Wind: Wind Exposure Categories

So this video is about wind exposure categories A, B, C, and D based on ASCE 7 minimum design loads issued by the American Society of Civil Engineers. This is based, these categories are based on surface roughness and they are used to calculate the wind pressure on buildings, such as this one. Exposure A has been deleted, and that one used to be densely built urban areas with tall buildings, and that was the problem. So it was deleted because of wind variability near tall buildings. That was too great of a variation. And it involves channeling and wake buffeting and other factors that are really difficult to predict and analyze. So wind tunnel testing was recommended instead. So here's some examples of wind damage in tall buildings. The example on the left is in Houston, and it was because of changes in the downtown area. Taller buildings arose, and wind was coming from a certain direction, and so it was buffeting and channeled towards this other building, and it lost all the glass panels. The picture in the middle is from Miami, and it's from Hurricane Wilma. And we all remember the example on the right, which is the John Hancock building in Boston, with all the problems that it encountered with the blue glass curtain wall. Anyway, if you're totally bored, there's a lot that can be read about that one. In the meantime, category A was basically deleted. So, exposure B is urban areas, urban and suburban areas, with closely spaced obstructions, typically 30 feet or less. So, it's the density of structures that is the main thing. And then we have, this is from the ASCE 7. So exposure B is suburban residential area with mostly single family dwellings, low-rise structures, less than 30 feet. In the center of the photograph have sites designated as exposure B. All of these are exposure B. Okay, here's some more examples of exposure B in an urban area. Again, closely spaced and represent surface roughness category B. Okay. Exposure C is open country, open terrain, grasslands with scattered obstructions, again, height less than 30 feet. Typically, see, the density here is a little bit less. And typically, airports, that's what this control tower is, airports are typically in exposure C. So here's an example of exposure C, flat open grassland with scattered obstructions having heights generally less than 30 feet. So here we have concentration, but then open grasslands, exposure C. So over here we have exposure B here, and over here in the back we have exposure C. Exposure D is the tricky one, and that one is flat terrain, unobstructed, near open waterways. That's what the definition of exposure D is. And so the wind is howling, and there's not much surface roughness to slow it down. So the wind speed is greatest for exposure D. And therefore, the resulting wind pressure will also be the greatest. so here's some examples I'm sorry I didn't have pictures I didn't think of pictures I just thought of these examples that I really like a lot the Stalneset memorial to the victims of the witch trials in Norway 2010 by Peter Zumthor and Louise Bourgeois here this had an eternal flame but anyway there's water here and there's a lot of wind speed coming and gusts and what have you without much surface roughness to slow it down. The other also example is Casa Malaparte in Capri in Italy, 1937 by Adalberto Libera. Again, we're near water and there's this beautiful, amazing villa and wind is hitting it at great pressures. So these are the four categories of exposure. And I'd like to say that D, of course, is the most severe and B is the least severe because there's a lot of surface roughness between trees, buildings, etc., density to slow down the wind. It's important to mention that snow is the reverse of this. So an exposure D has a lot of high winds, but the high winds would blow the snow off of a flat roof, for example. So although exposure D is most severe in wind, it's least severe in snow. And the reverse is true for exposure B. It's the least severe in wind, but of course with little wind comes more snow accumulation. So exposure B is difficult in snowstorms versus exposure D is difficult in windstorms.