Loads: 3a. Wind Loads

So we talked about this building and the loads on it. We described the dead load, the live load. Then in another video, I talked about the environmental loads, rain and snow. And now I would like to discuss wind. And just another reminder, rain, snow, wind and seismic depend on geographic location. So let's take a look at what's going on with wind. First of all, there is a predominant or a windward direction. And basically, let's say the wind is coming from this direction. So that defines a windward facade where the curtain wall, for example, is pushing into the building, causing positive pressure on the curtain wall components. But on the backside, there is suction. Suction on negative pressure on the leeward facade. So, positive pressure on the face that wind is coming from, on the windward face versus suction on the leeward side. Also, on the two sides, there is also suction. So, negative pressure on the side. So, we have one windward facade. We have three negative pressure facades. Very good. Also, on the flat roof, the pressure is upward or outward. So this is another facade, if you will, where there is uplift. So I'm going to write the word uplift on the flat roof. Very good. When we look at the sloped roofs, or in this case, the hip roof, we know for sure that that back roof is under suction. Therefore, the shingles, whatever it is, will be pulled out, the metal standing seam roof. And the same on these two sides. There is negative pressure on all of these sides. I'm using blue for tension, red for compression. So on the leeward roof and the two side roofs, the pressure is negative. But then on the windward roof, this one in the front, it could be positive. It could be negative. I don't know. So for the windward roof, windward roof, sloped roof, the pressure depends on slope. Now, the other three roofs doesn't matter what the slope is, but on the windward roof, if you have a shallow slope, then you're going to have uplift or negative pressure. But if you have a steep roof, I don't know what steep and shallow are. There's all kinds of descriptions in the code based on this degree or that degree. I really like the UBC, the old code pre-2000, because it said shallow is 3 in 12 or less. Steep is 9 in 12 or more. In between is intermediate. Now there's too many tables, but you should keep the general logic in mind. So for steep slopes, it is positive. And you have positive pressure. And then there's intermediate. So intermediate is going to be either positive or negative. Intermediate. So I don't know. It could be either. Now mind you, please. Oops. this is the windward roof. And let's keep in mind that wind is free to come from any which direction it wants. So if it comes from this direction, then that becomes the windward roof. The other three will be negative. Very good. So the three roofs are independent of slope, but the windward roof depends entirely on slope. The steeper it is, the more like a wall it is. And so it's going to be hit by the wind. The shallower it is, then it's more like a flat roof and there will be uplift. Excellent. So let's look at this in section or elevation or whatever I've drawn here. On the leeward side, The pressure is assumed to be constant. So it's this much. Whatever that pressure is. We can get it from the code based on location and many other things. But there is an outward negative pressure where the curtain wall is being pulled away from the frame, the structural frame. On the flat roof, same thing is happening. There is uplift. and there is a resultant of the leeward side. There is a resultant of the uplift. It's the sum of those multiplied. It's so many pounds per square foot times the area of the roof or so many pounds per square foot times the square footage of the leeward facade. Now, on the windward side, things are a little bit different. the code says up to 33 feet, which is three floors roughly, the pressure is assumed to be constant. So you have this much pressure on the windward side. But then as you go up, that pressure starts to increase. And there's tables and charts to give me how much that pressure is. But the taller it is, then the more the pressure. Unlike the leeward side, it's assumed to be constant. It changes, but it's not significant compared to the windward side. So typically, this is assumed to be a triangle. On the windward facade, the pressure is assumed to be a triangle. On the flat roof and the leeward side, it's assumed to be a rectangle. So you know from your other courses that when you have a rectangle, the centroid is in the dead middle, and that's where that resultant would be located, versus on a triangular distribution. The centroid, it's heavier on the top than it is on the bottom. So the centroid is going to be located at one-third and two-thirds, and not one-half and one-half. So one-third, two-thirds, one-third, two-thirds, one-third, two-thirds. So basically, the resultant positive pressure. So we have these three acting forces on this building due to wind and what this does is something called an overturning moment. So overturning moment due to wind, overturning moment and moment is force times distance and in the line of action of each of these forces is horizontal and the uplift is vertical. So the rotation actually happens about point D because we're