Wood: Wood Spanning Systems

In this video, in the next few videos, I would like to go over some structural systems in wood, steel, and concrete, and their spanning abilities and their traits and characteristics. But before doing that, I think it is essential to review one-way and two-way spanning logics. I know I've done this before, this slide, but now I'd like to get down into detail and talk about wood, for example, in this video. So when a bay is square or pretty close to square, then a two-way system is more efficient than a one-way system. So if we have dimensions that are 1 to 1.25 times, the aspect ratio is 1 to 1.25 or to 1.5 even, then that distinguishes a one-way from a two-way system. A two-way system has more, let me make sure I'm on the right, yes. A two-way system has more structure, so it has like four beams. And therefore, it'll end up being shallower than a one-way system that has only two primary members. So in a one-way system, we have something called a primary member versus something called a secondary member. And then there is a tertiary member, which is the deck or something like that. So tertiary. And that's what I want to discuss in this video pertaining to wood specifically. Then I will repeat the same discussion for steel and finally for concrete. It is important to say, though, that for floor systems, there is very little two-way systems in wood and in steel. There's plenty of two-way systems, floor systems in concrete. Now, for roofing systems, ones you don't walk on, yes, there's plenty of two-way systems in wood and in steel, and definitely in concrete. But for floor systems, wood and steel don't exhibit much two-way action. So in the case, for example, of this roof at the Metropole Parasol in Seville by Jürgen Mayer, this one is a coffered system of wood that is accreted together. Clearly, the bay is equal in two directions, the cell, and therefore it's a two-way system, but it's a roof, not a floor. Similarly, for the Dowland grid shell by Edward Cullinan, the structure also, this grid shell, is a two-way system, but it's not a floor, it's a roof. Likewise, a lamella system, which is a very nice system that was used by the rural studio in Alabama. This picture is not from the rural studio, but the system is nice because what it is, is it's short pieces of lumber that are doing a pinwheel. And they lock into each other and whatnot. Again, this is a roof, not a floor system. So we do have them in wood, but not in roof, rather only in floor. Very good. So let's take a look then at one-way systems in wood. And we're going to start with tertiary systems, which are basically the topmost level. We have plywood. We have OSB. These are usually a half inch or maybe five eighths, three quarter inch. Yes, there is a 3/8 and there is a 1 inch, but those are not commonly used. The 1 inch is for very heavy loads. The 3/8, I don't know, is too flimsy to talk about. So a sheet or a panel, plywood or OSB, goes on top of secondary framing, which is repetitive. But right now we're talking about the tertiary And typically, the secondary system is spaced 16 inches on center. That's what the plywood or OSB can span. It can span up to 24 inches and not more than 24 inches. But the most common spacing is, in fact, 16 inches. So we can also talk about tongue and groove decking, which also likes the 16-inch spacing of a secondary system underneath it. So the tongue and groove system looks like that. And finally, we have CLT, or cross-laminated timber. And this one comes always in an odd number of layers, a three-layer, a five-layer, seven, and a nine-layer. So if we look at it, we can see that CLT can do one-way or two-way. If you have beams going in two perpendicular directions and you have the CLT running on top, then that's a two-way system. Because the cross-laminated timber, we have 2x4s or 2x6s running north-south and then in the odd layers, and then east-west in the even layers. So we have structure in both directions. Therefore, this is a two-way system. If supported by beams in two perpendicular directions, if it's supported on beams only in one direction, perpendicular to the odd layers, then that one is a one-way system. So cross-laminated timber. There's a lot to say about this one. maybe I'll have a separate recording for that one. But basically what we're talking about is the spacing of the supports underneath the panel. So in a three-layer, sorry, in a three-ply CLT, we'd like to have these beams, glulam beams typically, spaced every eight foot to 10 foot. In a 5-ply, there's 12-foot to 16-foot spacing. In a 7-ply, 18-foot to 24-foot, and a 9-ply, 25-foot to 30-foot span between glulam beams. Now, these panels come in very long dimensions. If you want, whatever fits on the truck, so up to 60-plus feet is possible. The width, whatever the width of the truck is, 8-foot, 10-foot, is also possible. but we need to put beams underneath them every 8 foot, 10 foot, somewhere in that neighborhood for a three ply. Very good. Not in this, I don't know how I messed up here and did not include it, but there's something called nail laminated timber, which is basically two byes in this direction, and they're nailed together into a panel and then picked up and put on top of glulam beams or whatever. There is something called DLT, which is dowel laminated timber, which is exactly the same as the nail laminated timber duplicate. But in the case of dowel laminated timber, there is a dowel that goes through a hardwood dowel that goes through these two buys and this one is preferred the dowel laminate timber because you can put it on a CNC machine and cut out whatever you need and router and do all that stuff ahead of time in the fabrication shop versus NLT if there's nails in there you can't put it on a CNC machine okay so there we go. That's the tertiary system, the topmost layer. It could be tongue and groove, it could be OSB, it could be plywood, or CLT, NLT, DLT. Very good. As for the secondary system, the secondary system is going to be