Would You Cut Slots For Greater Strength?
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  1. #1
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    Would You Cut Slots For Greater Strength?

    Seems to me to be a good idea to cut slots that you can weld through to increase the length of welding bead and therefore increase the strength of the join.

    Just asking. Do you reckon that's right?

    Make myself clear: say you've got to weld a 4" square plate onto another larger plate, just flat down one on top of the other. So's you can weld all around the edge. You've got 16" of weld there.

    Now what I'm talking about is like cutting something like maybe two 1" slots in at right angle to the edge from each edge.

    And then you can weld down through those slots onto the underneath plate.

    You'd have a total of 24" of weld.

    A bit dopey there but perhaps in some applications it might be a good idea? Is that sort of thing commonly done in practice? I've never seen it mentioned anywhere.
    Last edited by abrogard; 04-12-2018 at 06:51 PM.

  2. #2
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    Re: Would You Cut Slots For Greater Strength?

    Not really different than adding a plug weld somewhere inside the perimeter of the plate. It increases the resistance to pullout.

    What a lot of people tend to do, is to measure the strength of a weld by the Ksi of the weld FILLER MATERIAL. What they forget is the base metal. You gotta consider just how strong the base metal is. A weld can't be any stronger than the base metal it sits on top of. A weld on top of A-36 steel,,,,, has the strength of A-36 steel. I don't care how you figure the strength based on the filler material..............it always meets the boundary of the parent metal, and the parent metal has to ultimately resist pullout, and shear.

  3. #3
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    Re: Would You Cut Slots For Greater Strength?

    So..........no........it's not a stupid question.

    The trick is to determine just how much weld is enough, and how much weld is overkill.

  4. #4
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    Re: Would You Cut Slots For Greater Strength?

    A lot of it would also relate to how thick the plates are? of course this goes back to strength of the materials...

  5. #5
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    Re: Would You Cut Slots For Greater Strength?

    Ultimately, I suppose so.

    You're looking at the plate itself. How resistant is it to buckling, or yield.......given its thickness. A great weld on top of 1/4 plate isn't gonna hold the Queen Mary

  6. #6
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    Re: Would You Cut Slots For Greater Strength?

    Thanks guys. So not such a silly idea. Just wondered. Never having seen a mention of it.

    For my own work the strength is fairly immaterial I think - I'm always way over what's needed. If my welds are good, that is, IF.

    I'm just kind of interested from general principles. I wander around the place and see things - like in the multi storey parking - and see how they've bolted and welded the girders. I see enormous weights and when I look it's all supported by a couple of 2" bolts or something.

    And I see large plates welded down and sometimes it seems to me there's just vast chunks of metal wasted. Like a 12" plate welded all around onto a base - another beam maybe - and that one foot square of steel seems to be 'wasted' because none of it is held down. Just the perimeter.

    So I wonder why they did that and figure it must be because of the 48" of weld they get by making it so big.

    And then I think well is there a way to get 48" of weld without using so much steel? And like that.... just curious, the curiosity of one without engineering training or education, just puzzling over things...

  7. #7
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    Re: Would You Cut Slots For Greater Strength?

    Quote Originally Posted by farmersamm View Post
    Ultimately, I suppose so.

    You're looking at the plate itself. How resistant is it to buckling, or yield.......given its thickness. A great weld on top of 1/4 plate isn't gonna hold the Queen Mary
    The Monarch or the Ship???

  8. #8
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    Re: Would You Cut Slots For Greater Strength?

    Quote Originally Posted by HT2-4956 View Post
    The Monarch or the Ship???
    The floaty thing

  9. #9
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    Re: Would You Cut Slots For Greater Strength?

    K'kins wasn't too thrilled about this. We're jumping the gun by a few days.............but then..............I'm taking it in the shorts on the upcoming Monday.

    Warm, in the 80's, sun out.........what's a guy gonna do (shrug) been one heck of a long Winter

  10. #10
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    Re: Would You Cut Slots For Greater Strength?

    Quote Originally Posted by HT2-4956 View Post
    The Monarch or the Ship???

    Darryl

  11. #11
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    Re: Would You Cut Slots For Greater Strength?

    Us old guys get a little mildewy if'n you don't drag us out in the sun once in a while

  12. #12
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    Re: Would You Cut Slots For Greater Strength?

    Quote Originally Posted by abrogard View Post
    Seems to me to be a good idea to cut slots that you can weld through to increase the length of welding bead and therefore increase the strength of the join.

    Just asking. Do you reckon that's right?

    Make myself clear: say you've got to weld a 4" square plate onto another larger plate, just flat down one on top of the other. So's you can weld all around the edge. You've got 16" of weld there.

    Now what I'm talking about is like cutting something like maybe two 1" slots in at right angle to the edge from each edge.

    And then you can weld down through those slots onto the underneath plate.

    You'd have a total of 24" of weld.

    A bit dopey there but perhaps in some applications it might be a good idea? Is that sort of thing commonly done in practice? I've never seen it mentioned anywhere.
    More weld does not make it stronger. A perfectly engineered weld joint should have the same strength as the base metal. If it is stronger, it creates a stress zone around it under load. Your idea would take base metal away, giving it less bonded area, meaning the load would be distributed through a smaller area.

