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Why Does a Skyscraper Sway?

Although it may seem impossible, skyscrapers do sway. This side-to-side motion protects the skyscraper from damage during high wind gusts and earthquakes. Despite the fact that the sway is sometimes several feet in either direction, you may barely notice the smooth, almost rhythmic movements. This is because engineers go to great lengths to design skyscrapers in such a way that the occupants inside feel no disruption.

Simple Harmonic Motion
- Just as a tree must sway back and forth in the wind to avoid snapping, a skyscraper must do the same. The force that controls the sway of the skyscraper is simple harmonic motion. In simple harmonic motion, the sway or oscillation is periodic and sinusoidal or wave-like in nature. Seconds is the unit of measurement used to determine the length of time that it takes for the sway to complete one full oscillation. The frequency of the sways per second are measured in hertz or Hz.
Frequencies
- All buildings vibrate during times of high winds and earthquakes -- even those made of concrete and steel. The fundamental frequency or natural frequency of vibration for a skyscraper ranges depending on height. While ordinary one-story buildings have a natural frequency of vibration of approximately 10 hertz, a building between three and five stories has a lower frequency, which averages 2 hertz. While tall apartment buildings that are approximately 10 to 20 stories measure 1 hertz, a skyscraper has the lowest frequency, often around .17 hertz.
Elasticity
- Despite their rigid look, skyscrapers are made of elastic materials that allow them to sway when necessary. Elasticity is also necessary to help the skyscraper hold its shape. Without elasticity, skyscrapers would remain tilted to one side after swaying. Steel is usually the material of choice used to build the skeleton of a skyscraper. Although steel is capable of remaining rigid under extreme stress, it is also elastic enough to bend with the wind when necessary.
Tuned Mass Dampers
- Some skyscrapers contain tuned mass dampers. These hydraulic systems control the sway of the skyscraper, keeping it in tune with the every blowing winds. A tuned mass damper is a complex system, which moves a 400-ton weight back and forth inside the upper portion of the skyscraper. The weight moves according to wind direction and wind strength. A computer system monitors and calculates the wind speeds and their effects on skyscraper sway. The tuned mass damper adjusts according to these calculations, straightening the skyscraper after each oscillation.

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