"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.
Engineers expect tall buildings to give a bit, and the Comcast Center will be no different. A 75-m.p.h. wind would move the top floors about 18 inches, said Anjana Kadakia, project manager for Thornton Thomasetti, the firm that did the structural engineering.
The damping system doesn't prevent this kind of deflection. Rather, it makes the motion subside more quickly. If you imagine the building's movement as a wavy line on a graph, the wave would flatten out sooner on a building with a damper.
In high winds, the tank should reduce by about one-third the acceleration that a person would feel on the upper floors, said Ferguson, project manager for the water tank.
Engineers around the world have begun to choose water for some damping projects in part because it is cheaper than steel, and because pumping water to the top of a building is easy. In areas prone to earthquakes, the water also can be used as a backup supply for sprinklers, said Leighton Cochran, president of the American Association for Wind Engineering. The Philadelphia tank will not serve that purpose, however.
The developer of another building with a water-tank damper, One Wall Centre in Vancouver, was somewhat skeptical of the idea - at first.
"I've been up there many times on a windy day, especially when we were getting it finished, to see if this thing actually works," said Bruno Wall, president of Wall Financial Corp.
Now his top-floor tenants say the building never sways enough to make a ripple in their morning cup of coffee.
"It makes a remarkable difference," Wall said.
The Wall building is about half the height of the 975-foot Comcast Center. Its tank contains 460 tons of water.
The Philadelphia tank will hold 1,300 tons, and has piqued the interest of the local engineering community. Kadakia spoke at the Union League on Thursday about the tank and other design features, addressing a dinner meeting of the Engineers' Club of Philadelphia and the local section of the American Society of Civil Engineers.
Because there's no sunlight in the sealed tank, algae won't be a problem.
Some tall buildings, such as Chicago's Sears Tower, have no dampers, said John Zils, structural engineer for that project. Older buildings tend to be thicker and stiffer and don't need them. And in New York the buildings shield one another from the wind, said Cochran, of the wind engineering association.
For buildings in earthquake zones, hydraulic dampers are sometimes used to offset vibrations. Water-tank dampers aren't ideal for that purpose, and are used merely to make people feel at ease.
And that's no small thing, said Cochran, a principal of CPP Inc. in Colorado.
"As one of my clients said to me once," he quipped, "people who can afford the top floor or penthouse also can afford a pretty good lawyer."
Contact staff writer Tom Avril
at 215-854-2430 or email@example.com.