For Skidmore, Owings & Merrill (SOM), designing the cladding for the record-setting 2,717-foot-tall Burj Khalifa required simplifying technology and pushing its limits. While the tower—which opened Jan. 4 but was closed at press time because of technical problems—is constructed from standard components, nothing about it remains strictly conventional when applied at such an unprecedented scale and height.
With a surface area of 1.2 million square feet, the unitized curtain wall comprises roughly 28,000 prefabricated panels of double-layer glass set within extruded aluminum frames. Spearhead-shaped stainless steel fins vertically accentuate the mullions and cover the angled joints between adjacent panels. How does this vast architectural skin modulate the light, heat, wind, and dust of the Dubai desert?
Adrian Smith, who was the skyscraper’s lead designer before leaving SOM in 2006, says the façade is curved in order to disperse sunlight. The reflective coating–Sunguard Solar Silver 20, which transmits only 20 percent of visible light and 15 percent of solar energy–would have turned a flat curtain wall into a blinding mirror. Since curved glass was beyond the budget, the rounded effect was achieved with flat panels whose angled joints are concealed behind the fins. To mitigate the inevitable buildup of dust, the panels have no horizontal ledges. Automated window-washing machines are housed at four heights along the elevation.
The curtain wall varies little from ground to top. “It’s a fully customized system, but cladding is one area where you don’t want to take too many risks,” says George Efstathiou, the managing partner at SOM’s Chicago office who directed the project. Typical panels on the Burj measure 4 feet 6 inches wide by 10 feet 8 inches high and weigh about 800 pounds each, growing wider at the building’s edges and taller toward the top. On the uppermost stories, each unitized panel was made narrower and taller to facilitate installation: Less width meant less susceptibility to wind on the hoist up, and the doubled height halved the number of crane picks.
Wind-tunnel simulations were completed using a 1:500 scale model (about 5 feet high), outfitted with more than 1,000 pressure sensors, at the labs of engineering consultant RWDI in Ontario. The tower’s ability to withstand wind loads of 110 pounds per square foot—in negative as well as positive pressure—comes less from the cladding itself than from the refinement of the building’s structural design and its tapering, trifoil form.
The curtain wall was one of the first Burj elements to take shape, says Smith, now a partner at Adrian Smith + Gordon Gill Architecture. SOM’s client, Emaar Properties, commissioned three full-scale mock-ups early on in order to establish the tower’s appearance. Later, the façade would indirectly cause major delays. When Swiss manufacturer Schmidlin went bankrupt in 2006, the fabrication contract was given to Hong Kong–based Far East Group, which rushed the components to Dubai for final assembly and heat-strengthening.