Loads: 1. Loads Overview| Gravitational Loads Dl & Ll

Loads: 1. Loads Overview| Gravitational Loads Dl & Ll

Loads: 1. Loads Overview| Gravitational Loads Dl & Ll - Full TranscriptionIn this video I would like to give an overview of different loads on a building. According to the code there is gravitational forces which are essentially up and down and we have environmental forces. The gravitational forces are essentially dead load and live load, whereas the environmental loads are basically on the roof. They are rain and snow. Also, these are downward. The lateral forces include wind and seismic and hydrostatic pressure. Hydrostatic pressure is basically soils on a basement wall or a retaining wall or similar. Now, these guys are horizontal loads and they are dynamic. Unlike dead loads, for example, those are static and snow loads and rain loads and live loads are predictable to a certain extent within a reasonable accuracy. But lateral loads are one, they're dynamic. Two, they're unpredictable. Three, they change directions. So it could be east-west or west-east or whatever it is, but sometimes it also includes up-down in the case of earthquakes. So let's start with our gravitational loads. I'd like to look at this ugly building and discuss the loads on its roof, on the floors, on the envelope, and then finally on the foundation. So we're going to fill out this matrix as an overview. Then we get into detail in future videos on each of these loads. Excellent. So on the floor, we're going to have a dead load and a live load. The live load depends on occupancy. And it's given by code because it includes people and the activities in the space, furniture, and anything that is movable, things that you move around, movable stuff. Whereas the dead load is based on the type of construction. So is it a type 1, 1A, 2, 2A, etc., type 5 type of construction? and it includes the structure, the materials, structure, material, and anything that is fixed, that is permanently attached to the structure. Very good. The roof, of course, is going to have a dead load and a live load. Now, the live load on the roof is going to be less than the live load on the floor and the people are, there's not many people on the roof unless it's an occupiable roof that changes everything. So if it's an occupiable roof, we might treat it like a floor. Dead load depends on the type of structure, and we'll get into that in a little while. But let's get started with the live load. It is based on occupancy, as we said, and so I'd like to look at this table, table 1607.1 from the IDC code. Don't worry about the detail. I just want to say if you have a lot of people per square foot, therefore assembly space, or an egress route. In a fire, people are congregating the hallway to get out, that's a high occupancy, a lot of people per square foot. So if you have an assembly or an egress route, the live load given in this table is 100 pounds on every square foot of space. So let's look a little bit at this table and we will see, for example, dance halls. That's 100 pounds per square foot. Corridors, 100 pounds per square foot. Fire escapes, again, egress route, 100 pounds per square foot. So let's take this 100 pounds per square foot as a benchmark for live loads. When you have a lot of people per square foot, it's 100 pounds per square foot. Now, if we look on this side, we're going to see residential. Residential, not a lot of people per square foot. So the live load is a lot less than 100 pounds per square foot. A private office, for example, is going to be 50 pounds per square foot. The point is there's 100 and there's less than 100. So if we look at residential again, we see public rooms and corridors serving them. That's assembly and egress. And that goes up to 100 pounds per square foot even in residential. Very good. So we established the datum as 100 pounds per square foot. I'm looking at this table and I'm seeing 150 here. And I'm seeing that this is for stack rooms. Yes, books weigh a lot and it's going to be more than 100. Where else is there something more than 100 right here? Light manufacturing, heavy manufacturing, both more than 100 pounds per square foot. On the other side of this table, it says if you have storage warehouses, clearly that is going to be more than 100 pounds per square foot. So with heavy storage, for example, at 250 pounds per square foot, we need to watch out for something. In the case of concrete, we need to watch out for something called punching shear because that's a huge load. Okay, sidewalks, vehicular, driveways, yards, subject to trucking, 250 pounds per square foot. Okay, let's not worry about the detail of this table, but let's go back here and say live load is 100 pounds per square foot for assembly space and egress paths. So that's what we have, 100 pounds per square foot, live load. I need you to understand that the live load depends on the occupancy, basically. Furniture people, things moving around versus fixed stuff. Dead load, on the other hand, is basically the weights of the materials and the structure and the type of construction and what's in that wall section, what's in the ceiling