The Pendulum Near the Roof

On a windy day, a tall building may move slightly from side to side. Engineers call this motion sway. A building is not perfectly stiff, just as a tree trunk is not perfectly stiff. Usually, the movement is small. However, people high above the ground may notice it. How can engineers reduce that motion without making the entire building thicker and heavier?

A Heavy Helper

One solution is a tuned mass damper. In one type of this system, strong cables support a huge weight near the top of the building. The hanging weight forms a pendulum, an object that can swing back and forth. Engineers often place it high up because a tall building's movement is usually greatest near the top. The weight has room to swing without bumping into nearby walls.

The system takes advantage of inertia, the tendency of an object to resist a change in its motion. When the building begins moving sideways, the hanging weight does not immediately follow in exactly the same way. It moves out of step with the building. At useful moments, the weight pulls against the building's motion rather than adding to it.

Finding the Right Rhythm

The weight cannot be just any size, and its cables cannot be just any length. Each building has a natural rhythm of back-and-forth movement. Engineers calculate how quickly the weight should swing to work with that rhythm. The word tuned means carefully adjusted to match. Like a musician adjusting an instrument, engineers make adjustments until the system responds at the right pace.

The goal is to reduce the amplitude of the building's motion. Here, amplitude means the distance the building moves away from its resting position during a sway. A smaller amplitude means less movement, not necessarily faster or slower movement. The building may still sway, but people inside are less likely to notice it.

Following the Motion

Picture a diagram with a tall building, a hanging weight, and arrows showing their movements. Its caption reads, "The weight helps reduce the building's side-to-side motion." The diagram also labels devices connected to the weight. Some of these devices contain oil that resists being pushed through small openings. They dissipate energy: they change some of the motion's energy into a small amount of heat that spreads into the surroundings.

A hanging weight is not needed in every tall building. Engineers choose different solutions for different structures. Where this system is useful, its carefully timed motion helps solve a comfort problem. A huge object moving near the roof can make the rooms below feel more still.