1977

Tuned Mass Damper

Sometimes engineers go to great lengths to solve problems, and the solutions are very cool. Then everything they do is hidden and no one gets to see it in action. The tuned mass damper found in many tall skyscrapers like the World Trade Center is an example of this phenomenon.

One way to think of a skyscraper is to imagine it as a long, flexible stick stuck in the ground. When the wind blows, the top of the stick will sway. The same thing happens with a tall skyscraper—the top of the building can sway in a strong wind. The wind is blowing against the side of the building and the building bends in that direction. Then the wind changes speed or direction and the top of the building shifts. Or the wind sets up a vibration in the building. No matter how stiff engineers make the building, this will happen to some degree. But the motion, especially on the top floors, can be uncomfortable. So how to lessen or eliminate it?

This is where the tuned mass damper comes in. The John Hancock building in Boston was the first building to use a TMD in 1977. It needed it because the top floors of the 60-story building swayed so much that they caused motion sickness. The tuned mass damper is a huge weight (typically hundreds of tons) that is free to move (imagine a pendulum) but is tied into the building’s frame with springs and/or hydraulic cylinders. Now when the top of the building tries to sway, the weight will try to stand still (an object at rest tries to remain at rest). With its springs and shocks, the weight’s stationary desire counteracts the sway, reducing it significantly.

One building where you can actually see the TMD is the Taipei 101 tower in Taiwan. A tourist caught the TMD in action during an earthquake and posted it to YouTube. Considering that the damper weighs nearly 1.5 million pounds (660,000 kg), seeing it move so freely is rather amazing.

Water also works. In one building, 100,000 gallons (380,000 liters) of water sloshes through baffles to counteract sway.

SEE ALSO Woolworth Building (1913), World Trade Center (1973), Burj Khalifa (2010).

Tuned mass damper on display in Taipei.