Science Puzzle
Why Is There a Giant Ball Inside Taipei 101?
Taipei 101 in Taiwan is 508 metres tall and stands where typhoons and earthquakes are common. Near the top, between the 87th and 92nd floors, a golden steel ball weighing about 660 tonnes hangs from thick cables. Visitors can go and look at it.
When a typhoon hits and the tower starts to sway, the ball begins to swing. How does a hanging ball help the building?
The Answer
The ball is a tuned mass damper: a giant pendulum that swings against the building's sway and soaks up its energy.
Every tall building has a natural rhythm, a time it takes to sway one way and back, just as a swing has. Gusts that happen to match that rhythm can build the sway up, push after push, the way a child on a swing goes higher and higher. The engineers made the pendulum's swing take the same time as the tower's sway.
When the tower leans one way, the heavy ball lags behind and ends up swinging the opposite way, pulling on its cables against the movement. It's like pushing the swing at exactly the wrong moment: each push takes energy out instead of putting it in. Hydraulic pistons around the ball turn its swinging into heat, so the energy is gone for good. The damper cuts the building's sway by up to 40%, which matters more for the comfort of the people inside than for the strength of the structure.
During Typhoon Soudelor in 2015, the ball moved about a metre. Its weight, though large, is less than one per cent of the whole building, so it couldn't hold the tower down; and it doesn't generate any useful power.
The principle: Resonance and damping. A structure sways most at its natural frequency; a mass tuned to the same frequency moves out of step with it and drains the energy of the swaying away.