Steel: Steel Spanning Systems

So much like in wood, structural systems in steel are mostly one way when it comes to floor framing. With roof framing, yes, there are some two-way systems, as we see here, for example, with the new National Gallery in Berlin that Mies designed. But it's very unusual to see a two-way system in steel or in wood, except for roof framing, for floor framing, it's not usual. Floor framing looks like this typically, and is a one-way, where you have the deck is tertiary, we saw in wood that it was OSB, plywood, or CLT, DLT, or NLT, or whatever other system was a tertiary system, then it sat on glulam beams or similar as the secondary and then they rested on a larger member for tertiary. So when it comes to steel they're pretty much much is the same but instead of a sheet or a panel we have only one thing which is the galvanized metal deck. It comes in two varieties. One has these surface divots and it has these protrusions and whatnot and that is because it's going to receive concrete versus a roof deck. A floor deck receives concrete and shear studs that I will talk about but on a roof deck you don't typically walk on it the live load is very small except maybe for a maintenance guy and some equipment but a floor deck has a lot more live load and so it needs to have the concrete topping and these divots are to make the concrete bond to the deck. So we're talking about a tertiary system that spans on top of either open-web joists as secondary or steel beams as secondary, which in turn frame into a primary system, a girder or a truss or something similar. So clearly the deck is the tertiary member and it's perpendicular to secondary members, which are perpendicular to primary members. So here's a picture of a roof deck, and we can see that the corrugations are more straight and the deck is smooth because it's not going to get concrete. Instead, it's going to get insulation and roofing material. So the profile of it looks more like this. It's straight. It's not trapezoidal. And it's sitting in this case on an open web joists for roof framing. It could sit on a steel beam in a nicer building. It would sit on a steel beam in a cheap commercial construction. Then the roof deck would sit on open web joists that are closely spaced. And as for the floor deck, this one is going to have all these corrugations and divots for the concrete to bond with the deck. And there is shear studs here that also allow composite construction so that the metal deck with the concrete bond to the top flange and brace it of the steel beam below. The top flange of the beam is in compression and we're worried about lateral buckling of that compression flange, but adding the sheer studs, the metal deck, and the concrete makes that wide flange, the top flange, a lot wider because now part of the deck is included with that flange. So if you walk on it, we're not going to see that we have open web joists. It is very unusual to have a floor framing with open web joists. Open web joists are typically for roof because the live load is minimal. So here's a sectional drawing of that deck. And usually this deck is probably around three inches of concrete on top of three inches of deck. And then your shear stud is in there. And it's welded through the deck to the top flange of the beam below. And then they might add an edge angle here. they'll weld an edge angle and then all of this is going to come into for example a steel beam sorry a girder which is in section and the beam in elevation and the girder in section and the edge angle basically contains the concrete pour okay so That's what I have to say about the metal deck. Okay. Let's talk about potato chips. Potato chips have gone through evolution. It started with a flimsy chip that always broke. And then they gave it some corrugations. And then they hired a physicist who told them the best thing, the best shape for dipping and to make your chip strong is a hyperbolic paraboloid. Happy days. And then they even ribbed it. And now it's a very strong chip because of its form, because of its corrugations. It can dip a lot easier. I'm sorry, this is not something I should be talking about, but it is. Because what we're doing with these corrugations is we're adding section modulus. We're adding depth. We're adding section modulus. We're adding moment of inertia. In the case of a rectangle, I know these are the formulas for a rectangle, but the logic is the same. You have depth square and depth cube, and now the chip is a lot stronger because of additional depth. So it's basically they added, I don't know, a sixteenth of an inch. that added a lot compared to when it was less than that. So it's exactly the same with the metal deck. What happens here is we have these corrugations and the profile is so that it is a lot thinner. So it comes in here, it slopes, it comes up, it slopes, it comes down, it slopes when it's trapezoidal. that's a floor deck. The roof deck is rectangular. The profile of the roof deck looks more like that. So anyway, so we can see these spot welds that are telling us the deck is welded to the top flange of the beam below. And then they go back and they add shear studs. And we're attaching the shear studs through the deck, welding them to make composite construction. Once this is poured, they will put, sorry, before they pour, they will put welded wire mesh. And then they will pour the concrete. And the welded wire mesh is not doing anything but key. It's temperature steel. It's against cracking because the deck is spanning from here to here. It's the deck that is doing the span. And then the concrete in there is just to give it mass, to dampen it against vibration, and to separate it for fire. So typical spacing, I think much like in wood, in steel, and in concrete, this spacing is somewhere between 6 foot to 12 foot. That's the comfortable spacing for a metal