A view from afar of any major city will yield a remarkable sight of the shapes and styles of that city’s skyline. Adding a skyscraper to a densely-packed downtown area can be a challenge, and high susceptibility to wind-loads which cause sway on the upper floors is one of these. Wind forces on the facade of skyscrapers can produce bending motions of the long slender cantilever causing nausea and discomfort to the occupants if not properly controlled. Today, careful structural designs are combined with vibration cancellation systems to reduce the sway of tall buildings so that occupants are more comfortable.
During the 1970s in Boston, the 60-story John Hancock Tower was completed. The construction of this project spanned eight years from 1968 to 1976. As the construction neared completion, the 4’ x 11’ panes of glass curtainwall, weighing roughly 500 pounds each, started popping out and falling to the street below. During the scramble to identify the cause as well as find a solution, the missing window panes were replaced with plywood. The slender cantilever shape would bend, twist and sway under wind loads of 45 mph or more. A pattern was detected by the positions of the plywood panels revealing modal shapes of vibrational peak amplitudes. The solution was to install two 300-ton lead and steel boxes that slide on a lubricated plate and are attached to the steel frame of the building with springs and shock absorbers. As the building sways, the dampers remain roughly in place and counterbalance the motions of the building. Credit for this solution goes to William LeMessurier, a Cambridge engineer who recommended this “Tuned Mass Damper” system.
Today, most skyscrapers incorporate objects of very heavy mass within their structures designed with multiple degrees of freedom to offset motions due to wind and seismic forces. Concrete bunkers filled with water have successfully been used as a multi-directional damper. Below are two examples that illustrate innovative ways to control skyscraper movement.
The Taipei 101 is a 101-story, 1671 ft supertall skyscraper in the Xinyi District and is designed to withstand typhoons and earthquakes common in Taiwan. To help mitigate sway on upper floors, a 728-ton steel pendulum designed as a tuned mass damper is suspended from the 92nd to the 87th floor. The pendulum sways to offset movements in the building caused by strong wind loads.
The Shanghai World Financial Center is a 101-story, 1614 ft supertall skyscraper located in the Pudong district of Shanghai. Its innovative structural design includes steel trusses that guard against the forces of wind and earthquake, and made the building lighter and more sustainable. To mitigate wind-induced vibration, two identical active tuned mass dampers (ATMDs) are installed on the 90th floor. Under wind loads, the active control feature of the ATMDs hoists the steel cables of the mass dampers to tune their response to the frequency and magnitude of the building’s motions.
The engineering involved in super tall skyscrapers is truly incredible, which changes the question, “how tall is too tall?”