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A skyscraper is a giant tuning fork. Give one a good knock — like with an earthquake or a heavy gust of wind — and it’ll start vibrating at its own natural resonance frequency (about seven octaves below the lowest notes on a piano). If you’re on the top floor of, say, the 1,667-foot-tall Taipei 101, you could find yourself swaying back and forth abruptly, a total of up to 2 feet within five seconds. Highly barfogenic.
So Taipei 101’s designers hung a pendulum inside the building — in this case, they used an equation like the one below to determine that the megastructure needed a 730-ton weight with giant shock absorbers bolted to its bottom.
I think that's an oversimplified equation, the real world analyses the designers do is probably much more involved, and these days it might even use finite element analysis.
Originally posted by anon72
I think the equation is way too simple for the twin towers too. They installed about 10,000 visco-elastic dampers between support columns and floor trusses throughout the building and I'm not sure such a simple equation accurately represents such a complex interaction of so many components.
So, I see this and I look it over and then, the novice in me in the area of building/structures asks: Was this equation utilized in any of the calulations performed in the Twin Towers investigation to take into account any Sway Factor from the wind occurring at that time?
Even when the planes hit, I doubt the buildings swayed much more than 3 feet, if that, did they? So apparently the dampening system worked.
The WTC crew ran extensive tests to find out just how much sway they could allow without disturbing the building occupants. They put structural models in wind tunnels and even lured unsuspecting test subjects to movable rooms hooked up to heavy hydraulics.
In the end, they designed the towers so they could sway about 3 feet in either direction. To minimize the sway sensation, they installed about 10,000 visco-elastic dampers between support columns and floor trusses throughout the building. The special visco-elastic material in these dampers could move somewhat, but it would snap back to its original shape. In other words, it could give a little and then return to its initial position, absorbing much of the shock of the building's swaying motion.
Even when the planes hit, I doubt the buildings swayed much more than 3 feet, if that
Originally posted by anon72
What if the building was already in a sway at the time of impact-say 2 feet sway. The planes hitting it and the resulting damage may have all factored into a building that was in the process of self correcting-or rather didn't have a change to begin the self-correcting... Thanks for the links.