Take the lift to the 88th floor of Taipei 101 and you can walk up to a giant steel ball hanging in the open. It is 5.5 metres across and weighs 660 tonnes. Visitors take selfies with it. But it is not decoration: it is there to keep the tower from swaying so much.
A tall building is a very large, very stiff spring. Push the top sideways and let go, and it sways back and forth at a steady pace of its own, like a child's swing. The taller the tower, the slower the pace, a matter of several seconds per sway.
That matters because wind is not steady. Gusts arrive in pulses, and past a tall building the air can peel off the corners in alternating swirls that tug it left, then right. If the pulses happen to match the tower's own pace, each one arrives just as the tower swings the same way, and the sway builds up. This is resonance, the same trick that lets you pump a swing higher with tiny pushes. The tower is not in danger, but the people inside feel it as nausea and creaking.
You could stiffen the tower, but that costs thousands of tonnes of steel. The cheaper answer is to hang a heavy weight inside it that swings at the same pace as the tower. This is a tuned mass damper.
Here is why it works. When the tower lurches to the right, the weight, being lazy, stays behind. It lags about a quarter of a swing behind the tower, so it is always moving the wrong way: its pull on the building opposes the building's motion. Meanwhile, dampers attached to the weight turn its swinging into heat. Energy that the wind pumped into the tower leaks into the ball and then out as warmth. The weight does not have to be huge, only a small fraction of the building's own weight, but it has to be tuned well.
Below, the same gusty wind blows on two identical towers. The left one has no damper. The right one carries a hanging ball near the top. Both start in the worst case for the wind, gusts matching the tower's own pace. Tap the picture to give both a kick.
Sliders change the wind rhythm, the ball's weight and its own swing rate. Ball motion is drawn shrunk to fit inside the tower.
First, leave everything alone. The no-damper tower sways a lot; the damper tower sways far less, while its ball swings widely to soak up the energy. Now drag the tuning slider away from 1.00. The ball swings at its own pace instead of the tower's, and the benefit shrinks fast. A damper that is out of tune can do little more than add dead weight.
Now sweep the wind rhythm slider slowly from low to high with the damper tuned. Without the damper there is one big peak in sway when the wind matches the tower. With it, the single peak is split in two smaller ones, one on each side of the tower's pace, with a dip between them. That is the whole point of tuning: it trades one tall resonance for two low ones.
Taipei 101's damper is a steel pendulum hung from the 92nd floor, built from 41 welded steel plates, each 125 millimetres thick. It was designed by the engineering firm Motioneering and cost around NT$132 million, about US$4 million. It can cut up to 40% of the tower's movement. When Typhoon Soudelor struck on 8 August 2015, the ball swung by 1 metre, the biggest movement ever recorded for it. Every metre the ball moved was movement the building did not have to make.
The same idea shows up all over the place. Boston's John Hancock Tower (completed in 1976) and New York's Citigroup Center (1977) both carry dampers. London's Millennium Bridge earned the nickname "the Wobbly Bridge" when walkers' footsteps made it sway, and it was fitted with dampers as a fix. In 2005 Renault even put one in a Formula 1 car to steady the car, until the sport's governing body ruled the device illegal.
Next time a building sways in the wind, remember it is not weakness. Somewhere up near the top, a heavy ball is being lazy on purpose.