Concrete: 1 Way Vs 2 Way

In this video, I would like to discuss the concept of one-way spans versus two-way spans. In a rectangle, typically, the members span in the short direction. They go this way. And they never go the long way. That is unacceptable. So they are more efficient and economical to span in the short direction. Why is that not working? Let's erase with an eraser. Very good. So this is the primary member. And perpendicular to it is the secondary members. And perpendicular to the secondary members, we're going to put some kind of decking. And the decking is the tertiary member. So it's like Jenga, you're stacking systems perpendicular to each other. So this is the tertiary, the deck. In a square, it's a lot easier to say, okay, I have a primary member, a primary member, a primary member, and another primary member. And with four beams, I'm going to end up a lot shallower than a one-way system. A two-way system is generally shallower than a one-way system. Because in a one-way system, I have two deep beams to span that much versus a lot shallower in a two-way system. I have a lot more structure. I have more joists, like a two-way joist system. Very good. So if you have a square or a square plus, which is 25% more one dimension than the other, so 100 feet by 125 feet is, for example, still a square. But 100 by 150 is clearly one way. Excellent. So the primary structure is in the long direction, and the secondary structure is in the short direction. This is the secondary structure. Excellent. And deeper for the same span versus shallower for the same span. I can, though, I can take this guy and make two squares out of it and put four beams on each square. And now I will have a two-way framing from a rectangle. Yeah, break it down into roughly a square. Very good. So here's the critical thing for floor systems, spanning floor systems. A two-way system exists only in concrete. Please notice I said floor, not roof. There are systems in wood and steel that do roof spanning in a two-way. But for floor systems, I'm not familiar with any two-way systems in wood or in steel. So concrete-only, one-way systems clearly could be wood, steel, or concrete. So let's look at some pictures. In this image, clearly this is a one-way system, and I have, let's increase a little bit the weight, I have steel beams going in this direction. I don't see anything of equal depth coming in a perpendicular direction. So this is the primary member. It carries the secondary. These are secondary. And the tertiary is what goes between the secondaries, which is essentially the metal deck. The metal deck is tertiary. Tertiary. Very good. Looking at the roof, I see open web joists all parallel to each other. And landing on this guy as a uniform load. So in that case, I am the primary. And you guys are the secondaries. And the deck is the tertiary, secondary. Very good. Please, a note here. If you walk on it, it should not be an open web joist. So looking at this image, the roof is framed with open web joists because there is no live load up there. Maybe an occasional maintenance guy or something like that. But if this were an occupiable roof and people go on that roof, then I need to use beams. Open web joists are only for very light live loads. Okay, so this is clearly a one-way system, and the primary member is spanning the long direction, and the secondary is spanning in the short direction. Excellent. This is cross-laminated timber that has recently made its way to the United States, predominantly in the Pacific Northwest, but it's starting to spread around on the East Coast a little bit. So what this is, is a piece of plywood, sorry, a piece of dimensional lumber here, and in the opposite direction. So there's some running north-south, there's others running east-west, and it's a cross-laminated, it's like a piece of plywood, but done this time with two byes. So layers one, three, and five versus layers two and four. Layers two and four are the shorter pieces. Layer one, three, five are going the long direction. So is this one a one-way or a two-way? I really don't know. Inside the panel, there's members going north, south, east, west. It looks like it's two-way. But then when you combine it in construction and you put another panel next to it, It's going to span in the long direction, and this is going to be a one-way system. Spanning in this direction, the outer layers determine the direction of the span. Very good. Let's see, what else do we have here? How about this one? One-way or two-way? This is formwork for a concrete pour, and there's these metal pieces of formwork. And this is clearly a one-way system. Because I see a lot of these. A lot of them. And not as many of these guys. There's much fewer in this direction than there is in the cross direction. So I'm going to call these guys something joist because it looks like it's repetitive. It looks like it's closely spaced. That's the definition of a joist. I'm going to call these guys beams or girders. There's fewer of them. They're deeper. They're carrying all the joists. We are the primary. You are the secondary. And the concrete pour will be the tertiary, and it goes from joist to joist. That's what the slab will have to do. The slab will go between secondary members. This guy is called a