If you check in at the Grand Hyatt San Francisco on a windy day, you'll get a friendly note at the front desk that the 35-story skyscraper can creak a bit as it gently moves back and forth in the wind. . While the hotel assures guests that this quirk is not an indication of any structural issues, the problem has prompted complaints from visitors.
“The RANGE building!” exclaims an exasperated, sleepless customer in his hotel review.1“Feels like you are on an old ship,” wrote another.2
From the bewildering to the dangerous, wind has always been a major consideration in skyscraper construction. Ever since the 10-story steel-framed Home Insurance Building, the world's first skyscraper, opened in Chicago in 1885, architects have had to think about wind stress, or "wind load," as they built more and higher.3Today, wind engineering is an integral aspect of the design of any new tall building, especially the tallest of all: the Burj Khalifa.
At 2,717 feet, the Burj Khalifa, formerly known as Burj Dubai, soars like lightning into the sky, towering over the surrounding skyscrapers. The tower, which opened on January 4, has become the tallest building in the world, surpassing the previous record holder, Taipei 101, by a staggering 1,046 feet. (The Burj is almost as tall as Taipei 101 with the Chrysler Building stacked on top.) More than half a mile from the base to the top of its tower, the tower redefines the term "supertall," a name often applies to skyscrapers. above 1,000 feet.
The Burj Khalifa was specially designed to conquer the wind, a goal that becomes increasingly important as altitude increases. The building rises to the heavens on several separate shafts, ending unevenly around the central tower. This rather odd design deflects wind around the structure and prevents clean eddies of air currents, or vortices, from forming that would shake the tower from side to side and could even damage the building. Even with this strategic layout, the 206-story Burj Khalifa will slowly sway from side to side about 2 meters at the top.
The Burj Khalifa's ability to "confuse the wind," as chief structural engineer Bill Baker calls it, is just one of the methods used to help super-talls resist wind stress.4More than four thousand miles away, off the coast of Taiwan, stands the Taipei 101 Tower, now a distant second at 1,667 feet. Inland, between the years 88ºmi 92North DakotaIn many stories, a giant pendulum known as a tuned mass damper wages a silent battle against deadly wind storms and typhoons. The 730-ton golden sphere gently sways from side to side, balancing the tower against the forces of the wind and ensuring the comfort of its occupants.5
The tuned mass damper, also used in the John Hancock Building in Boston and the Citigroup Center in New York, is a mechanism commonly used to reduce wind action on a skyscraper. The size and shape of the damper "adjusts" based on the height and mass of each particular tower. As the wind pushes the building in one direction, the shock absorber sways or slides the other way, reducing sway similar to how shock absorbers on a car smooth out bumps in the road. “You're adding a component to the building that will receive the motion rather than the building itself,” explains Jason Garber, a wind engineering specialist at RWDI, a leading wind tunnel testing company.6
When building a skyscraper, consideration of the wind is key, says Carol Willis, director and curator of the Skyscraper Museum in New York.7Throughout the design process, structural engineers and wind specialists work meticulously to alleviate wind stress, ensure structural stability, and ensure occupant comfort. Using structural solutions, such as the Burj Khalifa's "confusing the wind" method, and mechanical solutions, such as the tuned mass damper, the designers constantly battle the relentless wind.
The Burj Khalifa, says Bill Baker, is like a Swiss watch, each part working together to "resist the forces of nature like wind, earthquakes and gravity." However, forces like gravity are comparatively easy to handle. Gravitational forces pull the skyscraper in only one highly predictable direction: downward. But high-altitude winds swirl and churn in complex and uncertain ways, becoming eddies and eddies that put all sorts of stress on the structure.
As Garber explains, a building is like "a giant sail" with a large area that the wind can push against. “The wind is blowing the building, making it sway and twist,” he says. “For certain shapes, the wind can form a wake similar to what you would see behind a boat with vortices breaking, alternating on both sides and pushing the building from one side to the other.”8
“This causes a regular or periodic force,” Garber continues, “pushing the building back and forth against the flow of the wind. The frequency with which this happens varies with the speed of the wind, and if these vortices can align with the frequency with which the building wants to oscillate, it can develop very large forces.”
Like a guitar string, buildings have a nature, orresonant, frequency at which they are inclined to vibrate. Wind vortices will only have a significant effect on a building when their frequency is aligned correctly, much like an opera singer needs to hit the perfect pitch to break a glass of wine. If the vortices push back and forth at the same rate as the resonant frequency of the structure, they can generate enormous forces, as was the case with the Tacoma Narrows Bridge collapse in the 1940s. Skyscraper design is to disrupt the organized flow of wind around the building.