going to assume that this is a well-constructed building and that it is going to remain monolithic. It's going to remain monolithic. So it rotates like that. Now, you might think that it would rotate like that, but that would mean that the building broke at location B. Not a good idea. So we're going to assume that it's well built and it remains monolithic and stays together. So we're going to have to take extra precautions at location, oops, undo, at locations B and C. Because if this is a concrete retaining wall or basement wall, and then you come up with a steel structure, you must make sure that at B and C it is anchored properly and the steel remains on the concrete. So the pivot point of this moment is point D. Now, if this were an earthquake, then the ground would take this piece with it and move it to here. And you get something called base shear, whereas the upper part of the building wants to go this way. So one is moving to the left with the earthquake, and the other is moving to the right due to its inertia. Then you get something called base shear, or it breaks at sea because the building doesn't want to go, and the ground is going to the left, so we get base shear at the line AD. And then the earthquake reverses right away, and it reverses the direction of the building. It starts rocking. Very good. So back to overturning moment. I think this is a very important discussion. We're going to do it without numbers, so don't panic. But basically, I am making a moment about point D. I am not helping. I'm also making a moment about point D. and I am the biggest culprit and I am also rotating the building about point D. So we have three rotations that are all clockwise and somebody has to do counterclockwise and the somebody is called a stabilizing moment. So there has to be a dead load here and this dead load is at a certain distance from D and it makes a counterclockwise rotation versus the clockwise. The attack or the overturning moment was clockwise. The dead load of the building, W, the dead load, is going to create a counterclockwise moment to keep it from rotating. So we have a stabilizing moment versus an overturning moment. Stabilizing moment versus overturning moment. Very good. So it's not okay that those two be equal. It's more important that the stabilizing moment be a certain factor specified in the code more than what we suspect the overturning moment to be. So we find our location, we expect a certain force on the windward side, on the leeward side, and on the flat roof. We calculate the overturning moment, but then we throw in a factor based from code 1.67 to whatever it is then if you want to sleep at night you may take it up to four and calculate the stabilizing moment and make sure you have enough dead load dead load stabilizing moment is all about dead load so I have to have enough weight to counter the attack from wind so in Florida and other coastal areas if you have a tall building it has to be concrete in order weigh enough. It cannot be steel. So the taller the building, the more the overturning moment. The lower the building, the less the overturning moment. Obviously, it's a function of height. I have a picture here. Let me see where it is. These buildings have an aspect ratio that is ridiculously tall, but worse than these high rises, the high rises all about wind, everything. So worse than that is the skyscrapers, sorry, the pencil skyscrapers that are going up in New York City. And those have a crazy aspect ratio that immediately says wind. All of these tall buildings, they have to be concrete. Steel is too light, it'll overturn. They have to have five, six floors of basement to make the cantilever possible. And typically, they have a tuned mass damper at the top to reduce all the vibration from wind and the swaying. Okay, so back to our discussion here. We have overturning moment, stabilizing moment, very important to distinguish. I didn't do any But if you look at the videos of moment is force times distance, it's the same logic here. Very good. So let's take a look at the design of the curtain wall. On the windward side, I have panel number one, two, three, four, five, six. Since pressure increases with height, we need to be looking at the upper floor. In order to determine what's the worst scenario of compression or positive pressure on this curtain wall, and then we'll design all four facades to that level because wind is free to come any which way it wants. In structures, there is no prevailing wind. There is a prevailing wind, but we have to be safe and design for all four facades. So the answer is either five or six, but the corner is subjected to turbulence, to vortex shedding. The corner is a discontinuity. Wind doesn't like corners. The more aerodynamic the shape, the easier it is for wind to go around the corner. So six is more troublesome than five. And so the design would be based on the panel at the highest level in the corner. So corners are a discontinuity. Parapets are a discontinuity. Ridge, eave, rake, all of those are a discontinuity. Chimney is a discontinuity. Those are the first things to go in an earthquake. Sorry, in wind, hurricane, or tornado. Very good. By the way, the difference between tornado and wind, we'll see it in the next sheet. Let's look at the leeward side because we have to design