Confusing The Wind: The Burj Khalifa, Mother Nature, and the Modern Skyscraper
If you happen to check in to the Grand Hyatt San Francisco on a windy day, you’ll receive a friendly note at the front desk advising you that the 35-story skyscraper may creak a bit as it moves gently back and forth in the wind. Though the hotel assures guests that this quirk is not an indication of any structural problem, the issue has nevertheless prompted complaints from visitors.
“The building CREAKS!” exclaims one exasperated and sleepless customer in his review of the hotel.1
“It sounds like you’re on an old ship,” writes another.2
From the disconcerting to the dangerous, wind has always been an important consideration when constructing skyscrapers. Since the 10-story steel-frame Home Insurance Building, the world’s first skyscraper, opened in Chicago in 1885, architects have had to think about wind stress, or “wind loading,” as they’ve built higher and higher.3 Today, wind engineering is an integral aspect in the design of any new tall building, especially the very tallest of them all: the Burj Khalifa.
At 2,717 feet, the Burj Khalifa, formerly known as the Burj Dubai, rises like a bolt of lightening into the sky, dwarfing the surrounding skyscrapers. The tower, which opened on January 4th, became the world’s tallest building, outdoing the previous record-holder, the Taipei 101, by a staggering 1,046 feet. (The Burj is about as tall as the Taipei 101 with the Chrysler building stacked on top.) Over half a mile from the base to the tip of its spire, the tower redefines the term “supertall,” a name often applied to skyscrapers over 1,000 feet.
The Burj Khalifa is specially designed to conquer the wind, a goal that becomes more and more important as altitude increases. The building rises to the heavens in several separate stalks, which top out unevenly around the central spire. This somewhat odd-looking design deflects the wind around the structure and prevents it from forming organized whirlpools of air current, or vortices, that would rock the tower from side to side and could even damage the building. Even with this strategic design, the 206-story Burj Khalifa will still sway slowly back and forth by about 2 meters at the very top.
The Burj Khalifa’s talent for “confusing the wind,” as chief structural engineer Bill Baker calls it, is just one of the methods used to help supertalls resist wind stress.4 Over four thousand miles away near the coast of Taiwan, stands the Taipei 101 tower, now a distant second at 1,667 feet. Inside, between the 88th and 92nd stories, a giant pendulum, known as a tuned mass damper, does quiet battle with deadly windstorms and typhoons. The gold-colored, 730-ton orb swings gently back and forth, balancing the tower against the forces of the wind and ensuring the comfort of its occupants.5
The tuned mass damper, also used in Boston’s John Hancock Building and New York City’s Citigroup Center, is a commonly employed mechanism for reducing the wind’s action on a skyscraper. The size and shape of the damper is “tuned” based on the height and mass of each particular tower. As the wind pushes the building in one direction, the damper swings or slides the other way, reducing sway similar to the way shock absorbers on a car soften bumps in the road. “You’re adding a component to the building that’s going to take the motion rather than the building itself,” explains Jason Garber, a wind-engineering specialist at RWDI, a leading wind tunnel testing firm.6
When constructing a skyscraper, consideration of the wind is paramount, says Carol Willis, director and curator of the Skyscraper Museum in New York.7 Throughout the design process, structural engineers and wind specialists work meticulously to alleviate wind stress, ensure structural stability and guarantee the comfort of occupants. Using both structural solutions, such as the Burj Khalifa’s method of “confusing the wind,” and mechanical ones, such as the tuned mass damper, designers do constant battle against the tireless wind.
The Burj Khalifa, says Bill Baker, is like a Swiss watch, every part working together to “resist the forces of nature such as wind, seismic and gravity.” Yet forces like gravity are comparatively simple to deal with. Gravitational forces pull the skyscraper in only one, quite predictable, direction: down. But high-altitude winds swirl and jostle in complex and uncertain ways, whipping into eddies and vortices that put all different kinds of stress on the structure.
As Garber explains it, a building is like “a giant sail” with a great deal of area that the wind can push against. “The wind is blowing on the building causing it to sway and twist,” he says. “For certain shapes, the wind can form a wake similar to what you’d see behind a boat with vortices shedding off, alternating on either side and pushing the building from side to side.”8
“This causes a regular, or periodic, force,” continues Garber, “that pushes the building side to side across from the wind flow. The frequency at which that happens will vary with wind speed and if those vortices can align with the frequency that the building wants to oscillate at then you can get some very larges forces developed.”
Like a guitar string, buildings have a natural, or resonant, frequency at which they are inclined to vibrate. Wind vortices will only have a significant effect on a building when their frequency lines up just right, just as an opera singer has to hit the perfect pitch to shatter a wine glass. If by chance the vortices happen to push back and forth at the same rate as the structure’s resonant frequency, they can generate huge forces, as was the case in the Tacoma Narrows Bridge collapse in 1940. As a result of this effect, a key goal in skyscraper design is to disrupt the organized flow of wind around the building.Continued on Next Page »