Advances in material technology and structural efficiency are enabling Building Teams to produce structures that are lighter, more slender, and more architecturally innovative.
An unfortunate side effect, however, is that these buildings tend to sway in high winds — not enough to pose any danger of collapse, but more than enough to nauseate the tenants, especially in residential high-rises.
To control vibration and ensure comfortable conditions for occupants of tall buildings, Building Teams are turning to damping systems. In the process, they're often saving millions in costs.
Damping systems first received widespread attention in this country with an installation at New York City's Citigroup Center, a 59-story tower with a distinctive sloping top, which was completed in 1977. In earthquake-prone Japan, they are installed in some dozen buildings, where they are typically computer-controlled, active systems used to resist seismic movement.
A combination of stiffness and mass has traditionally been used to control, or "damp," the vibration of buildings and other types of structures, according to Brian Breukelman, general manager of Motioneering Inc., Guelph, Ont.
But designers are increasingly turning to mass damping systems to perform this function, says Breukelman, whose firm, a sister company of engineering firm Rowan Williams Davies & Irwin (RWDI), engineers and installs damping systems.
The pyramid-shaped top of the 67-story Park Tower in Chicago conceals a mass damper system that minimizes the impact of wind pressure against the building.
Building Teams are using three basic types of damping system. Passive systems include tuned mass dampers and tuned sloshing water dampers. Semi-active dampers "tune" the performance of passive systems through the addition of computers and instrumentation systems. Fully active dampers use computer-controlled actuators to produce forces that physically counteract externally applied forces. With all three systems, the damper is installed at the top of the building.
A damping system that utilizes 1,000 tons of water in two tanks recently came on line at Random House/The Park Imperial, a new 675-ft.-tall building in New York City that consists of 25 stories of office space topped by 25 stories of luxury residential condos. Skidmore, Owings & Merrill (SOM) was core and shell architect for the $170 million building, with New York-based Thornton-Tomasetti Group as structural engineer.
The office floors are steel-framed. At floors 26 and 27, the frame transitions to concrete for the apartments. The use of concrete for the residential portion reduced floor heights, allowing for more apartment floors.
The slenderness of the building — its residential portion has a footprint of only 9,000 sq. ft. — necessitated a heavy massing at its top to prevent excessive sway.
The original plan for the building did not envision incorporating a damper system. But when the Building Team put the design through a cost/value analysis, their calculations proved that installing such a system would reduce structural steel costs by several million dollars, according to Hamid Kia, SOM project manager. And even though the building's lateral movement would be imperceptible to its occupants, "you wouldn't want to be on the 40th floor with coffee that appears to be tilting in its cup," he added.
Dampers of the type used at Park Place are supported by cables at four corners. Four primary hydraulic cylinders dissipate wind energy.
The two water tanks, located close to the building's center of gravity, are oriented in east-west and north-south directions in space that would normally be allocated for mechanical equipment. "Architecturally, the only thing we had to do was to modify the penthouse to accommodate these two large 'swimming pools,'" says Kia.
A similar concept was used for a new building in Vancouver, B.C., where a local architect, Busby+Associates, designed the first Canadian high-rise to incorporate both hotel and residential floors. One Wall Centre is a slender glass tower with a 70-ft.-wide elliptical footprint and a 7:1 height-to-width ratio. To control building motion due to wind buffeting, a tuned liquid column damper was installed at the top.
According to Rob Simpson, principal in charge with the Vancouver-based project structural engineer, Glotman Simpson Group Consulting Engineers, the system utilizes two 50,000-gallon tanks that extend nearly the full width of the 48-story building. The walls of the four-story-high tanks are actually the sides of large beams that connect across the building, engaging four columns. Simpson says use of the water-based system saved about $2 million compared to the cost of conventional dampers.
Simpson offers a simple analogy of how the system works. "If you had a washtub full of water and pushed it one way, the water would tend to move in the opposite direction," he says. "The same thing happens when tuned liquid dampers are installed at the top of a building. They take energy out of building's vibration, and dampen building movement."
A fixed gate in the middle of the tank modulates the flow of water. If the water flow is "tuned" to match the movement of the building, "you'll get the greatest bang for the buck from the damper — the most resistance to building movement," Simpson adds.
Reelin' and a-rockin'
In Chicago, a tuned mass damper was installed atop the 67-story Park Tower, designed by local architect Lucien Lagrange and Associates. The Park Tower Hyatt Hotel occupies the lower 18 floors, with residential condominiums on the upper floors.
The Park Tower would not have needed a damping system except for its proximity to the 100-story John Hancock Center, says Breukelman, whose company designed the system and supervised its installation.
Wind blowing off Lake Michigan swirls around the Hancock building, creating high-speed venturi gusts that rock the Park Tower structure.
The 300-ton damper, which is 8 ft. wide, 18 ft. long, and 11 ft. high, is made up of 2-in.-thick steel plates stacked on top of each other.