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to hold the concrete foundation above in place. The next obstacle in erecting a superskyscraper, and perhaps the biggest one, is wind. Tall buildings actually sway in the breeze, in much the same way that a diving board bends under the weight of a diver. Building an edifice that doesn't topple over in the wind is easy enough. The real challenge is keeping the structure so stiff that it doesn't swing too far, cracking partitions, shattering windows and making the upper occupants seasick. As a rule, the top of skyscraper should never drift more than 1/400 of its height at a wind velocity of 150 km/h. Older buildings, like the Empire State Building, were built so that their core withstood all bending stresses. But structural engineers have since found that by shifting the bracing and support to the perimeter of a building, it can better resist high winds. The most advanced buildings are constructed like a hollow tube, with thin, outer columns spaced tightly together and welded to broad horizontal beams. Toronto's First Canadian Place and New York's World Trade Center towers are all giant, framed tubes. A superskyscraper would undoubtedly need extra rigidity, which you could add by bracing its framework with giant diagonal beams. You'll see this at Chicago's John Hancock Center where the architect has incorporated diagonal braces right into the look of the building, exposing five huge X's on each side to public view. Alternatively, you might design your building like a broadcasting tower, and tie it to the ground with heavy, sloping guy wires extending from the four corners of the roof to the ground. A control mechanism at the end of each cable would act like a fishing reel, drawing in the cable whenever the sway of the building caused it to slacken. Tall buildings also encounter the problem of vortex shedding, a phenomenon that occurs as the wind swirls around the front corners of the building, forming a series of eddies or vortices. At certain wind speeds, these vortices vibrate the building, threatening to shake it apart. In New York City's Citicorp Center, engineers have tackled vortex shedding with a 400-tonne concrete block that slides around in a special room on one of the