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weld designation on back to back channels
5

weld designation on back to back channels

weld designation on back to back channels

(OP)
I want to weld two steel channels back to back to hold them together for gravity loading. I don't think there is really a strength requirement but I want to keep them together. I was thinking of using a stitch weld (say 3-6) along the seam at top and bottom, but I am not sure what type of weld to designate. Is this a square groove weld symbol?

RE: weld designation on back to back channels

Different channels have different radii (rolled heels). Our shop will use the AWS D1.1 flare groove that states differing weld sizing for various radii and welding processes. (Pg. 85, 2010)

RE: weld designation on back to back channels

(OP)
I am not sure I understand your meaning holdpt. There is no radius at the back edge. I want to put these back to back and weld along the interface of where the webs meet at the top and bottom. It is like a butt weld but doesn't need to go through the full material thickness because it is a 10" channel. But I am not sure how to call out this weld.

RE: weld designation on back to back channels

Inconsistencies of the radii have caused us problems for other reasons than welding. If there is no radius to speak of, then my opinion is you can only refer to the square groove.

RE: weld designation on back to back channels

(OP)
So is the image of the weld callout shown correctly?

RE: weld designation on back to back channels

I usually spec it as a butt weld... two parallel lines on the welding symbol.

Dik

RE: weld designation on back to back channels

As shown in the preceding sketch, the weld is a square groove, welded from both sides (two arrows), and the required joint penetration is complete. I'm not sure that is what was intended.

I suggest AWS D1.1 be reviewed to determine the minimum weld size required based on metallurgical considerations. The back of the channels can be beveled to ensure the depth of the groove is sufficient to provide the required joint penetration dictated by AWS D1.1. The actual joint penetration is a function of the thickness of the base metal at the location where the welds are placed.

Best regards - Al

RE: weld designation on back to back channels

(OP)
I don't really need it or want it to be a CJP. I just want them connected with some weld. It is not required for strength. What is the best way to show that? Should I leave it as shown in the sketch, but arbitrarily add a 1/4" size to it, or what?

RE: weld designation on back to back channels

Replace the square groove symbol with a V-groove. To the left of the "V" symbol place the numbers (5/16) indicating the required joint penetration is 5/16 inch.

If you are the designer, not the fabricator, you can simply place "(5/16)" under the reference line indicating the required weld size (joint penetration). The fabricator would be responsible for the selection and detailing of the actual partial joint penetration groove.

Best regards - Al

RE: weld designation on back to back channels

(OP)
Thanks gtaw. I am the designer but obviously not a weld expert. In school the only thing we ever talked about was fillet welds. Just trying to figure out the rest as I go along.

But to make sure I am clear, are you saying that there is there no way to weld this without a bevel? I can't just set them back-to-back and weld along the top and bottom without beveling or gouging the channel first?

RE: weld designation on back to back channels

As a ruler of thumb (which means it doesn't really work) is to assume 0.001 inch of joint penetration per ampere. According to the "rule", you should achieve 1/8 inch of penetration if you weld with 125 amps. The truth is, you will not get that much. Some form of preparation ensures, or at least help ensure, the weld size is what the designer expects it to be. One would rather have more weld than necessary rather than find out there wasn't enough to hold the parts together.

Best regards - Al

RE: weld designation on back to back channels

You could put straps across the top and bottom or just a continuous strip and use fillet welds.

The reason for putting in a groove is that you can be certain that the parts are actually welded together. Without weld prep you can get anything from zero penetration to an unknown amount and they will both look the same.

The original weld concept is what I'd call a hope-weld, as in you hope it is welded.

Don't hope. Be certain.

RE: weld designation on back to back channels

You really need to rethink this.

You're putting welds of unknown penetration and thus unknown strength at the highest stress location of the beam.

And intermittent to boot. If there is any cyclic loading this is a certain recipe for fatigue cracks.

RE: weld designation on back to back channels

If all you wish to do is keep the two channels aligned under 1g static loading, then a double bevel stitch weld along both sides will be sufficient. The bevel depth probably only needs to be about 1/4", and 1" of weld per foot should provide more than enough strength. The only other consideration is whether the weld profile can be crowned or must be flush.

