Tall Stories Essays - Skyscrapers, Vortices, Structural Engineering

Tall Stories Picture in your mind the skyline of downtown Toronto. There's the CN Tower, of course, and the 72-floor First Canadian Place, the city's tallest skyscraper. Cascading from there are the assorted banks and hotels and insurance towers. Now, use your imagination to construct some new buildings, these ones reaching three, four and five times higher than the others. Top it all off with a skyscraper one mile high (three times as high as the CN Tower). Sound fanciful? It did 30 years ago when Frank Lloyd Wright proposed the first mile-high building. But not today. We are now said to be entering the age of the superskyscraper, with tall buildings poised to take a giant new leap into the sky. Skyscrapers approaching the mile mark may still be awhile off, but there are proposals now for megastructures soaring 900 m -- twice as high as the world's tallest building, the 110-story Sears Tower in Chicago. Suppose that you were asked to erect such a building. How would you do it? What are the obstacles you'd face? What materials would you use? And where would you put it? Building a superskyscraper, the first thing you would need is a considerable slice of real estate. Tall buildings require a large base to support their load and keep them stable. In general, the height of a building should be six times its base, so, for a skyscraper 900-m tall, you'd need a base of 150 square m. That much space is hard to come by in, say, downtown Toronto, forcing you to look for an undeveloped area, perhaps the Don Valley ravine, next to the Science Centre. Bear in mind though that the Don Valley is overlain by loose sand and silt, and tall buildings must stand on firm ground, or else risk the fate of edifices like the Empress Hotel in Victoria. This grand dowager, completed in 1908, long before the science of soil mechanics, has since found herself slowly sinking into the soft clay. Soil analysis is especially critical in facing the threat of earthquakes. The Japanese have learned many times the hard way what happens when an earth tremor shakes a high-rise constructed on soft, wet sand. The quake's enormous energy severs the loose connections between the individual grains, turning the ground into quicksand in just seconds and swallowing up the building. . Engineers have actually built machines that condense loose ground. One machine pounds the earth with huge hammers. Another plunges a large vibrating probe into the ground, like a blender in a milk shake, stirring up the sand so that its structure collapses and the individuals grains fall closer together. Anchoring a skyscraper in the Don Valley would best be solved by driving long steel piles down through the sand and silt into the underlying hard clay till. Or, if the clay till lies too far underground, inserting more piles into the sand. The friction between sand and so much steel would then be sufficient 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