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Thread: How strong is this kind of weld?

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    How strong is this kind of weld?

    Hello, I was needing to join two aluminum i-beams at about a 167 deg angle angle between them (joining them flat would result in a 180 deg angle between them).

    They would need to be cut at a slant to achieve this small angle. Once joined the beams will be loaded normally with vertical loadings.

    My question is, would this angle weld significantly reduce the strength of the overall beam? And if so, would it then be better to bolt the angle instead of welding it?

    see the attached picture for reference, thanks

    Jack
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    Last edited by Jack732; 03-28-2021 at 03:38 PM.

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    Re: How strong is this kind of weld?

    I'm not an engineer, but my gut says I might want to gusset the bottom chord (in tension) somehow to increase its tensile strength. Might even want to "box out" the joint by welding plates connecting the top and bottom flanges to help resist bucking of the top chord (top flange) due to compression.

    I suspect there's also going to be a torsion component -- it's going to want to twist and/or roll over -- so I'd want to make sure it was held in place laterally real well (with a "ridge beam" so to speak) if that makes any sense...

    An I-beam is really meant to be loaded in only one way, I believe -- aren't they generally weaker if you load them sideways rather than in line with the web? If you get any sideways deflection due to strain, then you're going to be side-loading it, seems like...and the more it deflects, the weaker it's gonna get.

    Where will this welded beam be supported from below? At each end? Any support in the middle?

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    Last edited by Kelvin; 03-28-2021 at 03:55 PM.

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    Re: How strong is this kind of weld?

    Kelvin, excellent idea, boxing out the joint in addition to the weld should increase the strength significantly. A gusset to the bottom chord is also a great idea, I think I might have enough space for such a thing. Thank you for you input Kelvin!

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    Re: How strong is this kind of weld?

    In addition to my previous response, see the picture attached- it would be supported from below. This is actually a conceptual idea for the main spar of a flying wing aircraft drone. The reason the angle is needed is due to necessary anhedral in the wing. You're right about the loading directions, I definitely rushed past that consideration. Perhaps a box or square beam would be a much better choice. There actually will be more than just vertical loading, there will also be loading in the z axis, or into the page. Your thoughts? Thanks Kelvin.

    Jack
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    Re: How strong is this kind of weld?

    Also remember that when you weld 6061 (and I am assuming you will be using a 6061-T6 I beam) you will have a weak spot in the heat affected zone, where the temper will be reduced to a T0, effectively cutting the tensile strength in that part of the I-beam by about 50%. https://www.thefabricator.com/thewel...sile-strengths

    You need to design your weld joints to compensate for this. I like the idea of gussets and plates to compensate.
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    Re: How strong is this kind of weld?

    I make benches from old extension ladders. They involve a joint like yours. I put a flat piece of 1/4" plate at the intersection. I then fillet weld all around the ladder rails. Never had a failure. These ladder sections are typically 6" from foot to foot, perhaps 4' flat plywood as the top. Maybe 18" tall, never more than 2' tall.
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    Re: How strong is this kind of weld?

    Whoa, you start talking dihedrals and dynamic loading and you're gettin way above my pay grade! Sounds like you might need an aeronautical engineer.

    Anyway, another option might be aluminum extrusions ... they come in all kinds of shapes and I suspect are very strong for their weight (and strong in all directions in some cases, depending on shape)...built like birds' bones and probably good for monocoque constructions...

    Not sure what type of aircraft or drone you're thinking about, but yet another option might be one of the many foam composites used in boatbuilding, with a skin (carbon fiber-reinforced plastic?) over top, maybe? There are all kinds of composites out there...in many cases, you could basically carve your shape out of something like styrofoam, and if you covered it with FRP, it would be strong as hell, with all of the strength in the skin and the styrofoam core serving merely as a "scaffold" or "web frame" to drape the skin upon...think "torsion box" ... after the exoskeleton was formed and hardened, you could effectively remove the core in many cases, making room for moar fuel and ordnance!

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    Last edited by Kelvin; 03-28-2021 at 08:46 PM.

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    Re: How strong is this kind of weld?

    Quote Originally Posted by Louie1961 View Post
    Also remember that when you weld 6061 (and I am assuming you will be using a 6061-T6 I beam) you will have a weak spot in the heat affected zone, where the temper will be reduced to a T0, effectively cutting the tensile strength in that part of the I-beam by about 50%. https://www.thefabricator.com/thewel...sile-strengths

    You need to design your weld joints to compensate for this. I like the idea of gussets and plates to compensate.
    Louie, thanks for your response. What you've said is quite invaluable, I did not know that aluminum was weakest at its heat affected zone. And I especially found this part of the article you linked useful: "The minimum tensile strength for butt welds is determined by the strength of the material HAZ, not the tensile strength of the filler metal. As an example, all codes require a minimum tensile strength of 24 KSI for welds in 6061-T6"

    Thank you,

    Jack

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    Re: How strong is this kind of weld?