assembly. All of that is going to determine the dead load. And we get it from tables, charts, manufacturers, literature, graphic standards, whatever it is. So zooming in here, let's look at how these specific materials are sold on a construction site. And what is the density of these materials? Please bear in mind, you don't need to memorize any of this. If you are an ARE candidate, it should be given to you. But let's understand that water on a construction site is sold by the gallon. Wood is sold by the board foot. So this is gallons, this is board foot. Steel is sold by weight. and just as a reminder, one ton is 2,000 pounds versus concrete. It's sold by the cubic yard and one cubic yard is 27 cubic feet. So one yard is three feet, one square yard is three times three, one cubic yard is three times three times three, which is 27 cubic feet. We need to talk a little bit about this board foot. I have a video specifically about board foot, but in this context, I would like to say that if you have a one foot by one foot by one foot cube, you can fit 12 board feet in it. So 12 board feet is equal to one cubic yard. 12 board feet equals one cubic yard. Sorry, one cubic foot. I apologize. So, very good. A formula to calculate board foot is very simple. You have a piece of lumber, and it has a width, it has a depth, and it has a length. And so the board footage is essentially B times D divided by 12 times the length times the number of pieces. So these have to be nominal. You don't take them into actual dimensions. You leave them alone as nominal. This one, on the other hand, has to be in feet. That's the definition of a board foot. So as an example, if you have a 2 by 6 that is 1 foot long, then it'll have this much board foot. Let me divide by 12. I leave these dimensions alone as nominal, not actual. And if I have one piece only, then this turns out to be 12 over 12, which is 1 board foot. versus if I had 2 by 12, which is 1 foot. I'll again divide by 12. And if I have one piece only, that's 2 board foot. So I can, let's say I have 2 by 6s. 2 by 6s. And they are 10 foot long. And I have 100 pieces. how much board foot do I have? How many board feet do I have? I divide by 12, that makes it 10, 1,000. 1,000 board feet. Versus if I had 2 by 12s that are 14 foot long and I need 50 pieces just as an example. Then I'm going to divide by 12 again and those cancel out. 28 times 50 is 1400 board feet. And so I can add these up now because they're a constant unit of board foot. And that gives me, as an example, 2400 board foot. Very good. Steel is sold by weight at 2000 pounds and concrete in the ready mix truck is sold by volume. Very good. So let's look at density. Density of water is 62.4 pounds for every cubic foot. One cubic foot of water weighs 62.4 pounds. Wood clearly floats. It's going to be less than 62.4. I don't know how much it is, but it depends on the grade, the species, and many other things, but it's less than 62.4. Concrete is important to recognize. I would like us to remember this number because this number is used in calculating stabilizing moment against overturning moment. And just dead weight of concrete is so much more than any other material, the dead load, not the density. So this one includes rebar. This is for reinforced concrete. So the concrete itself in this 150 number, again, I don't need you to remember the breakdown. I'm just trying to make a point that the rebar weighs nothing. It's the concrete that weighs a lot. So in a typical cubic foot, the concrete, oops, the concrete weighs the majority of the weight. Very good. And finally, steel. We think of steel as very light because we don't use a lot of it in construction. A column is a small profile. A beam is a small profile. A cable is a minute profile. But the density of steel is much more than the density, is much greater than the density of concrete because concrete has gravel, sand aggregates. It's vibrated. It's got a lot of air voids. So it's not as dense as steel. But in construction, a steel structure is much lighter than a concrete structure just because of its massiveness. A concrete structure is massive. In zooming out, I realized that I have a major mistake and my sincerest apologies. When I was looking at the video, I saw that I have this ugly mistake. It's not pounds per square foot. It's pounds per cubic foot. Sincerest apologies. Okay. So reinforced concrete weighs 150 pounds per cubic foot. Let's look at how this works. So reinforced concrete weighs 150 pounds per every cubic foot. So I'd like to look at a planar member, a wall or a slab, a linear member, a beam, and finally we'll look at this retaining wall. So, so far, I know that I have a live load. And what were the units of that live load? Let me look over. It said 100 pounds per square foot is a live load on an assembly. So the units are pounds per square foot. This is giving me a heartache, pounds per cubic foot. That's not useful. I need it to be pounds per square foot. So what I'm going to do here is I need pounds per square foot for a planar member. Likewise, in a linear member, I need it to be per every linear foot. So when you say, for example, in