deck. That's the comfortable spacing for cross laminated timber, as we will see with concrete. That's the comfortable span for a concrete slab, a one-way slab or a two-way slab. We'll see when we get to concrete. There will be a lot more two-way systems than we have seen thus far. So I tell my students, let's space the structure every 8 foot to 10 foot, 12 foot underneath the metal deck or underneath the CLT panels. We'll put some glue lamps every 8 foot, 10 foot, 12 foot. The same with the metal deck. We're going to put steel beams at that spacing. Very good. So now underneath the metal deck on the roof could be open web joists or steel beams. If we look at open web joists, let's switch to green. They come in three varieties, a K-series, an LH-series, and a DLH-series. LH is long span and DLH is deep long span. So the K series is up to 60 foot span and that one you can recognize it right away because it has a round web member. That's why it's called the bar joist. So when the web member is round it's called a K series and it's an open web joist it's a bar joist versus the LH series has a crimped angle or maybe it has a round bar in the middle but there's a crimped angle and that's what it looks like so it has more material and section modulus in the web and it's usually deeper than a K series member and whereas the k-series is good for up to 60 feet and a quick rule of thumb is basically take your span divided by 20 so to span 60 feet 60 divided by 20 is a three foot k-series member and they come in increments of two foot depth As for the LH series, a rule of thumb is take the span divide by 18. And then we have the DLH series or the girders, joist girders are big monsters and they can span up to 144 feet. Again, they come in increments. All of this is in manufacturer's literature. So it's L divided by 12. In the case of DLH, you have a pair of angles in the middle or a crimped angle, but it's a lot deeper. So we can see here a DLH member on the ground. It has a pair of angles for a top chord, a pair of angles for the bottom chord, and the web itself is also a pair of angles. So the spacing depends on the metal deck. And again, typically 6 foot to 12 foot is your typical spacing of secondary members. Now, looking at floor framing and sometimes roof framing in a nicer building, you're not going to have open web joists on the roof. You're still going to have steel beams, although the live load is a lot lighter than a floor load. So, typically it's picked up with steel beams, and the steel beam has a coping here. Let's switch to white. It has a coping where the flange stops here, but the web continues so that there can be another member coming in here to pick it up. So we have another member here, and it is able to pick up the beam, another member or a girder. Another option is a castellated beam, which is really fascinating. You take a wide flange, and you cut it on a certain pattern, and you shift it. So it used to be this deep, and once you shifted it, it got deeper for the same material. So that's an interesting way of making a beam as long as labor is minimal. So it's usually expensive because of labor, but if your machines or plasma cutter or whatever it is is doing the work and it's all CNC'd and programmed, then it becomes reasonably more affordable. Now, what's important about this profile and what I need to mention to you is that, oops, undo, sorry, is that the neutral axis is in the middle and it has no compression, no tension. Above it, the compression is increasing. Below it, the tension is increasing. But in the middle, there's very little stress. That's why this material can be removed. And you can run pipes or whatever it is through them. And depending on the way the cut is made, you can have a round opening or a hexagonal opening. And here they are at the fabricator. Here's the round cut. It's cut like this. And there's the other half behind it. And then they shift it. And they weld it back together. And you have a deeper member. The same with this hexagonal cut. And it's been welded back together. And this is a pretty deep opening. and we can run some systems through it. Very good. And finally, we have the primary members. And I should change this sketch because that could be the primary member and it's farther apart. And as long as the deck can span from here to here, if the deck cannot span from here to here, we will put purlins that can do that span and then run the deck perpendicular to those guys. So in that case, we would have a primary, secondary, and tertiary system. Very good. So that's what this would look like. And in this case, I have the deck, which is my tertiary. This is a floor deck tertiary. I have a secondary member perpendicular to the tertiary. And then I have a primary member, which is perpendicular to the secondary member. And if you notice these little holes here in the deck, that is when the concrete is cast, it's going to heat up and moisture is going to condense. Therefore, these holes allow that to drain. Please don't ask me what this member is doing because I don't know because it's parallel to the deck. So it's not carrying deck. This is the member that is carrying the deck. This one must be there's a load above it, a partition wall, something like that. And it's trying to pick that up and deliver a load to either side. So the primary support could be trusses. It could be castellated beams. It could be white flanges. But in this case, I have primary. And I have somebody who is repetitive. And it looks like it's closely spaced. I don't know what that spacing is. But this is the secondary. Secondary. And finally, we have something that is perpendicular to the secondary, that is the tertiary. And what I said earlier is this spacing is usually somewhere in that neighborhood for steel, for wood, for concrete. In this case, it looks a little bit less, but that's fine.