  13. #13
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    Re: Would You Cut Slots For Greater Strength?

    can't see that. exaggerate for the sake of simplicity:

    we have a piece of metal held to another by a bead 3 foot long.

    now we put down another 3 foot bead.

    Now we've got 6 foot of joined metal. That's surely stronger than 3 foot. Common sense. If the load was 3 tons there's now only 0.5 tons load per foot of weld.

  14. #14
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    Re: Would You Cut Slots For Greater Strength?

    Quote Originally Posted by abrogard View Post

    Make myself clear: say you've got to weld a 4" square plate onto another larger plate, just flat down one on top of the other. So's you can weld all around the edge. You've got 16" of weld there.

    Now what I'm talking about is like cutting something like maybe two 1" slots in at right angle to the edge from each edge.

    And then you can weld down through those slots onto the underneath plate.

    You'd have a total of 24" of weld.

    A bit dopey there but perhaps in some applications it might be a good idea?
    Interesting. Can anyone postulate one application where a "slot-weld" might be needed to provide added strength?

  15. #15
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    Re: Would You Cut Slots For Greater Strength?

    Quote Originally Posted by Weldordie View Post
    Interesting. Can anyone postulate one application where a "slot-weld" might be needed to provide added strength?
    You might find this interesting. If you want to jump straight to 'slot weld' it's about half way down I think.

    https://www.quora.com/What-is-the-di...d-slot-welding

  16. #16
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    Re: Would You Cut Slots For Greater Strength?

    Quote Originally Posted by abrogard View Post
    You might find this interesting. If you want to jump straight to 'slot weld' it's about half way down I think.

    https://www.quora.com/What-is-the-di...d-slot-welding
    Just a plug weld with an elongated hole. :-)

  17. #17
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    Re: Would You Cut Slots For Greater Strength?

    Quote Originally Posted by abrogard View Post
    Now we've got 6 foot of joined metal. That's surely stronger than 3 foot. Common sense. If the load was 3 tons there's now only 0.5 tons load per foot of weld.
    People get hung up about "strength." Besides the extra cost of welding time and welding materials, there are other reasons not to overweld, such as HAZ, warpage/distortion, and creating stress risers in members that are expected to strain (move/bend elastically), as TheBFA said.

    There's also nothing to be gained in many cases. If the base metal only has a yield strength of 36ksi (like A36), what is to be gained by adding so much weld deposit that it takes 1000 tons to break the welds, but only 20 tons to tear away the base metal? You're gilding the lily. A chain is only as strong as its weakest link. If something is made out of A36, you can add weld beads until you're blue in the face, but the weakest link in the chain will always be the A36.

    Let's say you're making a "T" joint in 1/4" plate. You could make a weld with a normal leg length, or you could lay down beads until the leg length was 6". The weld with the 6" leg length would be far, far stronger than the normal weld. But would it make any difference, when the base metal was only 1/4" thick?

    "Stronger" is not always necessarily "better" and often is "worse." If something only needs to hold 100#, why would you make it to hold 10,000#? I suppose you could make a hanging plant bracket out of 12" I-beams, but why would you? "Oh, honey, it's so graceful and delicate looking! I love it!"

    If one slot weld is supposedly better than "no slot welds," why not make 50 slot welds? Or 100? Where does it end? At what point does one say, "I think the welds are strong enough?"
    Last edited by Kelvin; 04-13-2018 at 06:40 AM.
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  18. #18
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    Re: Would You Cut Slots For Greater Strength?

    It also depends on what the plate is doing. For instance, if you're adding a tow hook to a bumper and you weld in a plate, then the hook, a couple plug welds will help keep the plate from pulling away. If you're plating over a butt welded joint to keep it from tearing, extra plug welds won't really add much.

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  19. #19
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    Re: Would You Cut Slots For Greater Strength?

    Quote Originally Posted by abrogard View Post
    can't see that. exaggerate for the sake of simplicity:

    we have a piece of metal held to another by a bead 3 foot long.

    now we put down another 3 foot bead.

    Now we've got 6 foot of joined metal. That's surely stronger than 3 foot. Common sense. If the load was 3 tons there's now only 0.5 tons load per foot of weld.
    My bad. I was literally smoking when I read the original post haha. For some reason I was picturing a T plate, that instead of welding a full straight bead, you cut a slot so you could wrap a weld around through the slot in the plate.

    But the slot weld you are describing is still subject to everything Kelvin mentioned. Picture trying to bend and break a broom stick with your hands. You probably couldn't do it. But if you used your knee to support it in the middle you could break it. That's the idea behind a stress riser. You are taking the natural flex out of the part and creating an area for the stress to concentrate itself. That's not saying you aren't ever going to get more strength, it's saying it needs to be engineered into the design, and just randomly adding slot welds may leave you with more negative effects than positive.
    Last edited by TheBFA; 04-13-2018 at 09:28 AM.

  20. #20
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    Re: Would You Cut Slots For Greater Strength?

    abrogard,

    As some of these other members are trying to tell you there's a whole lot more to making plug and slot welds than you might tend to think at first. Most welding codes have a lot to say on the who, what, where, when, why and how you can use and make them.

    These are screen captures from the index of the D1.1 Structural Steel Code.

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    Name:  plug.jpg
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