“What they deliberately did with the Burj Khalifa,” Garber explains, “is to keep changing the shape of the building with height so that the flow pattern can't organize itself. It's almost like you have a bunch of different tall buildings and each one of them has different vortices blowing at different wind speeds. All of these things can't happen at the same time, so what's left is very little vortex shedding."
If not properly addressed, wind stress caused by vortex shedding could theoretically cause major structural damage or even collapse. There's no need to get seasick, as today's skyscrapers are strong enough to withstand the most extreme wind speeds, making true structural failure nearly impossible. High-rises are designed according to a 50- or 100-year return rule, which means that, on average, engineers expect winds to reach structurally dangerous speeds only once every half-century or so. Just to be sure, designers increase the strength of the structure by about 60% more to compensate for the uncertainty in their measurements. When all of that is factored in, Garber says, "you're talking about something along the lines of a 500 or 1,000-year event." The bottom line, he says to her, is that these buildings are not in danger of collapsing.9
Even so, wind stress can still cause all sorts of problems in tall buildings. It can break window panes, damage exterior façade, stress construction joints, cause leaks, crack walls, and create a myriad of other problems. Plus, it can result in a bewildering and even nauseating swing.10
“If the building is moving a lot, sometimes you can hear it creak,” says Garber. However, “the most common concerns are excessive movement. It can make people complain that they feel like the building is moving or they can even feel sick.” That was what happened in the old Gulf & Western building in New York. As a result of the force of the wind, the 44-story building developed cracks on the stairs and interior walls. Also, office workers on the upper floors often complained of dizziness on windy days. To fix these issues, the owners invested over $10 million to add a massive steel brace to stabilize the structure.11
In fact, measures are taken to counteract the wind for both comfort and safety. Occupant happiness is an especially important issue for structural engineers, Willis says. “People are more sensitive to wind than structures. Tuned mass dampers, for example, are used to deal with the acceleration and discomfort of people and the roll response of buildings."
Wind stresses increase dramatically the higher you build. Not only does the speed of the wind increase with height, but the strength of the wind also increases with the square of its speed. This means a rapid increase in wind stress as the height of the building increases, which can cause even the most rigid skyscrapers to slowly sway back and forth.
"In any building," says Garber, "the amount of movement you would expect is on the order of 1/200 to 1/500 times its height." For the Burj Khalifa, that translates to around six to four feet. “Not much, but certainly enough to make residents feel dizzy if they feel that movement. For this reason, one of the main concerns of architects and engineers is acceleration, which can generate remarkable forces in the human body”.
In amusement rides, cars, and airplanes, physicists often think of forces in terms of "g," multiples of the force of gravity. "When we look at buildings," Garber explains, "we're talking about milli-g of force." As long as the occupants do not feel the movement of the building, some degree of sway is acceptable and even expected. Humans can feel acceleration as small as 5 to 25 milli-g, far less than the structure can actually handle.12In most cases, such as the John Hancock Building and Taipei 101, tuned mass dampers are installed not to ensure structural stability but to prevent motion sickness.
The skyscrapers undergo rigorous wind tunnel testing during the initial design phase. Rowan Williams Davis & Irwin Inc. (RWDI), one of the world's leading wind engineering consulting firms, has conducted testing on numerous projects around the world, including the Burj Khalifa and Taipei 101.
At RWDI, wind engineering specialists collaborate with the building's structural engineers from the very beginning. Prior to construction, wind engineering specialists receive complete architectural plans for the building and the RWDI team sets to work building a rigid and complex scale model for testing. These models are covered with small holes, called pressure taps, which are used to measure the effects of wind. The 1:500 scale model of the Burj Khalifa, for example, contains 1,140 separate pressure taps for collecting wind data.13
These elaborate replicas undergo several rounds of testing in a specialized wind tunnel. Unlike the tunnels used to test airplane wings, sports equipment, and other small projects, theseboundary layerWind tunnels are designed to simulate changes in wind speed with height and can replicate the changing wind environments in which buildings will be built. Inside the tunnel, the model rotates at all different angles and the effects of the wind are sometimes visualized using smoke. All this data is then fed into computer models for further analysis. In the case of the Burj Khalifa, wind tunnel testing led to a drastic design change: The entire building was rotated 120 degrees to reduce wind load. Ultimately, this wind testing process provides structural engineers with a nuanced understanding of wind loads.Continued on next page"
interviews
Garber, Jason. Rowan Williams Davies and Irwin Inc. – Senior Specialist in Wind Engineering. Interview by phone. November 12, 2009.
Gilchrist, Russell. Skidmore, Owings & Merrill LLP – Practice Leader. Interview by phone. November 9, 2009.
Glickman, Leon. MIT – Professor of Construction Technology and Mechanical Engineering. Interview by phone. November 10, 2009.
