271 lines
14 KiB
Plaintext
271 lines
14 KiB
Plaintext
ÜÜÜÜÜÜÜÜÜÜÜÜÜ ÜÜÜ ÜÜÜÜ
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ÜÛÛÛÛÛÛÛÛßÛßßßßßÛÛÜ ÜÜßßßßÜÜÜÜ ÜÛÜ ÜÛÛÛÛÛÛÛÛÜÜÜÜÜÛßß ßÛÛ
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ßÛÛÛÛÛÛÛÛÛÛÛÛÛÛÜ ßÛÛ ÜÛÛÛÜÛÛÜÜÜ ßÛÛÛÛÜ ßÛÛÛÛÛÛÛÜÛÛÜÜÜÛÛÝ Ûß
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ßßßÛÛÛÛÛÛÛÛÛÛÜ ÞÝ ÛÛÛÛÛÛÛÛÛÛÛßßÛÜÞÛÛÛ ÛÛÛÛÛÜ ßßÛÛÛÞß
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Mo.iMP ÜÛÛÜ ßÛÛÛÛÛÛÛÝÛ ÞÛÛÛÛÛÛÛÛÛ ÞÛÛÛÛ ÞÛÛÛÛÛÝ ßÛß
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ÜÛÛÛÛÛÛÛ ÛÛÛÛÛÛÛÛÝ ÞÛÛÛÛÛÛÛÛÝ ÛÛÛ ÛÛÛÛÛÛ
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ÜÛÛÛÛÛÛÛÝ ÞÛÛÛÛÛÛÛÛ ÞÛÛÛÛÛÛÛÛ ß ÞÛÛÛÛÛÛÜ ÜÛ
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ÜÛÛÛÛÛÛÛÝ ÛÛÛÛÛÛÛÛ ÛÛÛÛÛÛÛÛÝ ÞÞÛÛÛÛÛÛÛÛÛß
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ÜÛßÛÛÛÛÛÛ ÜÜ ÛÛÛÛÛÛÛÛÝ ÛÛÞÛÛÛÛÛÝ ÞÛÛÛÛÛÛßß
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ÜÛßÛÛÛÛÛÛÜÛÛÛÛÜÞÛÛÛÛÛÛÛÛ ÞÛ ßÛÛÛÛÛ Ü ÛÝÛÛÛÛÛ Ü
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ÜÛ ÞÛÛÛÛÛÛÛÛÛÛß ÛÛÛÛÛÛÛÛÛ ßÛÜ ßÛÛÛÜÜ ÜÜÛÛÛß ÞÛ ÞÛÛÛÝ ÜÜÛÛ
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ÛÛ ÛÛÛÛÛÛÛÛß ÛÛÛÛÛÛÛÛÛÛÜ ßÛÜ ßßÛÛÛÛÛÛÛÛÛß ÜÜÜß ÛÛÛÛÜÜÜÜÜÜÜÛÛÛÛÛß
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ßÛÜ ÜÛÛÛß ßÛÛÛÛÛÛÛÛÛÛÜ ßßÜÜ ßßÜÛÛßß ßÛÛÜ ßßßÛßÛÛÛÛÛÛÛßß
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ßßßßß ßßÛÛß ßßßßß ßßßßßßßßßßßßß
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ARRoGANT CoURiERS WiTH ESSaYS
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Grade Level: Type of Work Subject/Topic is on:
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[ ]6-8 [ ]Class Notes [An Essay on Skyscrapers ]
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[ ]9-10 [ ]Cliff Notes [ ]
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[x]11-12 [x]Essay/Report [ ]
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[ ]College [ ]Misc [ ]
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Dizzed: 09/94 # of Words:2152 School: ? State: ?
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ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>Chop Here>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ
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=======================================================================
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Tall Stories NEWSCIENCE
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-----------------------------------------------------------------------
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Picture in your mind the skyline of downtown Toronto. There's the CN
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Tower, of course, and the 72-floor First Canadian Place, the city's
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tallest skyscraper. Cascading from there are the assorted banks and
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hotels and insurance towers.
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Now, use your imagination to construct some new buildings, these
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ones reaching three, four and five times higher than the others. Top it
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all off with a skyscraper one mile high (three times as high as the CN
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Tower).
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Sound fanciful? It did 30 years ago when Frank Lloyd Wright
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proposed the first mile-high building.
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But not today. We are now said to be entering the age of the
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superskyscraper, with tall buildings poised to take a giant new leap
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into the sky. Skyscrapers approaching the mile mark may still be
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awhile off, but there are proposals now for megastructures soaring 900
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m -- twice as high as the world's tallest building, the 110-story Sears
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Tower in Chicago.
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Suppose that you were asked to erect such a building. How would
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you do it? What are the obstacles you'd face? What materials would you
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use? And where would you put it?
