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Friday / June 22.
HomeBuildingsThen there’s Building Sway…

Then there’s Building Sway…

swaying buildings

Clearly, if elevator shake in high-rise elevator shafts poses challenges, then the challenges posed by buildings themselves must be more so. After all, a too-slow or a too-fast elevator ride does not make the hair stand as much on end as the idea of a building’s tendency to sway in the wind. Wind – now and then high wind – always affects tall buildings, which toward the top can sway up to two feet back and forth in a high wind. Earthquakes are more problematic, but, as with hurricanes (except for Sandy), New York City has seen few, and none of consequence, thus far.

A swaying building can influence the comfort and even the safety of an elevator ride, but it can also cause sky-sickness in apartments toward the top. And it’s not so much wind pushing against a building: “Skyscrapers really start to rock,” writes Emily Badger in Gizmodo.com, “when the vibrations caused by vortex shedding bring the building’s motion into harmony with its natural frequency.”

New York’s second-tallest building, 432 Park Ave., whose 84 floors rise 1,396 feet, sways up to five feet in a high wind. A building half that height will sway a foot off axis near the top – that is, two feet from side to side – in a span of four seconds. A hundred-story building’s sway cycle might reach ten seconds. It is not the motion but its acceleration that most people feel. But people are variously sensitive to it. “We have this built-in mentality that some things move—like cars and elevators and airplanes,” one engineer tells Badger. “And other things do not, such as buildings.”

Still, the amount of wind prevents outdoor terraces in buildings above 40 or 50 stories, and windows open only a crack if at all, admitting a stiff wind always there that high. The resident of a 49th-floor unit in Chicago told SkyscraperPage.com: “I couldn’t figure out why I was feeling ill and went to the bathroom. As I was peeing I noticed the water was moving back and forth in the bowl.”

 

Fortunately, wind sway has focused the energy of building engineers, and for the tallest buildings solutions have been found that counteract sway. It might be a tank with sloshing water or a solid weight hanging on a steel cable. A damper is hung in the upper reaches of a building’s core and acts as a pendulum counterbalancing the effect of the wind. “When a building starts to move, that makes the damper want to move,” wind consultant Jon Galsworthy told CBC News. “And if you design the damper properly, it will lag behind the building.” That is, if it doesn’t accidentally swing in the wrong direction, doubling the sway. Of course, very highly sensitive control equipment helps to prevent that from happening.

 

Most people can live with these slight emotional inconveniences, since the distance views are great and the views down even better: the little people look even littler.