It's common knowledge that boats need ballasts to keep them steady in high winds. It turns out skyscrapers need them, too.
As Manhattan developers build ever taller, skinnier skyscrapers on ever smaller Midtown lots for multimillionaires seeking aerials views of Central Park, those buildings are more likely to sway at the top, the New York Times reports.
On your average breezy day, a 1000-foot-tall building can drift a couple inches, and on the rare day a 50-mile-per-hour wind blows through the city, it can move about half a foot. That motion isn't putting the building's occupants in any danger, but it might make them a bit dizzy.
The solution, developers and engineers tell the Times, is the installation of giant counterweights or dampening systems at the tops of buildings.
How do they work? One type of ballast, a tuned mass damper, is made of 300 to 800 tons of concrete or steel. Sitting at the apex of a high-rise, it acts like a huge shock absorber, pulling the swaying structure back to its original position by way of pistons and spring mounts attached to the walls.
Another type of ballast, a slosh damper or slosh tank, uses water as weight to keep a building steady.
The New York City building code doesn't require dampers and structural engineers stress that buildings are perfectly sound without them, so their purpose is all about providing penthouse buyers comfort. With top of the line apartments going for $100 million, $5 million spent on a ballast registers is a small price for level surfaces and unspilt champagne.
