Beyond Limits

By Josephine Minutillo

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.

The Burj Khalifa's organic form has a triaxial geometry. The Y-shaped building's three wings are connected to a central core. As the tower rises, one wing at each tier sets back in a spiraling pattern. Photo: EMMAR

"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.

Source:Continuingeducation