one-way pan. Pan is the formwork. This is a pan, this is another pan, this is a third pan. One-way pan, joist. Concrete system. Very good. If you are wondering about this beam, this beam specifically, this one here, I'm labeling it number one. That beam is parallel to the joist, but it's a little heftier. So we're calling it a beam, and its function is to tie beams number two and beam number three laterally to give them stiffness. So that's more of a lateral member than a gravity. If it is carrying gravity loads, don't get me wrong, but it also has a stiffening function to stiffen beam number two and three. Excellent. How about this one? This one is clearly whatever it is, it looks like it's a two-way system. That's a bad color. Let's use red. Two-way. Why is it two-way? Because it looks like there's a square. And it looks like it's the same in all directions. Yes, it's curving, doing all that stuff. It's a two-way system. This one clearly looks like there's some kind of joists or ribs or something repetitive. And they're in that direction. And they're spanning between the fewer, which is these guys. These are the beams or the primary or the girders or whatever you want to call them. We are secondary. And we are primary. Always they are perpendicular to each other. And the tertiary is perpendicular to the secondary. And that's going to be a concrete pour on top of these guys. So this one is clearly one way. This one, no discussion here. It looks like it's a two-way system. One, one, one. It looks like there's some kind of square there. So this is clearly the same in both directions. It's a two-way system. Here, it looks like there's an overlap, which is basically, there is an overlap requirement in the code. You can't just start a rebar, stop a rebar, start a mesh, start a mesh. You have to overlap it a certain amount given by the manufacturer or by the American Concrete Institute code. Very good. Okay, enough. Let's look at this one. This one I also discussed under post-tensioning. Right now I'm going to ignore the red cables. And I'm just going to say it looks like there is a lot in this direction. And not as much in the perpendicular direction. The green direction looks like it's primary. And the repetitive white direction looks like it is the secondary. Again, I see, can't see, if I zoom in, sorry, the resolution is not adequate. But those are metal pans, and this is a one-way pan choice system, which is post-tensioned. Very good. How about this one? This is the BWI Airport, Baltimore, Washington Airport. It looks like it's the same in two directions. It looks like it's a square here. But this is a roof system. It's not a floor system. So this is a space frame. A space frame is a two-way system. Could be in wood, could be in steel, could be in concrete. No concrete. Could be in wood, could be in steel. Same here. it's the same in all directions the bay is a square and then to prevent two-way systems are vulnerable to punching shear so to prevent that we're going to pick up multiple points over the column so that the whole space frame doesn't collapse we pick up we reduce the amount of year we increase the area of support. Same with the Crystal Cathedral in Garden Grove by Philip Johnson 1977. This is space frames on steroids on the wall on the roof on everything or the JFK library in Boston by I.M. Pei from 1979. You can tell the basic grid is a square not a rectangle. So this is a two-way system made of steel. Looking at this one, it looks like clearly this is a one-way system. So I'm seeing these guys. They're spanning in that direction. And there's enough of them. I'm calling this a secondary. And there's CLT cross-laminated timber on top of that. So this one is the tertiary. And who is supporting the secondary will be primary. It looks like this guy is the primary structure. And it supports the white secondary structures that support the cross-laminated timber sheets or planks. Not plank, but sheet. I'd rather say sheet. Please note, the column has no business being called primary, secondary, tertiary. It's just for, this is the primary, I didn't write it. It's just for spanning members, this terminology. Very good. Same here. That's the same thing. I see quite a few of these guys landing on somebody perpendicular to them. This is the girder, which is the primary system. And I see secondary here, which is these beams or joists or whatever they are. And the tertiary is the deck itself. Very good. How about this one? Is this a one-way or a two-way? Looks like there's members in two perpendicular directions. And what they're doing, this is a lamella system, is what it's called. A lamella. And this one is done in wood, steel, concrete, all of them. So what it is, is a member here, a perpendicular member to it. another one, and a third one. And they make a pinwheel arrangement, but this one will not work on a flat roof. Again, this is a roof, it's not a floor. So a lamella is a two-way system, in this case made of wood. I can show you lamella made of concrete. I forgot to put them in there. And let's not forget the rural studio. They did two consecutive years of beautiful lamella, but you can see it's the same in two directions, and it makes either a square or a contorted square, a parallelogram. So that's a two-way system. Okay.