Early designs placed the massive residential and hotel tower well above 2,000 feet. At that height, "vortex shedding"--eddies of wind, like the wake behind a boat--develops at a building's top stories. As air whips around the tower at speeds reaching 120 mph, low-
pressure zones occur on one side, then the other, setting up vibrations, known as resonant frequencies, that can literally shake the structure to death--which is what happened to Washington State's infamous Tacoma Narrows bridge in 1940, when high winds snapped a cable and sent the third longest suspension bridge in the world crashing into Puget Sound. Older skyscrapers like the Empire State Building are immune because they are built out of heavy steel. But to erect a tower more than twice as high requires a construction with even greater damping qualities. The Burj will be made of poured concrete that contains blast furnace slag and microsilicates--a material that's almost as strong as cast iron, yet more resistant to damage due to vibrations because the natural cracking in concrete dissipates the energy.