Skyscrapers
SkyscrapersPicture 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...
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