Imagine holding a yardstick at one end. When you flex your wrist even the tiniest bit, the far end of the yardstick whips back and forth. But if you hang a small weight on a spring at that far end, the weight's inertia absorbs some of the yardstick's kinetic energy and the whipping drops to a gentle sway.

Scale that up, a lot, and you have a mass damper, the system that's used to control movement in skyscrapers by reducing both the speed at which the building oscillates and the distance those oscillations cover. Mass dampers consist of large pendulums—usually steel plates bolted together to form a solid chunk—suspended from cables near the top of the building. When the building sways in a gust, the weight's inertia acts as a counterweight, pulling it in the opposite direction.

But a traditional damper wasn't good enough for the new Shanghai Tower. At 2,073 feet it's the second tallest building in the world, after the Burj Khalifa in Dubai. On the upper floors, where the flex is greatest, the oscillations would have been fast and wide enough to cause airsickness (not to mention paralyzing fear). So the building's architects and engineers came up with a solution. They installed a huge, tuned mass damper, the heaviest-ever flex-reducing weight in a building, and paired it with a magnetic system to create the first eddy-current damper used in a skyscraper.

Noah Sheldon

The "tuned" in tuned mass damper refers to an additional element: a control system to limit the weight's own motion, for maximum damping effect. In many buildings this is accomplished by passive damping control, but that has a couple of drawbacks: maintenance costs and the necessity for manual adjustments when vibration frequencies change. So Gensler, the architects of the Shanghai Tower, created the eddy-current damper, a masterpiece of simplicity. It consists of a 1,076-square-foot copper plate covered with 125 powerful magnets, mounted beneath the suspended mass damper. When the building sways, the 1,000-ton iron weight (360 tons heavier than in the previous largest damper) swings over the magnets, inducing an electrical current in the plate that, in turn, creates an opposing magnetic field, automatically counteracting the weight's motion and further amplifying its damping effect. No active control or outside power source is necessary. The magnetic flip occurs because of a version of Newton's third law called Lenz's law.

The result is not just elegant engineering but measurable benefit. According to Benedict Tranel, one of the architects of the Shanghai Tower, most people will never feel the building sway, not even in summer, when the typhoons roar in.

How it works

Sinelab

The Shanghai Tower's enormous tuned mass damper sits just below the roof and is five stories tall.

Noah Sheldon

The view from the top Noah Sheldon

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