Loads: 2. Environmental Loads

Let's talk about environmental loads. But before we do that, I'd like to bring to your attention the fact that these four loads, rain, snow, wind, and seismic, depend on geographic location. It rains a lot more in Washington State than it does in Arizona. Houston, it doesn't snow much. Alaska, you know the rest of the story. So it depends on location. And seismic-wise, there's the San Andreas Fault on the West Coast. It goes all the way up to Alaska and down all the way to Chile. So those depend on geographic location. Unlike dead load and live load, those are predictable with more precision. than rain, snow, wind, and seismic. Okay, let's get into environmental loads and look at rain loads first and then snow loads. So I've repeated the same building here. We're going to look at this building and we're going to say there is something concerning with flat roofs, long-span flat roofs. It's called ponding. And ponding is an accumulation of excess water on a flat roof, and it causes progressive collapse. And that's very dangerous. So they go in there and they figure out the roof drains they have and how many square feet each one will drain. They'll do all that design stuff. And then let's look at this a little bit in elevation or in section rather. So the closer I messed up. Okay, let's look at this picture. There's some additional rain accumulation ponding on this flat roof. I don't worry about this piece of equipment because we know it's there. It's part of the dead load and it's accounted for. There's extra structure underneath it. But this extra water, I don't know how many inches it is. But let's remember that water density is 62.4 pounds per cubic foot. I see a backup here or a scupper so that if there's excess water, then it drains through that scupper. So examples of long span flat roofs is basically in schools. You have a gymnasium, you have a theater, you have a cafeteria. These are, I don't know, 40, 50, 60 feet. In ponding or on a flat roof, that is a huge span. So we need to beware of excess water. So they will place, first of all, water drains at a quarter inch per foot. So if you have at least that much slope, then the water is going to drain nicely to the roof drain. But we have to have a backup system, which is a scupper, so that we get warning if the water is, if the roof drain is clogged, if this roof drain is clogged due to debris from a hurricane or whatever it is, then we need to make sure that we see the scupper and see that there is water coming out of the scupper. That's an overflow, so we need to do something about it ASAP. So here's a picture of two scuppers. Scupper A is flush pretty much with the roof, and water keeps coming out of scupper A. But if water is coming out of scupper B, then there is some accumulation of water, and we need to start worrying. So one inch of water is based on 62.4 pounds per cubic foot. Two inches, three inches, four inches, it gets ugly. So to think about it, three inches of water. Are you going to let me draw here? Oops. Three inches of water at 62.4 pounds per cubic foot. We have inches and cubic foot. That's not good. Let's make it into feet. Now, this one is feet times pound per cubic foot equals pound per square foot, which is where I want to be. And that turns out to be 15.6 pounds on every square of that roof. A typical unoccupiable flat roof is 20 pounds per square foot. Well, now with some accumulation, three inches of water, you're at 15.6 pounds per square foot in addition to the 20. So if you have four inches of rain, then you're looking at how much does this turn out to be? I did the math before starting the recording. That turns out to be 20.8 pounds per square foot. We've designed the roof for 20 pounds per square foot live load, but now it's raining and it's going to keep accumulating and this might get ugly very quickly. So we need that overflow scupper. Here's another example. In a butterfly roof, the main drain is pretty much flush with the roof. But then there's another one next to it that is a little bit higher. And that one is the overflow. So anytime water is coming through that drain, we have a problem. Okay, there's too much rain on the roof. So looking at this guy here with that additional rain, this is some of my former candidates from the ARE sent it to me. You can see the roof is starting to bow to give underneath that load. And what happened here was the open web joist, the bar joist, twisted or basically torsion. It twisted and it's no longer straight. And so it gave probably the depth of the joist is inadequate for additional load, or maybe they're too far apart and they need it to be a little bit closer so each joist carries a little bit less load so it can afford to carry additional rain load. So this roof, any flat roof experiences ponding and could