Clerks, January. Council on Tall Buildings and Urban Habitat - Research and Communications Manager. Interview by phone. November 4, 2009.
Employees, John (2North DakotaInterview). Council on Tall Buildings and Urban Habitat – Research and Communication Manager. Interview by phone. November 13, 2009.
Toukan, Abdelfattah. Council on Tall Buildings and Urban Habitat – Country Representative (United Arab Emirates). Interview by phone. November 4, 2009.
Willies, Carol. Skyscraper Museum – Curator and Director. Interview by phone. November 5, 2009.
Photos
"WTC the Bahrain". Wikimediacommons.org.http://commons.wikimedia.org/wiki/File:Bahrain_WTC_day.JPG. Consulted on November 17, 2009.
"Burj Khalifa." Wikimediacommons.org.http://commons.wikimedia.org/wiki/File:BurjDubai.01.vm.jpg. Consulted on November 18, 2009.
Other sources
CTBUH.org. Advice on tall buildings and urban habitat. "Tallest Buildings Database".http://www.ctbuh.org/HighRiseInfo/TallestDatabase/tabid/123/language/en-US/Default.aspx. Consulted on November 12, 2009.
1.) TripAdvisor. with. “Grand Hyatt San Francisco: Traveler Reviews.”http://www.tripadvisor.com/ShowUserReviews-g60713-d80999-r4715960-Grand_Hyatt_San_Francisco-San_Francisco_California.html. Consulted on November 17, 2009.
2.) TripAdvisor. with. “Grand Hyatt San Francisco: Traveler Reviews.”http://www.tripadvisor.com/ShowUserReviews-g60713-d80999-r2481265-Grand_Hyatt_San_Francisco-San_Francisco_California.html. Consulted on November 17, 2009.
3.) Moffett, Marian, Michael Fazio y Lawrence Wodehouse.A world history of architecture. . . . London: Lawrence King Publishing Ltd., 2004. Pp. 446.
4.) The Skyscraper Museum. skyscraper.org. “Tallest building in the world: Burj Khalifa.”http://www.skyscraper.org/EXHIBITIONS/BURJ_DUBAI/index.htm. Consulted on November 4, 2009.
5.) Eddy, Nathan. "Taipei 101's 730-ton Tuned Mass Shock Absorber".popular mechanics. May 2005.http://www.popularmechanics.com/technology/industry/1612252.html. Consulted on November 15, 2009.
6.) Garber, Jason. Rowan Williams Davies and Irwin Inc. – Senior Specialist in Wind Engineering. Interview by phone. November 12, 2009.
7.) Willis, Carol. Skyscraper Museum – Curator and Director. Interview by phone. November 5, 2009.
8.) Garber, Jason.
9.)Ibid.
10.) The Skyscraper Museum. “Tallest building in the world: Burj Khalifa.”
11.) Greer, William. "Rx to shake skyscrapers."New York Times. October 24, 1982.http://www.nytimes.com/1982/10/24/realestate/rx-for-swaying-skyscrapers.html. Consulted on December 1, 2009.
12.) Boggs, Daryl. “Acceleration Rates for Human Comfort in Tall Buildings: Peak or RMS?”CTBUH Monograph Chap. 13: Motion perception tolerance and mitigation. Council on Tall Buildings and Urban Habitat, 1997.
13.) RWDI. com. "Burj Khalifa."http://www.rwdi.com/project/burj_dubai. Consulted on November 17, 2009.
14.) “World Trade Center de Bahrain”.popular science.http://www.popsci.com/bown/2008/product/bahrain-world-trade-center. Consulted on November 10, 2009.
15.) Klerks, Jan. Council on Tall Buildings and Urban Habitat - Research and Communications Manager. Interview by phone. November 4, 2009.
16.) Gilchrist, Russell. "Towards Zero Energy: The Tower on the Pearl River".Smart glass solutions.Smart Publications Ltd.
17.)Ibid.
18.) Frechette, Roger E, and Russell Gilchrist. "'Towards Zero Energy': A Case Study of the Pearl River Tower, Guangzhou, China". Featured in Council Minutes on Tall Buildings and Urban Habitat 8ºMundial Congress. Dubai, United Arab Emirates.
19.) Gilchrist, Russell. "Towards Zero Energy: The Tower on the Pearl River".Smart glass solutions.Smart Publications Ltd.
20.) Gilchrist, Russell.
21.) Glicksman, Leon.
22.)Ibid.
23.) Glicksman, Leon.
24.) TIME. with. "Time's Best Inventions of 2008: 16. Dynamic Tower."http://www.time.com/time/specials/packages/article/0,28804,1852747_1854195_1854134,00.html. Consulted on November 13, 2009.
25.) Klerks, enero (2North DakotaInterview).
26.) Gilchrist, Russell.