    Quote Originally Posted by Willie B View Post
    I make benches from old extension ladders. They involve a joint like yours. I put a flat piece of 1/4" plate at the intersection. I then fillet weld all around the ladder rails. Never had a failure. These ladder sections are typically 6" from foot to foot, perhaps 4' flat plywood as the top. Maybe 18" tall, never more than 2' tall.
    Hi Willie,

    The experience you describe is a powerful proof of concept, I am more confident now in using plates to augment the strength of the joint. You describe using a flat plate, but the joint I'm thinking of requires a slight bend to it as I described in my beginning posts. I can only think of one way to do it- by taking two flat plates and welding them at said angle and then laying it flush on the joint and then fillet welding around it.

    What do you think?

    thanks,
    Jack

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    Re: How strong is this kind of weld?

    Quote Originally Posted by Kelvin View Post
    Whoa, you start talking dihedrals and dynamic loading and you're gettin way above my pay grade! Sounds like you might need an aeronautical engineer.

    Anyway, another option might be aluminum extrusions ... they come in all kinds of shapes and I suspect are very strong for their weight (and strong in all directions in some cases, depending on shape)...built like birds' bones and probably good for monocoque constructions...

    Not sure what type of aircraft or drone you're thinking about, but yet another option might be one of the many foam composites used in boatbuilding, with a skin (carbon fiber-reinforced plastic?) over top, maybe? There are all kinds of composites out there...in many cases, you could basically carve your shape out of something like styrofoam, and if you covered it with FRP, it would be strong as hell, with all of the strength in the skin and the styrofoam core serving merely as a "scaffold" or "web frame" to drape the skin upon...think "torsion box" ... after the exoskeleton was formed and hardened, you could effectively remove the core in many cases, making room for moar fuel and ordnance!

    Whatcha makin'?
    Hello Kelvin,

    Yes indeed aluminum extrusions are my main focus for all the structure components, right now I'm thinking a square/box AL extrusion should be used for the two main spars, but it won't be a full depth spar (the spar reaches the top and bottom skin of the wing) in that case due to weight problems, so I'm not sure how to reconcile that fact..

    I actually want to stay away from composites and stick to aluminum , it's just a little too complex to manufacture the right shape for composites- I might use carbon fiber for some stuff though. The torsion box is my go-to by riveting the aluminum skin to the AL ribs and spars.

    I already said- flying wing drone :^}

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    Re: How strong is this kind of weld?

    Quote Originally Posted by Jack732 View Post
    Hi Willie,

    The experience you describe is a powerful proof of concept, I am more confident now in using plates to augment the strength of the joint. You describe using a flat plate, but the joint I'm thinking of requires a slight bend to it as I described in my beginning posts. I can only think of one way to do it- by taking two flat plates and welding them at said angle and then laying it flush on the joint and then fillet welding around it.

    What do you think?

    thanks,
    Jack
    Jack, if you weld two pieces together you may have a weak spot where they are jointed together. Rather than welding two flat plates together I'd suggest that you cut the entire shape needed from a sheet of aluminum - probably 1/4" or so. I'd cut it so that it fits inside the flanges and weld it flush to the outside.

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    You can make the cuts in the plate with a jigsaw (unless you have access to a plasma torch). If the blades get clogged with aluminum, use wd-40 as a lubricant while cutting.
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    Re: How strong is this kind of weld?

    +1 on SCSmith's gusset design.

    You can also generally use a bandsaw, table saw or whatever woodworking equipment you have to cut the aluminum, using the same safety procedures -- cut speeds are similar.

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    Re: How strong is this kind of weld?

    Quote Originally Posted by scsmith42 View Post
    Jack, if you weld two pieces together you may have a weak spot where they are jointed together. Rather than welding two flat plates together I'd suggest that you cut the entire shape needed from a sheet of aluminum - probably 1/4" or so. I'd cut it so that it fits inside the flanges and weld it flush to the outside.

    Name:  aluminum gussett.jpg
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    You can make the cuts in the plate with a jigsaw (unless you have access to a plasma torch). If the blades get clogged with aluminum, use wd-40 as a lubricant while cutting.
    I agree with scsmith42
    Up to a point .
    If the joint was in a "Portal " frame ie no bottom cord , yes.
    If the joint was in a simple truss ie no "King Post" , yes.
    But with a King post the gusset should extend to include the post to "Pin" the joint .
    A good guess is better than a bad measurement

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    Re: How strong is this kind of weld?

    You haven't said what you are building. Lots of facts unknown.

    What I described was avoiding butt welding, favoring placing a flat plate between to provide a higher volume cross section. Aluminum weld is never as strong as the material it joins.

    Figure out a way to reinforce.