lead to progressive collapse, The sink could come down. So, okay, that's enough on rain loads. Yeah, that's enough. Okay, let's talk about snow loads. So, just to recap a little bit, water. Oops. Why can't I write? Let me write, please. You're not going to let me write. Okay. Okay, let's come back and see if we can do it now. Nope. Okay, I didn't have a color. I probably had white. That's why. I'm sorry. So rain loads, rain or water is 62.4 pounds per every cubic foot. That's the density. density of light snow or fluffy or just snowed is more like 20 pounds per cubic foot versus heavier snow or not heavy but packed snow could be maybe 40 pounds per square foot which is double 40 pounds per cubic foot. I apologize. If you include ice, heavy snow plus ice, then you're looking, I don't know, that much pounds per cubic foot. Mind you, this is water. This is light snow. Ice, ice still floats. Think of the icebergs, but it's way closer to water density-wise. So ice is maybe 57, 58 pounds to every cubic foot. Very good. So those are some parameters regarding rain, snow, and regular water. So let's talk about snow loads. Designed for rain, snow, earthquake, wind, They all share one thing in common. They're based, two things in common. They're based on geographic location. That's one we talked about. The second thing is there's something called the importance of the occupancy. So the importance factor says a fire station, a police station are more important than a shopping mall or a residence. So you up the design based on the importance of the occupancy. An airport, a nuclear facility, a water supply. All these things are more important than individual, especially if they have high occupancy, like a designated shelter or something like that. Then we're going to up the design because we might have too many fatalities if something collapses. Very good. So the importance factor, geographic location, these apply to all of the lateral loads and environmental loads. Excellent. So at the airport, there is an anometer and there's instruments that measure wind speed, rainfall, how many inches, snowfall, and a seismograph that measures earthquakes in a seismic zone. Very good. So let's talk about snow loads. Snow loads. So we're going to get a generic ground snow load from the airport for the certain location. Ground snow load. Using GIS, you can get your specific exact snow load for the zip code your project is in. So that is not an unknown. We know the ground snow load. We know the wind speed. We know the annual precipitation. We know the seismic zone we're in. So all of this is known to us from the building code that is constantly updated. Every three years, the IBC changes all those values when appropriate. So we have a ground snow load, but we're interested in a roof snow load. So the roof has some properties that we need to address. One is it has a slope. Clearly, the steeper the slope, the quicker the snow gets off, but it could drop on a lower roof and therefore we have snow drifts that we need to be cognizant of. In addition to slope, there is the material of the roof. It could be a standing seam roof, metal, okay, the snow slides off quickly. It could be slate, it could be glass, compared to shingles, shingles have more friction, and therefore in a snow zone, you're more likely to see standing seam roof, for example, or a slate roof. So material and how quickly the snow gets off the roof. Then there is a hot or a cold roof, which means is the space under this roof heated or not? An atrium is going to be heated versus an attic is going to be cold. So clearly, the more temperature you have, the quicker the snow will get off. So very good. So these are some of the issues we need to watch out with with snow loads and rain loads. And of course, you're not studying to become an engineer, but you need to know just big picture issues. So there's something about wind loads that affects snow loads. And clearly, that is something called exposure. Exposure. There's three exposures in the IBC code. Exposure B, exposure C, and exposure D. D is near water. Near water means the wind is blowing at a high speed, higher speed, because there's less friction with the water. So the higher the speed of the wind, the more it removes snow off the roof. Versus an exposure B, that one is urban. Therefore, a lot of tall buildings and whatnot to slow down the wind speed. But the more the wind speed is reduced, the more the snow stays on the roof. And then you have exposure C, which is basically suburban. But most airports are in an exposure C. which means in a downtown area you're less than exposure C. If you're near water, you're greater than exposure C. C is the default, is what the airport reads. So exposure is an important thing in wind, but of course it influences snow as well. Excellent. - Thank you.