repetitive. It's going to be closely spaced, typically 16 inches on center. It carries only uniform loads and not heavy loads and the spacing as I said is typically 16 inches on center. Now we have 2x6, 2x8, 2x10, 2x12, that's typical dimensional lumber and if you place them at 16 inches on center in a floor and you put plywood or OSB on top of them then you have a one-way framing system. Now, typical rule of thumb for floor joists in dimensional lumber, carrying normal residential loads, or maybe anything less than 100 pounds per square foot live load, a rule of thumb is take the depth, nominal depth, and add four to it. And that gives you an idea of the span. So a 2 by 8, add 4, 8 plus 4 is 12. That's a possibility of a 12-foot span versus a 2 by 12 plus 4. That can go in the neighborhood of 16 feet. I should put a line here because these are secondary framing. But then when you start sistering them or you do a built-up, now you're looking for a beam or a primary. a primary structural member. So we can put two by twelves together or we can put a piece of plywood or OSB in between two two by's two by eights with a half inch piece of OSB to make a header to span an opening. The idea here is this is an inch and a half for a two by this another inch and a half for a two by and in between them is a half inch which gives me one and a half plus one and a half plus one half a total of three and a half inches which works with a two by four wall another option to make a primary beam is to put a flitch plate or this piece of steel instead of a piece of plywood, put a piece of steel and it's bolted together. Therefore it needs a structural engineer seal. But a flitch beam is a nice solution for the 20 to 30 foot span and it can be incorporated into a 2x4 wall. If you have two 2x and a piece of steel half inch in between, that still gives me three and a half inches. And if I have a two by six wall, then I can add another piece of steel and another two by to make five and a half inches. Okay, so flitch beam. And then there is a box beam, which is huge. And it has two bys and two sheets of plywood or OSP on either side. and it has blocking in the middle every 16 inches on center, and this one is a big monster beam that can also serve as a primary, as one of these members to carry several other two-byes, Joyce, two-by Joyce. Okay, otherwise, there is some options here. How come I don't have the glue lamp? I thought I had glue lamp. Okay, anyway, so under structural composite lumber, the typical widths for these guys is one and three quarters or multiples thereof. Therefore, if you put two one and three quarters together, you get three and a half, which works with two by wall or two by four wall, or else maybe another one and three quarters to make three layers of one and three quarters, which gives me five and a quarter to work with, to buy six wall. So under structural composite lumber or SCL, we have LSL, we have LVLs, and we have PSLs. So LSL is laminated strand lumber. It's pretty ugly. It's got OSB in a beam, basically, and it uses wood flakes or shreds. They're shorter strands of wood, unlike LVL. It's ugly, therefore it's typically concealed. Versus LVL or micro-lam is laminated veneer lumber, and this one is more expensive, clearly. And it has parallel veneer layers. And this is what we were talking about earlier. This is typically 1 3⁄4 inch, and it could be 2 times 1 3⁄4, or it could be 3 times 1 3⁄4 to make 5 1⁄4. Another option is the parallel strand lumber, or PSL, which is even more expensive because this one will take stain, and it's possible to paint it. It's possible to use it outdoors. It's also called Paralamp. Very good. So those are the structural composite lumber. Then, of course, we have our iJoyce, which is basically a piece of, over here, micro-lam that is routered to allow for a sheet of OSB to go in between. and this guy will span in the 20 to 30 foot range, very popular solution for slightly bigger spans in residential construction. If not, then there's a truss joist floor system made of two by fours and this one also can span 20 to 30 feet. It's also called a gangnail truss, gangnail, because that's what this is. It's a gangnail plate that joins the two-by-fours together. So all of these are placed typically 16 inches on center, and then they get plywood or OSB. The same with the two-bys, but not these guys. These guys are not, we're not going to put them every 16 inches on center. These are primaries and not secondaries. It's just they're made of dimensional lumber. That's why I include them here. Very good. So as for primaries, oh, there's the glulam. As for primaries, then we have glue laminated timber, and that one can go in the neighborhood of 20 foot to 60 foot span. And we're going to place them based on whatever they're carrying. We're not going to place glulam beams every 16 inches on center. No, we'll leave that for the eye joist and the truss joist and the two by. So for these guys, we're going to place the glulam beams, the primary, at a farther distance. And a rule of thumb for glulam beams is take the span and divide by 20 to get an idea of the depth. This is the depth. So for a 60 foot span, I'll divide by 20 and I get three, which is three foot. So a three foot glulam will span 60 feet. And typically the width is equal to half the depth, a third of the depth, somewhere in that neighborhood. So one foot by three foot, somewhere in that neighborhood to span 60 feet. This primary member could be a truss or it could be a load bearing wall. But the point here is there is a primary, there is a secondary, and there's a tertiary. And please note that this terminology is for framing in plan, not in elevation. There's no discussion of the columns here. Although I have a load bearing wall, it's just receiving a bunch and we're calling it primary. So, tertiary is the topmost layer, below it and perpendicular to it is secondary, and below secondary and perpendicular to it it's like Jenga is the primary structure. I hope this helps.