for suction. We have to design for positive pressure. We decided panel number six. Now, looking at the side, we have panel A, panel B, C, D, E, and F. So, height doesn't matter here. It's just on the windward side that it matters. So on the leeward and the two sides, it doesn't matter. So I don't know which one controls here, but I assure you that it must be corner versus non-corner. So I'm not sure if it's panel C or panel F. I have to look at codes. There's plenty of tables that tell me how much the pressure coefficient is at the corner of the leeward side, at the corner of the side side, or on the windward side. Fine. Unimportant. So we're going to design it for that much tension and that much compression. Excellent. Very good. I covered all of these. I'm looking at my notes. Excellent. So let's see what wind pressure depends on. We looked at rain and snow, and we saw they depend on geographic location. Obviously, with wind, it's either coastal or inland. If it's an island, there's a lot more wind, and therefore less friction from the water. So it's going to be much more severe. So that's one factor, obviously. But then let's look at this sheet and try to make some sense of it. There's a lot of information here, but let's just focus on the big picture. You're not doing the PE exam. You just want to be able to talk and to argue with your engineer. Very good. So the intent of the code is to protect buildings and inhabitants, reduce the potential damage to property. Buildings and structures resist wind loads, not wind speed. That's a loaded statement. Wind speed is what the airport is measuring, and it's in miles per hour. And there is a map of the United States. It has wind speeds on it. That's not what we designed for. What we designed for is the pressure that results from the wind speed. The higher the wind speed, the more the pressure. How much? I have no idea. So there is this equivalence between wind pressure and wind speed. So you take your speed, you square it, and you multiply it by a factor here. It's the Bernoulli factor. And that gives you wind pressure. It is extremely important to understand that the wind speed is squared. Therefore, if you double the wind speed, you double square, or you get four times the wind pressure. It's not a direct relationship. It's a square relationship. So I've done the math here for you. 90, 90 square times.00256 gives me pounds per square foot. 90 miles an hour is 20 pounds per square foot. 100 is 25, 110, et cetera. I've added some here because the code has increased some of the wind pressures. So 150 mile an hour wind will exert roughly 60 pounds per square foot on the building. And 200 mile an hour is 100 plus pounds per square foot. So important to understand that we are designing for the wind load that results from wind speed. And the relationship here is you square this one in order to get the other one. Very good. Wind is assumed to come from any horizontal direction. It is assumed to strike perpendicular to the surfaces, shielding by taller buildings around your project. Not permitted. Designing for tornadoes is not required by code. It's not. Because it is so much more of a wind speed. But, of course, a good designer is going to have a safe house. Because in tornado, let the structure go. Life safety is what's important. With hurricanes and wind, you've got to minimize the damage and, of course, save lives. Very good. The main wind force resisting system must be provided in the north-south and east-west. They can't all be parallel, otherwise you have a bunch of dominoes. And the main wind force resisting system is basically a moment frame, a brace frame, or a shear wall. Those are my lateral strategies. Post and beam will not survive in wind. Excellent. So I think this has changed, but there is a minimum wind pressure. You cannot say I don't have wind pressure at all. Therefore, it's zero and there's no moment frame, brace frame, or shear wall. The minimum is 10. I think now with the current code, it's more like 20 pounds per square foot. So there's something called the allowable story drift and the allowable floor to floor drift and the allowable total drift. And it's much more than the deflection allowed for gravity loads. So it's a lot higher. So it's much stricter because I don't want the curtain wall to break. Okay. So, please watch the words here. Adequate strength. Strength means breaking. So, too much shear in an earthquake will cause the building to break this way. But too much bending from wind, bending moment, will cause it to fail. The connection would fail on the tension side. So, strength has to do with breaking versus stiffness. Stiffness has to do with movement and deflection and horizontal deflection, which is called drift. So there is a limit on how much one floor can move with respect to the other floor. And then there's an overall limit on how much drift. And it's a very small number. You cannot tolerate too much movement. Otherwise, inhabitants get dizzy, things break, etc. So the third condition is to make sure that the stabilizing moment compared to the overturning moment, and there is a factor of safety specified by code. So it has to be 50%, 60%, 80%, whatever the code says. But it's more than one and a half times for life safety. Okay, and then keep the moment frame, brace frame, shear