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Building a superskyscraper, the first thing you would need is a
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considerable slice of real estate. Tall buildings require a large base
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to support their load and keep them stable. In general, the height of a
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building should be six times its base, so, for a skyscraper 900-m tall,
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you'd need a base of 150 square m.
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That much space is hard to come by in, say, downtown Toronto,
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forcing you to look for an undeveloped area, perhaps the Don Valley
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ravine, next to the Science Centre. Bear in mind though that the Don
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Valley is overlain by loose sand and silt, and tall buildings must
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stand on firm ground, or else risk the fate of edifices like the
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Empress Hotel in Victoria. This grand dowager, completed in 1908, long
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before the science of soil mechanics, has since found herself slowly
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sinking into the soft clay.
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Soil analysis is especially critical in facing the threat of
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earthquakes. The Japanese have learned many times the hard way what
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happens when an earth tremor shakes a high-rise constructed on soft,
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wet sand. The quake's enormous energy severs the loose connections
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between the individual grains, turning the ground into quicksand in
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just seconds and swallowing up the building. .
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Engineers have actually built machines that condense loose
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ground. One machine pounds the earth with huge hammers. Another
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plunges a large vibrating probe into the ground, like a blender in a
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milk shake, stirring up the sand so that its structure collapses and
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the individuals grains fall closer together.
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Anchoring a skyscraper in the Don Valley would best be solved by
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driving long steel piles down through the sand and silt into the
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underlying hard clay till. Or, if the clay till lies too far
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underground, inserting more piles into the sand. The friction between
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sand and so much steel would then be sufficient to hold the concrete
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foundation above in place.
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The next obstacle in erecting a superskyscraper, and perhaps the
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biggest one, is wind. Tall buildings actually sway in the breeze, in
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much the same way that a diving board bends under the weight of a
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diver.
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Building an edifice that doesn't topple over in the wind is easy
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enough. The real challenge is keeping the structure so stiff that it
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doesn't swing too far, cracking partitions, shattering windows and
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making the upper occupants seasick. As a rule, the top of skyscraper
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should never drift more than 1/400 of its height at a wind velocity of
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150 km/h.
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Older buildings, like the Empire State Building, were built so that
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their core withstood all bending stresses. But structural engineers
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have since found that by shifting the bracing and support to the
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perimeter of a building, it can better resist high winds. The most
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advanced buildings are constructed like a hollow tube, with thin, outer
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columns spaced tightly together and welded to broad horizontal beams.
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Toronto's First Canadian Place and New York's World Trade Center towers
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are all giant, framed tubes.
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A superskyscraper would undoubtedly need extra rigidity, which you
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could add by bracing its framework with giant diagonal beams. You'll
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see this at Chicago's John Hancock Center where the architect has
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incorporated diagonal braces right into the look of the building,
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exposing five huge X's on each side to public view.
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Alternatively, you might design your building like a broadcasting
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tower, and tie it to the ground with heavy, sloping guy wires
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extending from the four corners of the roof to the ground. A control
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mechanism at the end of each cable would act like a fishing reel,
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drawing in the cable whenever the sway of the building caused it to
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slacken.
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Tall buildings also encounter the problem of vortex shedding, a
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phenomenon that occurs as the wind swirls around the front corners of
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the building, forming a series of eddies or vortices. At certain wind
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speeds, these vortices vibrate the building, threatening to shake it
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apart. In New York City's Citicorp Center, engineers have tackled
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vortex shedding with a 400-tonne concrete block that slides around in a
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special room on one of the upper stories. Connected to a large spring
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and a shock absorber, and riding on a thin slick of oil, the big block
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responds to oscillations of the building by moving in the opposite
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direction.
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Other ways to disrupt vortex shedding include making several large
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portals in the upper part of the tower, through which the wind passes
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freely. In New York City's World Trade Center, vibrations are dampened
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with special spongelike pads sandwiched in its structure.
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The price tag on a superskyscraper is going to be enormous, but
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one way to cut costs is with high-strength concretes. Ordinary concrete
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is much cheaper than steel, but lacks steel's rigidity, and could not
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withstand the huge burdens in a superskyscraper. But recent
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experiments with chemical additives, called superplasticizers, have
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whipped up double and triple-strength concretes that could make
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superskyscrapers an economic reality.
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Once you've built your superskyscraper, there still remains the
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job of servicing it -- providing water, electricity, fire protection,
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ventilation and cooling. Servicing also means controlling stack effect.
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If you've ever been up in a skyscraper and heard the wind moaning and
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whistling by the elevator -- that's stack effect. In any tall building,
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the difference in temperature and air pressure between the outside and
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inside the structure pushes air up the stairwells and elevators, like
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smoke up a chimney. Strong, cold drafts blowing up the building create
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heating problems and make it difficult to open doors into stairwells.
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To control stack effect, buildings must be as airtight as possible,
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with ventilation ducts extending only part way up the building, and
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revolving doors at ground level.
