When the World Trade Center towers were being planned, the builders discovered that nobody really knew how much sway occupants of tall buildings could tolerate. Wind tunnel tests showed that the (relatively) light buildings would sway more than expected and the builders feared that any publicity about this would scuttle the project.
They began conducting two secret research programs. One investigated methods of dissipating energy to reduce sway. The other determined how much a room could move before occupants noticed it.
This second investigation was conducted under the pretense of "free vision exams." Participants were led into a room that was (unbeknownst to them) mounted on rails and moved by hydraulic rams. The amount of simulated sway was increased until somebody spoke up.
The engineers eventually determined that the sway could be brought within acceptable (though still detectable) levels by installing viscoelastic dampers between the floor joists and the building's perimeter columns.
The whole story is told in the 2003 book City in the Sky, which is a fascinating read.
> Inside the building, on those top floors, the oscillation is what unnerves us. A forty-story building may sway a foot to the left, a foot to the right. The span of that period might last around four seconds. A hundred-story building, by comparison, may move on the order of two-and-a-half to three feet to each side, cycling through a ten-second period. Typically, the taller the building, the longer the period of its cyclical motion.
And the implicit question of how much movement we feel:
> Acceleration is what causes the body forces that might tip us off our firmly planted feet, or nudge us back into the passenger seat of a car pulling away from a stoplight. Fighter pilots experience acceleration at many times the magnitude of gravity—“4 Gs” or more. The top of our hundred-story skyscraper accelerates through its period, as it sways from one side to the other, at a mere fraction of what a fighter pilot feels: maybe ten milli-g’s, or one hundredth of the force of gravity.
Surely it's not actually acceleration that you feel most of the time. A constant acceleration is, after all, indistinguishable from constant gravity, and so small acceleration vectors which, when added together with the local gravity vector, are still just a little more or less than 1 g, aren't really detectable - they just feel like gravity is pointing in a different direction. Obviously larger accelerations are detectable as being 'not quite like normal gravity'. When it takes different-to-normal effort to move your limbs, your body gives you feedback about it.
What you can feel is change in acceleration - a sudden change in the direction your body perceives as 'down'. Braking in a car doesn't feel much different to driving down a hill, but when the car's speed reaches zero and suddenly stops decelerating, the jerk as gravity snaps back to vertical is definitely noticeable. It's the jerk, not the acceleration, that throws you off your feet when you're standing on the subway and it pulls into or out of a station.
For a fascinating read on the intersection of buildings, wind dynamics, and a litigious society, see "The Fifty-Nine Story Crisis" [0]. The article describes the response of the architect/engineer of the Citicorp Tower, when he realized that the building would topple like a domino in winds that could be expected at least once every 16 years.
Why did you link through Wikiwand? It's some annoying overlay on Wikipedia that makes the window really small?
Here's the direct Wikipedia link: https://en.wikipedia.org/wiki/Taipei_101
Not much of the exterior shells of these super tall buildings are concrete. Those are the areas that are experiencing large deflections, i.e. are bending in response to the wind. The floor slabs are concrete, but they are essentially rigid and do not see much bending that would cause cracking. The notion of a shear wall and/or shear core allows for the strength of reinforced concrete in shear to be taken advantage of, while not exposing the concrete to deflections that would induce cracking.
Not a "vertical transportation" guy, but elevators are often placed in the center of buildings, surrounded by shear walls precisely for the same reason that shear walls are. Less bending, less chance for displacements.
A building is say 500 feet tall. The top moves say 2 feet.
In order for this to happen every 10 feet of the building doesn't shift sideways a certain amount. Every 10 feet of the building bends a certain amount. A VERY small amount. Tiny fractions of a degree.
So the bottom 10 feet of the building bends say 0.001 degrees. The bottom is flat, the top is tilted 0.001 degrees.
The next 10 feet bends an additional 0.001 degrees, but its base was already tilted 0.001 degrees so its top is tilted 0.002 degrees.
The next 10 feet bends an additional 0.001 degrees, but its base was already tilted 0.002 degrees so its top is tilted 0.003 degrees.
Repeat this 50 times (for 500 total feet) and you've got 0.050 degrees of tilt at the top which might be noticeable. Further, you have to add up displacements the whole way from the bottom to the top as well.
Partly because the materials in question aren't infinitely rigid, and partly because the torsion is spread somewhat uniformly across the entire structure, and never concentrated at any one point.
Concrete isn't entirely inflexible either, and it's typically reinforced with steel. Lifts as well - the materials have some ability to flex, and the rails for lifts have some headroom for imperfection. That's all quite intentional because we expect the lift to be in a building that will flex.
