"This is almost like a large waterbed," said John Gattuso, senior vice president for the developer, Liberty Property Trust.
At least a dozen such tanks have been installed since they came into vogue in the 1990s, but other kinds of dampers have been used for several decades. As architects have designed their skyscrapers to be ever loftier and more slender, engineers have resorted to a variety of means to keep the willowy wonders in check.
Some - such as Taiwan's Taipei 101, the world's tallest building - use a giant pendulum. Others use huge chunks of steel attached to springs.
Another option is simply to make a structure stiffer, as the Eagles are doing with the ramp that swayed on several occasions as fans left Lincoln Financial Field.
But a damper is an elegant design solution for today's svelte skyscraper. The key is that the pendulum or water moves back and forth at the same "natural frequency" as the building it's in - but in the opposite direction.
The Comcast Center, for example, is expected to oscillate once every seven seconds when deflected by the wind, Motioneering's Guy Ferguson said. The water will slosh in the opposite direction in the tank's twin U-shaped chambers.
The motion of the water, pendulum or other mass helps offset any wind-induced acceleration felt by people in the building.
In addition, energy from this motion is absorbed by some sort of attached device, such as a hydraulic cylinder, and dissipated as heat. Otherwise, the building would rock longer. A car's shock absorbers work on the same principle.
In water tanks like the Comcast Center's, the energy is dissipated by vertical steel vanes, or louvers, that impede the back-and-forth flow of water.