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The one invention that, above all, has enabled buildings to climb
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higher is the elevator. As skyscraper populations have grown, elevator
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manufacturers have handled larger loads with double-decker cars -- one
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car piggybacking another, with each one stopping at alternative floors.
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Another innovation is the sky lobby system, in which passengers take
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one car to a floor part way up the building, and then transfer the next
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flight up to another car in the same elevator shaft for the rest of the
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journey.
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Elevators will probably never move any faster than they do today,
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since the human ear can only endure a descent speed of 600 m per
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minute. So, an elevator ride in a superskyscraper might be comparable
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to a subway trip, with several transfer points and extended waits
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between cars.
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Which brings designers to the inevitable question: Will office
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staffs and tenants stand for such long rides? Indeed, will they
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tolerate all the other shortcomings of skyscrapers -- the feelings of
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entrapment inside them, the dark, windy canyons between them, and the
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congested traffic below -- made worse by higher heights.
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Developers now claim they've worked most design bugs out of the
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new megastructures Whether or not people will actually want to occupy
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them should prove if the sky is really the limit.
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Don Valley -- loose deposits of sand and silt overlying deep
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deposits of cllay. Soft deposits. -- or is sand cover on top of clay.
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terms: loose sand, loose silt, soft clay. Increase surface area of
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piles.
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Perhaps the most critical servicing job is protecting the
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building's occupants from fire and smoke. Today's skyscrapers are
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equipped with ultra-sophistated fire-control systems: automatic
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sprinklers help douse the fire while exhaust fans suck out the smoke
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from burning areas, preventing it from escaping into other floors and
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stairwells.
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Feeding the sprinkler systems are huge water storage tanks that sit
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on the top floor or roof. These are the same tanks that Paul Newman
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blew up to douse the rampaging fire in "The Towering Inferno".
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Exploding tanks undoubtedly made for exciting climax, but they could
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never contain that much water to put out a skyscraper fire.
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Built in the early Seventies by I.M. Pei, one of America's foremost
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architects, the "John Hancock" towers majestically over the Back Bay
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area of Boston.
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Over time, it developed the bad habit of letting its windows fall out
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on windy days. This problem grew so serious, that police had to cordon
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off the leeward side of the skyscraper to keep unsuspecting
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pedestrians from getting beaned by falling glass. In fact, the
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situation became so dangerous that doormen were escorting workers in
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and out of the building during the daily invasion and exodus, keeping a
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wet finger to the wind and an eye peeled for falling glass.
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And what was the foundation of this perplexing and disturbing
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window-popping habit? As it turned out, the foundation was to blame; it
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and what is known as Bernoulli's Principle, ( which states that the
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pressure of a gas falls as its velocity increases.)
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What happens is this: a light wind comes along and has to get around a
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large slab of building. It pushes against the front of the tower, and
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then speeds up to get to the edges of the building so it can keep up
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with the rest of the wind, (this is why the areas around tall buildings
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and groups of tall buildings become very windy). The back side of the
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skyscraper, because of all the fast air on its sides, develops an area
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of low pressure, as predicted by Bernoulli's Principle, and because the
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air pressure inside the wall is suddenly higher than that outside,
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there is the potential for windows blowing out
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This is obviously what was happening to Mr. Pei's building; but why was
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it happening with such frequency? After all, this building was becoming
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a lethal weapon! The search for the solution would have to start from
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the ground up, and the design team began with the history of the
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site...
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As is the case with many cities built beside a body of water, Boston's
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downtown area expanded rapidly during the last century, and its bay
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was filled in to provide more building space. Because this land was
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built on more or less right away, it didn't have the chance to compact
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and provide as much support as land that had been settling for
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thousands of years.
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The design of the "John Hancock" took into consideration the condition
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of the soil on which it was built, and the engineers did their best to
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allow for settling. What they couldn't accurately predict was how the
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building would settle, so they planned for a uniform settling of the
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building. Instead, they found that the building had settled unevenly!
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The result of this settling caused an unequal surface tension on the
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curtain wall, which, as all curtain walls are, had been designed only
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to serve as an envelope for the building, and to support no weight
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other than its own. This meant that it was nearing its maximum strength
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limit even without any wind blowing on it. The suction of the low
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pressure area on the leeward side of the building caused the wall to
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billow out and pop windows like buttons.
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The mechanical engineers, realizing that the negative air pressure was
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too much for the wall, decided to fight that negative pressure with
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negative air pressure of their own. Using the fact that all
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skyscrapers are completely sealed, the perimeter air supply system of
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the whole building was monitored with regards to the exterior air
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pressure, and then air was supplied or removed to balance the tension
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on the curtain wall. Quite literally, they would make the building
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suck in its billowing stomach to keep from popping buttons.
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Simple, huh?
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This tale ends with a moral and with a warning: the moral of the story
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is to look up when you're around tall buildings on very windy days ;
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the warning (for local folks) is that all the land south of Front
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Street is infill!
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