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Truss basics

How does a truss in a stadium help to distribute forces efficiently?

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Video transcript

- [Instructor] So I have here a picture of a stadium, and the piece of engineering that I want to look at today is this little triangular pattern of steel beams that you see. So I kind of have a zoomed in version over here. And you see these beams arcing all across the stadium, and each individual one has this interesting sort of triangular pattern with a bunch of smaller little steel beams making it up. Now, this kind of pattern, anytime you have an arrangement of straight pieces like this, all connected at their ends, it's called a truss. So most commonly, a truss looks kind of like what we have up here, where you have, you know, two long metal beams, and then in between them, this triangular pattern of smaller beams. And the question to answer today is, what's the point of this? Why have this pattern of steel beams? You see this, you know, not just in stadiums, but if you look out for them, you'll notice them all over the place. Now, the problem that this is trying to solve, and this happens with a lot of structures, is that you want to take the force that's coming down on one place, so let's say the force, you know, of the weight of the ceiling of this entire stadium, and somehow turn that into a force acting on a different part of the structure, where it can actually handle it. 'Cause what you would optimally want would be a beam straight down from the point on the ceiling where you're trying to support all the weight. But of course, with a stadium, that would be completely impractical, you'd have all of these columns hitting the middle of the field, and you can't have that. You need to make sure that the force that that beam would support is instead supported off on the border of the stadium, where you can actually have thick columns and strong structures without worrying about running into a field. So a related problem to this that'll help us kinda start thinking about trusses is if you were to build a bridge. So let's say you've got one column over here, and then you've got another column over here, and then you want some kind of bridge that'll connect the two. And let's say I just give you a bunch of sticks to make this bridge, this is out in the forest somewhere. So if you just lay a stick flat across, now that could be something you could walk across, right? You might be standing in the middle here, crossing that bridge, and in effect, you are applying a force straight down here. And in effect, what this stick does is, it transfers the force that you're pushing down here over to the columns on the side, where it can actually handle it. But the problem with this is that, unless this is a very strong stick, you're probably gonna bend it down a bit as you're pushing in the middle there, and it might kind of bow down. Or you might even break it, right? It would snap in half. And I think the instinct for this is pretty clear. If I give you some kind of stick and I tell you please break this in half, what you're gonna do is you're gonna try to apply pressure right to the middle of it. And once you apply pressure to the middle, maybe it kind of starts to bow a little bit, and then eventually it would snap in half. So sticks are not very strong if you start pushing them right against their middle like that. So one solution could be, rather than using just one stick for your bridge, you pile up a whole bunch of them. And maybe you use a really thick log or something like that. And with enough of them, this might work, and you would have a valid bridge. But that's very inefficient, you have to use a lot of material to be able to support the kind of weight that you want to. So the cleverer solution, using this idea of a truss, would be, here, I'll kind of draw the same setup. We've got two columns in some way, and you want to draw a bridge across them. So you might have your initial stick, and then, you might make this truss pattern with triangles, connecting two different sticks across there. And now, the advantage of this is that, when you're standing on the middle of the bridge here, that downward force that you're applying is not pushing in the middle of a stick like it was in the first attempt over here. But instead, you could say that it's distributed across these two different sticks here, and it's pushing along the stick itself. The force is going along the stick. And then, when you consider what the forces along these two sticks do to the points they're connected to, in effect, they'll be trying to kind of pull apart the stick down here. And for every stick in the bridge, there's either gonna be a force that pushes along it, compressing it in, or forces that pull it apart, stretching it out. But that's okay, because sticks are very good at handling that kind of force. If, once again, I gave you a stick and I said, please break this, but instead of pushing down on the center, you're not allowed to do that, you're only allowed to push in on the ends of that stick, or you can try to pull out on the ends of that stick, it would be really hard to break. I mean, you could try this with a pencil right now, pulling it apart or pushing it in. The pencil's not gonna break. It's really strong in that direction. So in both of these examples, we had a force from your body in the middle of the bridge that was pushing downwards, and ultimately, we wanted to turn that force into something that was acting on the columns over here, because those nice, thick columns can support it. And similarly, up on the stadium, you might have a force acting on the roof. And because you can't have a column right there, you want to transfer that to a force acting somewhere on the edge of the stadium. Even though both of these examples do that, this first one is really inefficient, really inefficient. If you wanted to find enough material to handle the kind of weight that you need, you'd have to have a whole bunch of it. But over here, this pattern, because it's taking advantage of the fact that sticks are hard to break by pulling apart, and hard to break by pushing inwards, this one is really efficient. You can use fewer resources to support the same weight. So I think that's a really clever piece of engineering.