" published by the Lincoln Arc Welding Foundation.

 

[mock1]

Is that one better than the Hobart book?