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What device in Taipei 101 helps it sway less in wind and quakes?
A giant steel pendulum ball hangs near the top.
A tuned mass damper is a large mass attached to a structure through springs, cables or a pendulum, and it is tuned to the building's natural swaying frequency. When wind or an earthquake sets the tower swaying, the damper's inertia makes it move out of step with the building, so it pushes against the motion and absorbs energy. This lowers the size of the sway and the discomfort for people inside. The principle is described in engineering literature, including analyses of the frequency tuning that makes it effective.
In Taipei 101 the damper is a steel sphere hung as a pendulum in the upper part of the building and visible to visitors at an observation level. Public sources commonly give its mass as about 660 metric tons, though this figure is quoted from general reporting rather than from the pages consulted here. The building's operator describes the device as absorbing slight swaying and suppressing the worst of the movement during strong winds or earthquakes. Typhoon winds and seismic activity are both significant in Taiwan, so the damper is a central part of the design.
The damper works alongside other features. The operator lists a foundation of 380 piles, outrigger trusses that tie the outer and inner columns together, and large sloping columns that add stiffness. Dampers do not prevent a building from moving. They reduce the amplitude and duration of sway, and they are most effective when the building shakes near the frequency they were tuned to. Because of this, it is a mistake to think the ball makes the tower immune to earthquakes.