If the channels are long, you might also want to provide instructions to the welder as to the sequence the welds should be made. With long parts, the weld sequence should start at the center and alternate side-to-side and end-to-end, in order to minimize distortion.

RE: weld designation on back to back channels

I don't know your application, but you should consider reviewing the requirements of AWS D1.1 Structural Welding Code/Steel.

Best regards - Al

RE: weld designation on back to back channels

Why don't you just specify an I-beam and stop welding all together?

Bill

RE: weld designation on back to back channels

Shaneelliss:
If your loading condition distributes the load to both channels equally, no weld would be required, except that you want the two channels to brace each other and act in unison. You have to use some engineering judgement in determining what connecting weld is required. If your load is applied to only one channel, and you expect it to transmit half that load to the second channel, then the weld must transmit that shear flow to the second channel. In the first case, I think I would grind or nibble a .125 or .1875" chamfer on both heel corners of one channel and fill it (concave) with weld, paying some attention to sequencing the welding to control final camber. If there are concentrated loads or real load transfer needed then some weld design is needed.

RE: weld designation on back to back channels

(OP)
Thanks all for the comments. I am doubling a channel because one channel is already in place but it is not enough, by about half. So I am simply adding an identical channel next to it. Some steel grating is resting on the channels and theoretically each channel will take half the load and they don't need to be connected at all. As dhengr points out, I just want to connect them to brace each other and to act mostly in unison. I think the take-away I am getting here is that I can't really just have them do a surface weld at the seam, that I must have them create at least a one sided bevel for even the small welds I would like to see.

I just looked up AWS D1.1 and the cost for that shocked me a bit. And I thought the AISC steel construction manual was expensive.

Thanks again.

RE: weld designation on back to back channels

You have it exactly:
A) Without some bevel, 'face' welds have too little strength to use in a design load.

B) AWS D1.1 is a Code Book and thus quite expensive, just like the rest of 'em.

RE: weld designation on back to back channels

2
Punch some holes in the web of your new channel and ring weld or plug weld to the web of the in-place channel.

RE: weld designation on back to back channels

I like MintJulep's suggestion. This approach sounds like it provides the easiest access and least amount of prep work for making the welds in-place.

As a side note, I'm always a bit puzzled when I hear people complain about the cost of technical reference materials needed to perform their work. The AWS D1.1 document is probably only a couple hundred bucks, which is a rather modest sum in relation to the total engineering costs of even a small project. A couple hundred bucks is nothing compared to the incredible value of all the excellent "free" engineering advice you have received from this forum!

RE: weld designation on back to back channels

RE D1.1: at first glance, is is a $500 paperback that will go out date [theoretically] in a couple of years.

The reality is that the design and welding knowlege contained in D1.1, plus the underlying causes and thinking shown in the Commentary in the back, exceedes any two collegeate engineering classes any of us took. Makes the $500 look pretty cheap to me. and if your D1.1 is less than 20-years old, keep using it - facts are facts, and welding is welding

RE: weld designation on back to back channels


Quote (shaneelliss (Structural) )

25 Mar 14 16:09
Thanks all for the comments. I am doubling a channel because one channel is already in place but it is not enough, by about half. So I am simply adding an identical channel next to it. Some steel grating is resting on the channels and theoretically each channel will take half the load and they don't need to be connected at all. As dhengr points out, I just want to connect them to brace each other and to act mostly in unison. I think the take-away I am getting here is that I can't really just have them do a surface weld at the seam, that I must have them create at least a one sided bevel for even the small welds I would like to see.

I'm going to strongly disagree with you - not exactly with the intent of your solution, but your execution is gong to break.

Failure mode will likely be: Assume you grind a weld prep "v" into BOTH sides of the joint on both top and bottom sides evenly and perfectly. And then you weld both sides together evenly and perfectly so there are no stresses or uneven heating and cooling and exactly even penetration on both sides of both channels evenly on both top and bottom exactly offsetting each other so - yes! - both beams will carry half of the load.

If that all happens, then - yes! - both beams split the loads.

