Structural concrete gives way to steel at the tower's top section, where a structural-steel spire utilizes a diagonally braced lateral system. Photo © Samar Jodha/Spectra Maxima

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.