    In my case I use a ladder section intended to span 28' and downgrade load to 6'. I found butt welds on dirty aluminum to be difficult, so I do filet welds to plate from each direction. It reinforces, (more cross section) It gives torsional support to the I beam.

    I am a vertically challenged electrician. I can spend time moving stepladders many times, climbing many times, or I can stand on a bench. The step ladder serves as a tool tray and it gives me a place to step when I kick the bench. I climb down to pee. If my bench isn't quite tall enough, I grab a piece of 3" blue styrofoam. Only downside is carpenters & plumbers try to steal my benches! My standard answer: find me a busted ladder you can have this one.
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    Re: How strong is this kind of weld?

    Mr. Smith, thank you for your response.

    I quite like your idea, it appears to be a great reinforcing measure to take. I'll definitely keep in my mental notes as I continue to explore designs. And thanks for the tip on using WD-40!

    Jack

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    Re: How strong is this kind of weld?

    Brett, that's a very good point you made. I hadn't considered it. The only downside to having the gusset extend to include the king post (that I can see) is that it won't be between the flanges of the I-beams which would give added support. Do you think that that loss in strength would be somewhat negligible?

    Thanks,
    Jack

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    Re: How strong is this kind of weld?

    Hi Willie, I appreciate your follow up.

    For right now, I believe my best most reinforced path comprises of a few things, first I can cut the aluminum so that it can be butt welded at the angle (I'll make sure it's real clean), then I can use your plate/gusset idea to increase the volume cross section in combination with a king post styled truss. Aluminum extrusions will be used as much as possible to increase strength. Your thoughts?

    And strong note you made about the fillet plate welding method giving torsional resistance!

    Jack

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    Re: How strong is this kind of weld?

    Quote Originally Posted by Jack732 View Post
    Hi Willie, I appreciate your follow up.

    For right now, I believe my best most reinforced path comprises of a few things, first I can cut the aluminum so that it can be butt welded at the angle (I'll make sure it's real clean), then I can use your plate/gusset idea to increase the volume cross section in combination with a king post styled truss. Aluminum extrusions will be used as much as possible to increase strength. Your thoughts?

    And strong note you made about the fillet plate welding method giving torsional resistance!

    Jack
    You still haven't said whether you are welding stools, or platforms supporting tractor trailers. Few of us are engineers.
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    Re: How strong is this kind of weld?

    Depending on how big the I beam is, I would take a pie cut out of the underside, heat the top a little bit if needed, put a chain puller on the beam and bend it. That way you don't have a weld at the top to contend with and rely on. Then I would gusset just the bottom with a angle bracket so its in pull and only weld down the middle and sides so there's no cross weld HAZ to crack in half.

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    Re: How strong is this kind of weld?

    Since you are fabricating the main spar of a flying wing drone, wouldn't your principle loading be up (i.e. lift) rather than down? Maybe I am looking at it wrong? There would likely be some downward load as well when on the ground. I think you may need to account for both directions.

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    Re: How strong is this kind of weld?

    Quote Originally Posted by Willie B View Post
    You still haven't said whether you are welding stools, or platforms supporting tractor trailers. Few of us are engineers.
    Flying wing drone kind of project Willie.

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    Re: How strong is this kind of weld?

    Quite right Somorris, lift will definitely not be the only force applied on this drone, I will be accounting for all other forces, but thank you for pointing it out. It is very important.

    Jack

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    Re: How strong is this kind of weld?

    Wow, you've got an imagination ferrret! I don't think this would be my go to method however as the angle required must be as accurate as possible (+/- say 1 degree) and I'm not so sure how accurate I can bend an aluminum I-beam to that end, even if I can get the angle I wonder if it would still be straight on the other side. Thank you for the suggestion, I will keep this also in the back of my mind as I go forward with the design.

    Jack

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    Re: How strong is this kind of weld?

    Quote Originally Posted by ferrret3238 View Post
    Depending on how big the I beam is, I would take a pie cut out of the underside, heat the top a little bit if needed, put a chain puller on the beam and bend it.
    Good idea, but I would make the saw kerf on the top, leave the bottom flange unkerfed, then bend what's left (the bottom flange only), then insert a wedge-shaped piece into the gap and weld it out.

    Seems to me the "failure mode" (like if you tried to pull out of a too-steep, too-fast dive too quickly) would be for the wings to break upward at the wing tips, and the weak point -- where it probably would break under tension -- would be the welded seam on the bottom flange at the vertex. Bending, rather than welding, the bottom flange would eliminate this weaker weld joint. Then most of your weld joints would be under compressive, rather than tensile, loads.
    Last edited by Kelvin; 04-03-2021 at 06:26 AM.

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    Re: How strong is this kind of weld?

    That thing's gotta be huge if you're considering a welded aluminum frame. As I understand it, that type of construction is just too heavy for most small hobby stuff.
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