The Aluminum rods of British Pavilion in Shanghai are void or soild?

Thursday, September 23, 2010

Burj Khalifa

Skeptics question the logic behind building a supertall skyscraper in the middle of the desert. Others are less interested in why the Burj Khalifa exists than how it was built. The secrets to its construction might surprise you. While the Dubai landmark dwarfs its closest rival in the competition for world's tallest building by more than 1,000 feet, it doesn't flaunt its architectural muscle. Rather, its design is as straightforward and logical as it gets.

At the heart of that logic is the building's triaxial geometry. "The Y-shaped plan is ideal for a residential building because it gives plenty of surface area per unit, and structurally, it is much better than a cruciform or linear tower," explains Adrian Smith, FAIA, former design partner at Skidmore, Owings & Merrill (SOM) in charge of the project through the completion of construction documents. And though SOM's competition-winning design for the Burj far exceeded the approximately 550-meter (1,800 foot) height called for in the brief to make it the world's tallest, the scheme — originally at about 700 meters, or 2,300 feet — was selected based on its appearance and construction feasibility, according to Smith.

The center of the structural-concrete tower features a hexagonal core that surrounds the elevators. Since the core is not big enough to rise to such extreme heights on its own, it is buttressed by the three wings. While the core functions as an axle to keep the building from twisting, 2-foot-thick corridor walls on either side of each wing act like the web of an I-beam; cross walls like the flanges. Round columns are located at the pointed end of each wing between ordinary flat plate slabs. The result is a tower that is extremely stiff laterally and torsionally.

"These are very conventional systems, just arranged in a unique manner," says William Baker, structural engineer partner at SOM. The driving force behind the structural design was wind. "Tall building design is dominated by wind forces, even in most seismic areas where earthquakes are a major concern," Baker says. Since wind velocities increase with height, it was an even greater concern here. Consulting engineers Rowan Williams Davies and Irwin (RWDI) carried out extensive wind-tunnel testing over the course of two years in its renowned facilities in Ontario, Canada. First, balsa wood models of the slender tower were subjected to force balance tests. Later, more sophisticated aeroelastic tests were conducted. RWDI studied the building's six important wind directions — the pointed end, or nose, of each of the three wings, and the areas between two wings, called tails.

The most significant change to come from RWDI's analysis did not significantly affect the building's design but rather its orientation. Since analysis indicated less excitation in wind patterns blowing at the nose, the tower was rotated 120 degrees from its original position so that the noses faced into the wind. RWDI also suggested that the Burj's different tiers be made more regular. "Initially, the building spiraled much more dramatically," says Smith. "But each time it steps back, it changes how the wind reacts. To keep the wind from organizing into vortices, we evened out the setbacks."

While changes to the design were being made, so too were changes to the building's use. Originally meant to be all residential, the client, Dubai-based Emaar Properties, added offices to the program. Corporate suites were located at the top of the tower, which, with floor areas as low as 5,000 square feet, is more ideally suited for apartments, the original intent for those floors. But the program was not the only element to be in flux. The tower's final height remained a question mark until rather late in the game. It wasn't until after the foundation was in place and construction of the superstructure began that the magic number — 828 meters, or 2,717 feet — was finally determined. "I hated the proportions of the shorter tower and kept pushing for it to be taller," recalls Smith. The economy was on Smith's side at that point, and the client agreed it looked better taller.

The tower's strategic design allowed flexibility in terms of changes in program and height. The addition of offices required an extra set of elevators, which were accommodated in one of the wings. The other two wings would then house elevators for the apartments and hotel, respectively. Issues of proportion and scale were paramount in the Burj Khalifa's design. Unlike the Willis (formerly Sears) Tower, which scales by the cube, the Burj scales by the square. So whereas doubling the height of the Willis Tower would increase its area eightfold, doubling the height of the Burj only increases its area fourfold; its wings would get longer but not wider.

Nevertheless, most of the extra height was in the spire, which Baker calls "a nest of steel triangles that sits on the hexagonal walls." At the opposite end, at the very bottom of the building, a 12-foot-thick concrete mat, or raft, foundation rests on the surface of a calcisiltite rock mass. It was constructed in four separate pours — one for each of the three wings and the center core. Then, 194 5-foot-diameter piles were driven 140 feet below the mat. Most of the piles are located toward the edge of the mat, with very few at the center. "It's all about decreasing wind forces and managing gravity," says Baker. "By the time you get to the bottom, everything is in compression, so you don't need much reinforcing. The reinforcing there is similar to what you'd see in an average 20-story building. We're very proud of that."

None of this would have been possible without recent material advancements. "We discovered this new material called concrete," Baker jokes. "It is so different from the stuff we used to call concrete." While in the past, slump tests were used to measure how hard and consistent a sample of concrete was, the chemicals in the ultra-high-performance concrete used for the Burj make it so flowable that it forms a puddle. (Silica fume and fly ash are its main ingredients.) The quality control comes in measuring the diameter of the puddle.

Regardless of the concrete's 100 MPa (14,500 psi) strength, all concrete changes dimension over time. Fifteen separate three-dimensional finite-element analysis models predicted the effects of creep, shrinkage, and foundation settlement. "We made precise calculations with data that is very rough," says Baker. "It's all going to shrink. The problem comes when one part moves differently from another."

The key to minimizing that kind of differential movement was to use the same concrete in every vertical element, and to ensure that columns and walls had similar volume-to-surface ratios so that they dried at the same rate. There are virtually no transfers within the concrete structure. Designers adhered to a strict 9-meter (29.5 foot) module. Where a wing sets back and the columns at its nose drop off, the next set of columns appears directly over the walls beneath it.