There's a really fascinating and relevant 99% Invisible [1] on a building with insufficient sway damping leading to potential for collapse. If I recall, a student discovers the issue while running calculations for a class and reached out to the architecture firm. Worth a listen if you haven't heard.
[+] [-] matthewmcg|10 years ago|reply
They began conducting two secret research programs. One investigated methods of dissipating energy to reduce sway. The other determined how much a room could move before occupants noticed it.
This second investigation was conducted under the pretense of "free vision exams." Participants were led into a room that was (unbeknownst to them) mounted on rails and moved by hydraulic rams. The amount of simulated sway was increased until somebody spoke up.
The engineers eventually determined that the sway could be brought within acceptable (though still detectable) levels by installing viscoelastic dampers between the floor joists and the building's perimeter columns.
The whole story is told in the 2003 book City in the Sky, which is a fascinating read.
[+] [-] wldcordeiro|10 years ago|reply
[+] [-] paulannesley|10 years ago|reply
> Inside the building, on those top floors, the oscillation is what unnerves us. A forty-story building may sway a foot to the left, a foot to the right. The span of that period might last around four seconds. A hundred-story building, by comparison, may move on the order of two-and-a-half to three feet to each side, cycling through a ten-second period. Typically, the taller the building, the longer the period of its cyclical motion.
And the implicit question of how much movement we feel:
> Acceleration is what causes the body forces that might tip us off our firmly planted feet, or nudge us back into the passenger seat of a car pulling away from a stoplight. Fighter pilots experience acceleration at many times the magnitude of gravity—“4 Gs” or more. The top of our hundred-story skyscraper accelerates through its period, as it sways from one side to the other, at a mere fraction of what a fighter pilot feels: maybe ten milli-g’s, or one hundredth of the force of gravity.
[+] [-] jameshart|10 years ago|reply
What you can feel is change in acceleration - a sudden change in the direction your body perceives as 'down'. Braking in a car doesn't feel much different to driving down a hill, but when the car's speed reaches zero and suddenly stops decelerating, the jerk as gravity snaps back to vertical is definitely noticeable. It's the jerk, not the acceleration, that throws you off your feet when you're standing on the subway and it pulls into or out of a station.
[+] [-] firethief|10 years ago|reply
This is not so much a quirk of our species as a physical impossibility
[+] [-] dmlorenzetti|10 years ago|reply
[0] http://people.duke.edu/~hpgavin/cee421/citicorp1.htm
[+] [-] nsxwolf|10 years ago|reply
[+] [-] sosuke|10 years ago|reply
The small scale demos just don't do it justice.
https://www.wikiwand.com/en/Taipei_101
[+] [-] Gravityloss|10 years ago|reply
[+] [-] boardwaalk|10 years ago|reply
Impressive and unsettling.
[+] [-] stox|10 years ago|reply
[+] [-] mschuster91|10 years ago|reply
[+] [-] rmxt|10 years ago|reply
Not a "vertical transportation" guy, but elevators are often placed in the center of buildings, surrounded by shear walls precisely for the same reason that shear walls are. Less bending, less chance for displacements.
https://en.wikipedia.org/wiki/Shear_wall
[+] [-] msandford|10 years ago|reply
In order for this to happen every 10 feet of the building doesn't shift sideways a certain amount. Every 10 feet of the building bends a certain amount. A VERY small amount. Tiny fractions of a degree.
So the bottom 10 feet of the building bends say 0.001 degrees. The bottom is flat, the top is tilted 0.001 degrees.
The next 10 feet bends an additional 0.001 degrees, but its base was already tilted 0.001 degrees so its top is tilted 0.002 degrees.
The next 10 feet bends an additional 0.001 degrees, but its base was already tilted 0.002 degrees so its top is tilted 0.003 degrees.
Repeat this 50 times (for 500 total feet) and you've got 0.050 degrees of tilt at the top which might be noticeable. Further, you have to add up displacements the whole way from the bottom to the top as well.
If you took a picture of the building sideways it would look like this: http://www.codecogs.com/users/23287/Cantilever-Beams-101.png
[+] [-] rosser|10 years ago|reply
[+] [-] munchbunny|10 years ago|reply
[+] [-] iaw|10 years ago|reply
[1] - http://99percentinvisible.org/episode/structural-integrity/
[+] [-] adam74|10 years ago|reply
https://www.youtube.com/watch?v=HB2jgJJG2is
[+] [-] z3t4|10 years ago|reply