wall symmetrical. We'll see this one with seismic, but if the lateral system is not symmetrical, then the building twists and it does torsion. And that's not a good thing to have, especially in an earthquake. So here are some scales, and I've gone in and written the current code here. The enhanced Fujita scale for tornadoes versus the Sapphire Simpson hurricane scale. unimportant. There is a map in the code. It gives me the wind speeds I know based on occupancy. So the importance factor is incorporated into the wind speed charts now. So let me talk about that very briefly. The old code used to talk about the importance factor of the occupancy. So there was a category for substantial hazard to human life in the event of failure. Sorry, this is four. This one was three. But the categories were substantial hazard to human life in the event of failure versus essential facilities. That one is even more strict. It's a police station, a fire station some facilities that need to do search and rescue after a disaster so they have to be designed to a higher standard versus other is basically residential shopping center things like that agricultural you can reduce a little bit the design let the cows fly unimportant very good so based on the importance factor there's different charts and different wind speeds but what's important to recognize is the coast, the Atlantic coast, has very high wind speeds. And the west coast has other issues than wind to deal with. There's seismic stuff. But then there is some wind concerns on the Pacific coast because there's also hurricanes that come from the Pacific coast. Okay, so what else do we need to say? Don't panic, but there is an equation. All I need from it is this. The wind pressure, Q, is proportional to the square of the wind speed. We talked about this. I think this is very important because everything else is a factor, the importance factor. This is an old formula, by the way. Now, the importance factor is called a risk category, and these charts are based on risk category. You're not an engineer. You're not studying for the engineering exam. You need to understand that the police station is more important than your house and mine in a disaster. Sorry, importance factor is for snow, for rain, for wind, and for seismic. It's for all of them. Very good. So what is all this stuff? Let's look at it. These are factors that are going to affect how much design you're going to incorporate. So we talked about exposures when we talked about snow loads. Exposure C is the airport where wind speeds are measured. And then exposure D is near water. And there's very little structures around. And the water is not resisting by friction. exposure D is worse than exposure C for wind, but better for snow because the wind blows away the snow. Exposure B is urban and suburban areas, many closely spaced obstructions, and exposure B is less than C. So we find exposure C, and then we reduce it based on exposure B or increase it based on exposure D. Typically, you can go into GIS and enter the zip code of your project and it'll give you exactly the exposure factor and all these things in that equation that I'm not worried about. Another factor is a topography factor. If you have a hill or a ridge or an escarpment and you're building at the top of it, well, you better increase your design to account for wind speed up. It just gets higher as you go on these topographic factors. Very good. I'm done almost. I just want to say that this profile of building is more aerodynamic. The wind can go around it more easily. There's very few corners. There's no corners. The more corners you have, the more the wind does turbulence and vortex shedding and whatnot. And then one facade longer than the other facade, that's the critical facade, the longer one. We talked about this one, and we said 33 feet or 10 meters is where the anometer at the airport is set up at that level. It's a standard, universal standard. Everybody does it. And then the pressure increases with height on the windward side, but on the leeward side, it's constant. On the flat roof, it's constant. We said here are the main wind force resisting systems. They are the same as the seismic force resisting systems. That's who is lateral and who can resist wind and seismic. Very good. So here's a graphic of this thing, and 33 feet or 10 meters is the datum. Now, if you have an opening, for example, a garage door that blew out, and the wind is coming from this direction, we know that we have two sides and one leeward, and the pressure on the outside is in the direction shown. But now the wind is coming in and making it worse on these facades. It's pushing further. And in section, it's pushing up on the roof. It's pushing on the back wall. It's pushing down on the floor. Versus if the opening, or let's say the garage door blew out and it was on the leeward side. Now the pressure is this way, but everything in here is being sucked out. And so that's the direction of the internal pressure as shown. This one just shows that if this is my windward roof, the pressure could be positive or negative, but the others are outward or negative. Clearly, a gable roof, this one, versus a hip. Which one is better? clearly the hip because it kind of deflects the wind versus the gable roof is hitting that entire facade.