steel that a W12 by 58, 58 is pounds per linear foot. That's the weight of the steel beam per foot. The 12th is the nominal depth, but the 58 is pounds per foot. So it would be nice to have pounds per foot for this concrete beam. So back to the slab, 150 pounds per cubic foot. Well, what we need to do here is essentially we need to look at the thickness. I have a piece of concrete that is six inches thick. So if I multiply pounds per cubic foot, the density, and I multiply it by foot, I will end up with pounds per square foot. Please watch the units. I think they can answer a lot of questions on the ARE. Very good. 150 pounds per cubic foot, multiply that by the thickness of the slab. Six inches? No, we need to make it feet so that our units are consistent. So now it's saying it's half a foot times 150 pounds per cubic foot. That gives me 75 pounds to every square. So now this one square is going to have a live load of 100 pounds per square foot plus a dead load of 75 pounds per square foot for a total load of 175 pounds per square foot. Dead load plus live load. Very good. So that's how we calculate the weight of a slab. It's per square foot. Likewise, if I had a wall, a concrete wall that is four inches thick, I don't know its height, unimportant, because I would like to know how much one square is going to weigh. So in this case, if this is a reinforced concrete wall, then it is four inches thick. Let's make that into feet, and let's multiply by 150 pounds per cubic foot. And that gives me, this is a third of 150, that gives me 50 pounds for every square. This 50 pounds, this another 50, this another 50. Now give me the dimensions of the wall, the height and the length. I'll tell you the total weight of the wall. A beam, in contrast, wants the weight per foot, wants the dead load per linear foot. So what we're going to do here, just let's think of the units. I have pound per cubic foot. That's my density. If I multiply it by cross-sectional area, then I would end up with pounds per linear foot. So let's see. I have this beam. It's 12 inches wide. It's 24 inches deep. It's made of reinforced concrete. Then its density is 150 pounds per cubic foot. So this guy has a dead load of 12 inches, make it into feet, because the density is in pound per cubic foot, not per cubic inch. And its depth is 24 inches, make that into feet, at a rate of 150 pounds per cubic foot. So it's 1 foot times 2 foot times 150 pounds per cubic foot gives me 300 pounds per every linear foot. So 1 foot of this beam is going to weigh 300 pounds. The next foot is 300 pounds. Tell me the length of the beam and I'll give you the span of the beam and I'll give you the total weight. So let's say it's a 20-foot span. Then its total weight is going to be 300 pounds per every linear foot, and we have 20 linear feet. So it turns out this guy is going to weigh 6,000 pounds total. The 6,000 pounds is not as important as the 300 pounds per foot. Okay, let's move on to this retaining wall. It's trapezoidal, and it has its width at the top is 4 feet, at the bottom it's 10 feet, its height is 18 feet, and its length is 100 feet. And I would like to calculate the volume of this wall in order to place a ready mix order. Very good. They taught you in elementary school that the area of a trapezoid is rectangle plus triangle. And that's very good to know, but it's very cumbersome. I would much rather think of the area of a trapezoid as the average of the two parallel sides. Their average is this much. And the area of this trapezoid, this cross-sectional area, is equal to that width times the height. The average width times the height. So let me get an eraser and get rid of this hatch. Very good. So what we need to do here is to find the average width, which is essentially 4 feet plus 10 feet, divided by 2. So this is 14 feet divided by 2, 7 feet. So the average of 4 and 10 is 7. And the area of this cross-sectional trapezoid is 7 feet. times 18 feet, or 126 square feet of cross-section. Now, the volume is equal to 7 times 18 times 100 feet long. So that's foot times foot times foot, or cubic foot. So it turns out to be 12,600. What are the units here? I'm losing it. because this is a different software. Okay, 12,600 cubic feet. I don't want cubic feet. I want cubic yards because that's how concrete is sold. So let me divide by 27 cubic feet in one cubic yard. Turns out it's 466.6 or 467 cubic yards. Very good. You might add some because of losses because there's going to be a cylinder test, there's going to be a slump test, and you don't want to run short in a concrete pour. So that might go up a little bit. I'm not going to add any extra because I'm just trying to illustrate basic concepts here. So I have 467 cubic yards. One concrete, one ready-mix truck. it averages between 9 to 12 cubic yards. So I'm going to call it 10 cubic yards per truck. So I'm going to need 46.7 or 47 or might go up to 50 cubic yards. So if concrete costs 100, I don't know how much it costs, 100, 200, 300, 500 if you're in a remote area. So let's say 100 just to keep the math easy. If it's $100 per cubic yard, then the total cost is going to be 467 times 100 or $46,700.