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The Strength of Wind


GrandMasterK
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What does it take?

 

To level an upper middle class home with brick walls and a quality foundation?

 

To knock a building down with steel frame?

 

To drag 2-3 ton cars around?

 

To flip an 18-wheeler over?

 

 

I live in the south suburbs of Chicago. We get tornadoes here frequently enough. Every year at least one tornado (that is spotted) touches down in our county. One of the worst tornadoes ever happened right here not 3 miles east: http://en.wikipedia.org/wiki/Plainfield_Tornado#Tornado_touchdown.2C_path.2C_and_damage

 

 

I live in an upper class housing development. A couple years back when the development was still being finished on the other side we had a storm that managed to knock a completed house down. Two story house with thick big white brick walls, standing alone by itself at the end of a coldesac (spelling?). My friend and I road our bikes down that street the next morning. The house was completely leveled, wood and brick and glass scattered all over the place as far as 200ft in every direction. Luckily no one had moved in yet. We came around when one of the constructionists was standing outside the ruined home, kicking the tires on his truck and swearing as loud as he could. This storm happened in the middle of the night when it was raining so thick and hard you couldn't see the street or tell if it was raining water or hail. Lightning was flashing constantly, sometimes several times a second, in different colors, but it didn't help my field of vision at all. The sirens were going off all night, which meant that they spotted rotation on the radar or that somebody actually spotted a tornado in town or near town.

 

It was blamed on a microburst by the neighbor folk as no one really caught wind of an official analysis (if one was even done). I'd like to know how much wind it takes (and how long) to completely flatten a good house like that?

 

I studied micro bursts a little bit and they are described as producing straight lines winds outward in every direction from the area the burst came down at. Whats weird is the only other houses that were damaged were on that road leading to the coldesac. One or two garage doors were completely torn off, front and back lawn light objects were deformed and displaced. One garage door was curled up most awkwardly. It was curled up like a fruit roll-up, hanging from the top of the frame like window blinds. The bottom had obviously been sucked (why sucked?) out with such force that it broke loose of the frame, then managed to start curling up. How in the hell did wind do that? Seperate incidents from what leveled the house maybe but the houses at the end were only two lots away from the house standing by itself. A microburst can be strong enough to tear a house down in an area as small as a lot and be weak enough not to do anything to houses two lots away?

 

How much can a good building take? Lets take the Sears Tower for instance since I live in Chicago. Part of it's design for handling wind is that it sways. But could it stand a direct hit from an F5 tornado with 300+MPH windows without collapsing?

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that's a nice description---a graphic piece of writing

 

I don't claim expertise about wind effects but I can help get the ball rolling on this by mentioning a simple fact.

 

A wind blowing against a wall, the force it exerts goes up as the SQUARE of the wind speed. So if you triple the windspeed the force goes up by a factor of 9.

 

That is because if you triple the speed, then three times as many cubicfeet of air hit the wall per second, and also each cubic foot is going faster so it carries three times as much momentum. So nine times as much momentum is being stopped by the wall, per second. Hence 9 times the force.

 

But that is just a simple analysis for the case of a steady wind. Like in a hurricane.

 

There are also effects of sudden drop in PRESSURE. A building can IMPLODE, I think maybe. So tornado effects would be a whole different topic. It is probably not just a matter of calculating the force exerted by a wind of some speed like 150 mph.

 

Then there are also effects of FLYING DEBRIS, crazy stuff like lamp posts that the wind could pick up and throw at your house.

 

My college English teacher lived in a brick farmhouse in the country outside of town in the middle of Ohio. He got bored and took a job at an Ivy League and moved his family to East Coast. Good thing. That summer after they left a tornado leveled the whole house flat.

 

Masonry has it's disadvantages. Brick doesnt do well in earthquakes either.

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Wind is messy fluid dynamics stuff. When you're calculating the energy available for a wind generator the energy available goes up by the cube of the wind speed. (Mass X Speed^2) When you double the wind speed as the previous poster noted twice the mass passes that point so a wind twice as fast has the potential of 2(mass) X 2(speed)^2 for a total of 8 times the energy. However in the case of the wind hitting a house it gets messy... The way a house is shaped and the landscape around it would dictate how much of that energy is transfered to the house. It's possible that it could be modeled on a computer if you included the neighborhood... A good wind tunnel test would give you more info though. I think the car and truck questions may be answerable, but the building questions I think would have to be modeled building by building with the surrounding buildings in the model.

 

Just my thoughts.

 

-Ben D

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like has been stated, this is a very complex question (about the building), without knowing the circumstances. when there's an EXTREME low pressure system hovering right outside of an enclosed house, the house will attain a higher pressure...release that pressure and poof your house implodes...the pressure can be released in any number of ways as well, a tornado throwing a car through the front wall...all the windows simultaneously giving out, the roof getting ripped off by a tornado...etc...once it implodes and breaks apart, it becomes little pieces of debris, and can be tossed around like breadcrumbs. tornado's are truly a masterpiece of natures terrifying beauty. I live in Massachusetts, so unfortunately i've never seen a rateable tornado, of course, most people who have would call that a good thing.

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From a somewhat superficial reading of the U.K. building regs pursuant to building a new house for myself, it seems that the country is divided into areas according to maximum expected wind speeds, which alters the way in which walls and roofs should be built (e.g. max. single thickness wall height with different buttress spacing).

 

Existing walls can vary in wind resistance according to age and strength of morar joints.

 

For wind strengths required to move or break objects and structures in the U.S., why not look at how tornado strength is judged in the absence of known wind speed data? The old and trusted Beaufort wind scale relied on such practical observations, and it was good enough to circumnavigate the world in wood and sail ships.

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