Video Transcript
Welcome to The Mentored Engineer. In this video, I’m going to show you a way that you can make me cringe. Okay, so we’re going to talk about gussets, and gussets are one of those areas where I’m just like, ooh, this is so simple. Why do people keep doing it wrong? And it makes me sick.
All right, so let us look at a cantilever beam system right here. Let’s say it’s like a street light or something. And we have a square tube butting up with another square tube. Now, as you can imagine, I will load it here. It’s going to be a cantilever beam and then a constant moment cantilever beam all the way down to the ground.
All right. Right here at this joint is where it obviously causes us problems. Now, the cause right here is stress flow and it’s everywhere. I mean, we’re just stresses need to change direction. And when you change it 90 degrees, that’s it’s hard to do without extra pieces of steel and extra design time.
OK, so I’m going to show you how a couple different ways to make this joint better and one that I really hope that you don’t do because it makes me cringe okay so if I look closer at this area right here here’s what I’m I would want to see all right I’m going to have this tube here and it’s going to have its wall I want to have it transition to this one. The first thing I’m going to want to do is I’m actually going to want to set this tube just a little bit higher so that I can get good stress flow.
The top of this vertical tube is going to be right here, and I’m going to have my dotted lines. When I think about stress flowing, it’s flowing in the top of this beam very nicely. It’s all uniform. It’s all going. You’ve seen this on an FEA plot where, man, it’s just going smooth.
All of a sudden it comes to this tube and there’s nowhere for it to flow. We’re going to change that. The first thing we want to do is we want to cap the end of this plate. And we’re going to use that with the same thickness as this tube, if not thicker. And then we’re going to come back and fill this spot up with weld.
And now I have a nice path for my stress to flow. through this joint all the way back here to this back wall and as it goes through this plate it’s actually going to distribute to the sides of this beam very nicely and you’re going to have a great transition so do that another thing you can do is actually have this plate come over and cap this one but then you have to worry about different stress transitions with transverse welding and stuff like that so that actually may have more complications than it’s worth so I would do this All right, so if I have the room, I can get down in here and weld this, weld another plate down there.
I want to do that. All right, and there’s a very specific way you want to do that. You actually don’t want to line it up with the material at all. It’s kind of weird. All right, so I’m going to put in a fillet weld right here.
All right, and I want to actually have my plate come down to the toe of this fillet weld. right there now you think that’s kind of weird but it actually makes the stress flow great so I’m going to put a fillet weld right here because that’s likely the only place I can get in with a welding torch is coming down right there and keep in mind this actually has to be quite a big tube to do this all right
and I can see that my stress flow will flow through here come down here into that weld and into that plate and it makes a nice smooth line and that’s what we’re looking for stress flow is like water if the if there’s enough material and the stress flows your water is nice and calm and clear on the top as it gets shallower and moves quicker, I’m going to see a lot more hot spots and a lot more motion in water. So that’s why we say stress flow flows like a river. All right, so nice, even stress flow through that material.
Okay, now if you can’t do that, you’re going to have to look to gussets. So, let’s redraw this and look at some gussets here. all right now the first thing and this is what’s going to make me cringe is I come in here and see a gusset of that nature may not be exactly that profile but if I look at the end here I see a tube and I see another tube and that gusset centered right in the middle of the tube don’t do that it makes me very upset okay very upset all right there’s no reason to do this
all right so first of all you have you have nothing behind this gusset on this plate to take load it’s just going to bend that plate it’s already your highest stress surface right there why would you add another plane of stress there so don’t do that okay it’s just dumb you’re going to push up you’re going to bend that thing and it’s already bending in another direction Don’t do it.
Stop it. All right, so the next solution, and I like this one somewhat better. If I go in here and actually modify this a little bit, my tube actually has a radius corner. All right, so what I can do is go in here and add a gusset, and I can weld right back in here where that J-bevel would be and do the same thing over here. And I at least have something on both surfaces where I’m pushing directly into a wall behind it.
Much better. Good job. Give yourself a round of applause for thinking of that all by yourself. Okay. And that’s good.
It’s better. It’s not the best. Alright, the problem with this is that you still have your highest stress right on that tube, on that outer surface. And you’ve added a lot more area moment of inertia with that gusset. And drastically.
And that’s going to put a hot spot right here. Especially right there. It’ll be a lesser one right there. And that’s… No matter what you do, you’re still going to have that.
If you drag this down, you’re still going to have it. In fact, almost anything you do is still going to have it, but we can minimize that as much as possible. And the way we do that is by placing the gusset outside of the beam, or outside of the tube, not on the edge of the tube. All right, so we’re going to want to start with our gusset and it’s going to go on the side, each side of the tube on the outside. And it’ll go right down here because this is what, it’s the neutral axis.
And when we pick that up, we’re going to have a stress concentration at the end of that. But if it’s on the neutral axis, who cares? All right, so we’re going to bring this down and it’s going to go this way. down this beam a lot farther than it will down this way. Alright, and we’ll round that up.
We’re going to bring it down to the edge of the tube, and we probably want to do so at a 30 degree angle or so. It doesn’t need to be very wide, you know, 15 to 30 degrees is probably where you want to be. And right as you get here, to this edge, you’re going to want to make it flat for just a little bit. You’re going to make a nice smooth transition. You’re transitioning grate into the radius area.
You’re going to have a little bit of a lateral weld there. And you’re going to be increasing your section modulus without a dramatic increase in your area moment of inertia. I could go into a whole video on that as well. So, I won’t bore you with that here, but that’ll probably come out pretty soon. Now what we’re going to do is we’re going to start going back on our 30 degree angle here.
And then we’re going to make our transition here and we’re going to radius that back up. and go to our point now this one doesn’t need to be as as critical as this one this one is trying to get that in there this this whole tube it just acts differently I can’t explain it very well but when you look at it in FEA it’s just not a problem so a lot more forgiving on this side of the equation than on that side all right so that is how you make a great gusset all right don’t make me cringe I don’t want to have to find your design know
that oh my gosh you watch this video and you still did that so I implore you to make your gussets as much like this as you can and do it every time a lot of times this will save you even having to do any FEA on it just because it’s such a great design all right well thank you for watching this episode.