Braced Frames: 3a Toys Braced Frames

Leave the pin connections alone and we put a diagonal between them. Now, I always forget to say this. After all my years of teaching, I forget to say this. Brace frames are typically steel. You're not going to have a brace frame in concrete. In wood, more often than not, it'll be a sheer wall of plywood or OSB. So these examples, the braced example and the moment frame, the brace frame, let's talk about the brace frame. The moment frame could be concrete or steel. The brace frame is going to be steel, basically. So looking at this one, it looks like the diagonal is kind of flimsy. It's called the tension-only diagonal. And when you load the beam, it bends and deflects. It is no different than the post and beam. It does exactly the same thing under a gravity load. But whereas the post and beam could not handle a lateral load, this one can handle a lateral load as long as the diagonal goes into tension. It's tension only. You put it in compression, it freaks out. So, this is not a good, very good solution for a seismic area because load is coming from either direction and so that member cannot handle compression. No good. So, what can we do about tension only bracing? We can, of course, go in there and make the diagonal a lot beefier. So with tension only, it's either a cable or a tie rod or a strap, galvanized metal strap in light frame construction. That's what that is. It cannot do compression. But if you bring in more section modulus, more moment of inertia, more area to that diagonal, then we're talking -- this diagonal wants to be as symmetrical as possible because it's going to receive axial load, so you want symmetry. So an angle, they can brace with angles, but they would rather have a pair of angles to make it a little bit more symmetrical, or a pair of channels, or maybe a wide flange. But the preferred shape, not for detailing, but for structural performance, is the more symmetrical square tube. That one is more efficient. There's the same amount of material in the x and y directions. or better even is around a pipe member. Those are the most efficient axially, but maybe not the easiest to detail and probably not the cheapest. So that's why you'll see a lot of double angle or a lot of double channel. It's a lot cheaper to make those. But anyway, they have more area and section modulus than tension only brace such as a cable or a tie rod. So now we have a more serious brace, and if you load it with gravity, the diagonal does zero. It's not doing anything. It's not there for gravity loads. Those are beneath the diagonal. The diagonal does lateral load. So if you push on it, it goes into compression. It can handle compression. You see it's buckling a little bit, just a little bit because I'm overdoing it to demonstrate that. Or if you push in this direction, then you're sending this guy in tension. Or if you pull on this corner, you're sending it in tension. If you pull on this corner, that guy goes into compression. So I'll pull on this corner. You can't really see there it goes into compression. So a single diagonal that is beefy can handle tension and or compression, whatever the earthquake sends to it. If you put two diagonals, then it becomes indeterminate because one of them is working, the other one is not working. So that's one remedy for the tension-only situation. We can put a beefier diagonal, costs more money, or else another solution for the tension-only situation is to have... Where is this toy? Here it is. So with tension-only bracing and you have a single brace, it cannot do compression and so in an earthquake, it's really in trouble. But one of the remedies is to go with two tension-only diagonals. They're not connected together, they're separate. Now, if you give it a gravity load, the diagonals are doing nothing. They're not here for gravity, they're here for lateral loads. The post and beam can handle gravity. But if you give it a lateral load and you have two diagonal members, then one of them goes into compression and is useless, but the other is working. And if the load reverses because it's an earthquake or wind load or lateral load, then this one goes into compression and the first one goes into tension. There's two members, but one is working at one time, the other is doing nothing, depending on the direction of earthquake. So, this guy is real happy. It pulled out, very good. It demonstrated that it was intentioned. I can't get it in back in live. So, you get the idea. That's it. Okay, I'm going to stop here and then we'll go to paper and explain all of that. Продолжение следует...