Skyscraper sway: rattling elevator cables, a 660-ton pendulum, the inner ear limit
When Skidmore, Owings & Merrill finished the Burj Khalifa back in January 2010, they didn't just build an 828-meter rigid tower. They engineered it to sway up to six feet at the peak. Six feet. Hey, Grace. But six feet of movement, that breaks the illusion right away.
You stand on the sidewalk and look up, and you assume these tall buildings are just solid columns of concrete and steel. Hey, Salt. Yeah, if they were perfectly rigid, the wind would snap them. They have to bend. But the the flexibility creates a really practical problem inside.
If the top is leaning over by a few feet, what happens to a straight vertical elevator shaft? Right, because the rails the elevator rides on are, you know, bolted to the walls. If the walls curve, the rails have to follow. You can't just have a straight metal line inside a curved tube. Which doesn't always go smoothly.
In February 2021, reports from The New York Times and The Guardian came out about 432 Park Avenue in Manhattan. It's one of those super-skinny pencil towers, and the wind sway was so bad that that elevator cables were just rattling against the shaft and the rails shifted so the cars shut down. You're kidding. It's a real issue. The internal mechanics have to accommodate the bending.
Plumbers put in, um, expansion joints for the pipes, and elevator engineers have to leave tolerances so the cab doesn't get jammed when the building leans. Okay, so say the elevator is working fine. What does that sway actually feel like if you live up near the roof? Do you notice it? It's a slow, um, low-frequency rocking.
You don't feel the building leaning over, you feel the acceleration as it changes direction. It's a very distinct kind of motion. Oh, like being on a boat. Right. It triggers the same inner ear response.
People get seasick in their own living rooms, staring out a window while the horizon seems to shift back and forth. Wait, so is there a standard— like, building codes handle structural safety, but do they care if a resident is just throwing up all day? They do. There's an international standard, ISO 10137, that sets thresholds for human comfort in tall buildings. The engineers have to prove the sway won't cross that line for, say, a standard one-year wind event.
How do they fix it if it does cross the line? The most famous way is a tuned mass damper. Instead of stiffening the building, you— well, you put a heavy weight near the top and hang it like a pendulum. That's the one in Taipei 101, right? The big gold sphere, yeah.
It weighs 660 tons. When the wind pushes the tower one way, the pendulum lags behind and pulls it back the other way. It's kind of wild that it worked during that earthquake in Taiwan, the 7. 4 magnitude one in April 2024. The footage of that sphere swinging around is incredible.
It just absorbed all that seismic energy so the building itself avoided structural damage. And that's a passive system. It just relies on gravity and oil shock absorbers. But you can also build active systems. Active meaning it has its own motors?
Right. An active mass damper uses real-time computer controllers and hydraulic actuators to actively push a weight in the opposite direction of the sway. Since when? Since 1989. An engineer named Takuji Kobori put the first one in a building in Tokyo, working for Kajima Corporation.
They used, you know, computer-controlled actuators to read the sway and fight it. Wow. I'd sort of assumed that was newer tech. 1989 computers running a building's balance system feels pretty early. It was, and the modern versions are getting much more sophisticated.
In late 2025, there was a system co-developed by a researcher, Paolo Calvi, and a startup called ISAAC antisismica. They installed it on the new Torre Piloti in Genoa. The port tower? Yeah, it's the new control tower for the harbor. The system they put in actually won the international 'THE PLAN Award' for how effectively it stabilized the structure.
Okay, but what happens if the power goes out during an earthquake? Does the computer just die and leave the building unprotected? That's the catch with pure active systems. So now engineers use hybrid setups. During a standard wind storm or a mild tremor, the active controllers do the work.
But if a severe earthquake hits, the system is programmed to cut the power. Huh. So it protects its own machinery from getting torn apart by the extreme forces. Exactly. It just reverts to a purely passive mode and lets gravity take over for the really big hits.
It makes you wonder about the ceiling of all this engineering. As we keep pushing these tall structures higher into the atmosphere, the structural math is mostly solved. It won't fall. The steel holds. Right.
The limit is just our own biology. We're reaching the point where a a human inner ear just can't tolerate the horizon moving that much. It's an interesting line to bump up against. Thanks a lot for listening to Daybrain.
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