Ain't gonna happen in real life. Worse, what will happen in real life is that the upper weld bead WILL vary unevenly in height above the flat top of the two channels, and in penetration below both sides of the two channels.

Now, when uneven loads are put on the gratings ABOVE the uneven weld above the two channels, the higher of one side of the one of the stitch welds is going to get ALL of the concentrated loads not from just the local weight, but all of the weight on the whole section of grating. It has to: That single little location is the only place where the grating touches the channel. That one half of one stitch weld is going to yield (force the half-beam down) or break.

Once that happens, your two channels begin reacting unpredictable as each section yields to a higher stress than it can handle. (You've already concluded that you need to double the existing channel to handle the expected loads, right?) so, this combination of single point yielding has noting really to stop each half-weld (of the two stitch welds) from catching all the actual loads at that point. The final will have the two half-beams sagging enough so they share the residual load in tension - if the two ends of the two channels are restrained by bolts or angle iron connections. If one is "tighter" than the other, then it will try to stop all of the load itself and likely fail. Then, all of the load falls to the remaining beam - which is also expected to fail.

Better, in my opinion, is NOT to weld the two beams. In particular, I strongly recommend NOT welding ANYTHING on the upper surface of either back-to-back channel! Rather, drill through a 5/8 or larger high tensile bolt every 2-1/2, 3, or 3-1/2 feet, and bolt the two channels together.

RE: weld designation on back to back channels

racookpe1978- you make some valid points. Obviously, we would need to see more details about how the loads are transferred from the grating to the beams, and how the beams are supported. The OP stated that gravity loading was the only concern, a later post stated that the weld strength was not a concern, and the last post stated the beam must support some steel grating and the existing channel was "not enough by about half", whatever that might imply (strength? stiffness?).

If strength is an issue, then load sharing between the channels is a big concern. If the grating transfers load equally to both channels, then stress at the welds should not be a problem. But say only the inner channel cap supports one free end of the grating, and the grating is loaded mid-span, then initially 100% of the load will pass thru the inner channel, and the contribution of the outer channel will be limited by the strength/stiffness of the welds connecting them.

If we accept that the existing single channel did not provide "about half" the strength needed to support the grating, then it would seem critical to ensure the pair of channels share the grating load equally. On one hand, it would seem difficult to produce a finished weld joint between the mating channel faces that had sufficient strength to reliably transfer loads amounting to 50% of one channel's capacity. So in this regard welding would seem problematic. The other approach would be to loosely constrain the channels (maybe loosely bolting the webs as suggested?) and allow them to independently deflect to distribute the grating loads.

Ultimately, the real problem with any approach is being able to show by analysis that the beam structure is not overstressed, including applying the appropriate factors of safety.

RE: weld designation on back to back channels

Forget the plug welds and bolt them together.

RE: weld designation on back to back channels

If it does not need to look nice, you could consider a small offset (say 5/16) in the height of the existing and new channels, then use sufficient fillet welds to handle:

1) longitudinal VQ/I type shear (almost none)
2) sufficient vertical shear to load up the lower beam.

I bet that once you actually size these welds for your factored loads, you'll find that almost any weld size will do. Best of all, there's no guessing on the load path, no drilling, no chamfering...

tg

RE: weld designation on back to back channels

If you want these channels to split the loads, you need to "unload" the existing channel before installing the new channel. Otherwise, the load the existing beam is already seeing will always be there and the new beam will only take 1/2 of any load applied after the modification.

I would unload the existing beam (by physcially providing temporary supports and jacking them to unload the existing channel). Or by jacking the existing channel itself.

Then install the second channel. Preferably by bolting, if it will be allowed by physical constraints and the owner.

RE: weld designation on back to back channels

(OP)
Thanks for the comments. I think it is a good idea to have them bolted. Just to ease everyone's mind, the existing channel is failing in deflection, not strength. One channel can handle all the load without being over stressed, it just deflects too much. The existing connections at the end of the beam can handle all of the load as they are. And the beams will be fully unloaded prior to the addition of the second channel. So none of those catastrophic failures predicted above would have occurred even if we had them weld it. It is still good to get your input though. Thank you.

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