weld designation on back to back channels 5

[shaneelliss]

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?

 

[tbuelna]

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!

 

[Duwe6]

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

 

[racookpe1978]

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.

 

[tbuelna]

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.

 

[chicopee]

Forget the plug welds and bolt them together.

 

[trainguy]

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

 

[mihmb]

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.

 

[shaneelliss]

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.