"You verify as much as possible through computer programs and calculations, but it's not an easy thing," Baker admits. "In the end, you walk the building and look for cracks." So far, the building has settled about 2 inches.

Samsung Corporation was responsible for making the design a reality. The Seoul-based contractor used an automated self-climbing formwork system to build the concrete structure. Specially developed pumps brought the concrete to heights of 600 meters (1,970 feet). The structural steel spire was constructed from inside the building and jacked to its full height of over 200 meters, or approximately 700 feet, using a hydraulic pump.

Seven two- to three-story-high mechanical floors are distributed throughout the building, about every 30 floors or so. "It's really a series of 30-story buildings stacked on top of one another," Baker says. "There would be too much pressure in the pipes, and ducts would get too big, if you try to move things too far."

The mechanical floors house various equipment, including water tanks and pumps, air-handling units, and electrical substations. Track-mounted building-maintenance units, used for window washing, are stored in garages within the structure. "We were very aware of the sand problem," Smith says. "The consistency of the sand in Dubai is more like talcum powder. It sticks to everything." The building was kept as flush as possible, and ledges were kept to a minimum to reduce the number of areas where sand could settle. Window washing is expected to take place every few months.

Over 26,000 low-E, antiglare glass panels were used in the exterior cladding of Burj Khalifa, which features more than 1.8 million square feet of glass. Eight-inch-long wing-shaped, stainless-steel mullions occur at every glass joint. "We originally designed the exterior wall with steel tubes, but it looked too industrial," recalls Smith. "The sheen of the vertical stainless steel, especially in the horizontal sun of morning and evening, makes the building sing."

While the building's structure and its exterior, including the cladding, were designed to resist a variety of forces, forces of a different kind needed to be addressed inside the tower. According to SOM's Luke Leung, "There is a tremendous amount of pressure in a building of this height, both on the water side and on the air side."

The typical system pressure for water is 300 psi. The Burj has one of the highest water pressures in the world at up to 460 psi. "Imagine a water pipe that is 800 meters tall," says Leung. "You don't want to be standing under that."

Pressure breaks are typically added in high-rise buildings to alleviate the forces. In the Burj, SOM created some of the highest pressure breaks ever in a building, consisting mainly of heat exchangers to isolate one riser from another. The tower's water system supplies an average of 250,000 gallons of water daily.

Cooling the water presented another challenge for SOM's engineers. "When we first started coming to Dubai, we noticed that the hot water in our hotels was very hot, and that the cold water was also very hot," says Leung. "Imagine getting hot water out of the cold faucet at the Armani Hotel!"

Since Dubai has limited fresh water and relies on the sea, the water had to travel through the very hot ground during the salt evaporation process. Instead of following that scenario, SOM took advantage of the area's high humidity and the large amount of condensation that results. "The moisture is so high that if you collect condensate in the air during a cooling period, you get a significant amount of water in the 55—65 degree Fahrenheit range," explains Leung. This water is collected and drained in a separate piping system to a holding tank. The system provides about 15 million gallons of supplemental water per year. A sitewide graywater collection system collects water for use in landscape irrigation.

The effects of air pressure are more noticeable to the average visitor to the Burj. There is an enormous amount of air movement going through the building. Due to Dubai's high temperatures, reaching 115 degrees Fahrenheit and higher in the summers, the stack effect is reversed. Instead of hot air rising, it is sucked in from the top of the building and directed downward because the inside of the building is cooler than the outside.

Stack effect is a function of both the building's height and the temperature difference between the inside and the outside. Both are extreme in this case. When you enter the building in the heat of summer, the air will feel like it is trying to push you out. "In Chicago, for instance, it is 75 degrees inside and as high as 95 degrees outside on a summer day," Leung explains. "In Dubai, the temperature difference in summer can be more than 40 degrees Fahrenheit."

What is not so apparent from the building's height is the amount of power it consumes. As electricity travels through the building, which in essence is a stack of five 30-story buildings, it loses voltage similar to the way water loses pressure flowing through a small pipe. To supply these massive loads efficiently, the Burj's electrical mains are supplied with 11 kV, 23 times higher than the 480V typically used in the U.S. Transformers located at each of the mechanical levels reduce the voltage to intermediate levels for heavy equipment and to 220V, the normal voltage used in the U.A.E., for office equipment and appliances.

Fire-and life-safety issues are a vital concern in high-rise buildings, particularly one of unprecedented height such as this. The Burj contains 57 elevators, some of the fastest in the world, serving different building zones, though no one elevator travels more than 500 meters (1,640 feet). According to Baker, the longest elevator ride takes under two minutes — with the express elevator to the observation deck on level 124 taking much less time. Baker also admits to walking down the full height of the building (at a leisurely pace) in about 45 minutes. A typical floor contains three sets of concrete-encased fire stairs, one in each wing.

In case of fire emergency, the building deploys a "defend in place" strategy. Fire-rated, air- conditioned refuge areas accommodate building occupants until further instruction. Some elevators are equipped with cameras so that elevator shafts can be inspected remotely.

Despite the challenges involved in designing the Burj Khalifa, and the criticisms leveled against it in the wake of Dubai's subsequent financial meltdown, Adrian Smith staunchly defends it. "The Burj was an important piece for Dubai at the time it was built," says Smith. "Dubai wanted to be recognized as an international player on par with other major world cities, and it needed an international landmark to do that."

1 comment:

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