Skyscrap(A) Essay, Research Paper
Tall Stories NEWSCIENCEPicture in your mind the skyline of downtown Toronto. There’s the CNTower, of course, and the 72-floor First Canadian Place, the city’stallest skyscraper. Cascading from there are the assorted banks andhotels and insurance towers. Now, use your imagination to construct some new buildings, theseones reaching three, four and five times higher than the others. Top itall off with a skyscraper one mile high (three times as high as the CNTower). Sound fanciful? It did 30 years ago when Frank Lloyd Wrightproposed the first mile-high building.But not today. We are now said to be entering the age of thesuperskyscraper, with tall buildings poised to take a giant new leapinto the sky. Skyscrapers approaching the mile mark may still beawhile off, but there are proposals now for megastructures soaring 900m — twice as high as the world’s tallest building, the 110-story SearsTower in Chicago. Suppose that you were asked to erect such a building. How wouldyou do it? What are the obstacles you’d face? What materials would youuse? And where would you put it?Building a superskyscraper, the first thing you would need is aconsiderable slice of real estate. Tall buildings require a large baseto support their load and keep them stable. In general, the height of abuilding 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 Valleyravine, next to the Science Centre. Bear in mind though that the DonValley is overlain by loose sand and silt, and tall buildings muststand on firm ground, or else risk the fate of edifices like theEmpress Hotel in Victoria. This grand dowager, completed in 1908, longbefore the science of soil mechanics, has since found herself slowlysinking into the soft clay. Soil analysis is especially critical in facing the threat ofearthquakes. The Japanese have learned many times the hard way whathappens when an earth tremor shakes a high-rise constructed on soft,wet sand. The quake’s enormous energy severs the loose connectionsbetween the individual grains, turning the ground into quicksand injust seconds and swallowing up the building. .Engineers have actually built machines that condense looseground. One machine pounds the earth with huge hammers. Anotherplunges a large vibrating probe into the ground, like a blender in amilk shake, stirring up the sand so that its structure collapses andthe individuals grains fall closer together.Anchoring a skyscraper in the Don Valley would best be solved bydriving long steel piles down through the sand and silt into theunderlying hard clay till. Or, if the clay till lies too farunderground, inserting more piles into the sand. The friction betweensand and so much steel would then be sufficient to hold the concretefoundation above in place.The next obstacle in erecting a superskyscraper, and perhaps thebiggest one, is wind. Tall buildings actually sway in the breeze, inmuch the same way that a diving board bends under the weight of adiver. Building an edifice that doesn’t topple over in the wind is easyenough. The real challenge is keeping the structure so stiff that itdoesn’t swing too far, cracking partitions, shattering windows andmaking the upper occupants seasick. As a rule, the top of skyscrapershould never drift more than 1/400 of its height at a wind velocity of150 km/h.Older buildings, like the Empire State Building, were built so thattheir core withstood all bending stresses. But structural engineershave since found that by shifting the bracing and support to theperimeter of a building, it can better resist high winds. The mostadvanced buildings are constructed like a hollow tube, with thin, outercolumns spaced tightly together and welded to broad horizontal beams. Toronto’s First Canadian Place and New York’s World Trade Center towersare all giant, framed tubes.A superskyscraper would undoubtedly need extra rigidity, which youcould add by bracing its framework with giant diagonal beams. You’llsee this at Chicago’s John Hancock Center where the architect hasincorporated 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 broadcastingtower, and tie it to the ground with heavy, sloping guy wiresextending from the four corners of the roof to the ground. A controlmechanism at the end of each cable would act like a fishing reel,drawing in the cable whenever the sway of the building caused it toslacken. Tall buildings also encounter the problem of vortex shedding, aphenomenon that occurs as the wind swirls around the front corners ofthe building, forming a series of eddies or vortices. At certain windspeeds, these vortices vibrate the building, threatening to shake itapart. In New York City’s Citicorp Center, engineers have tackledvortex shedding with a 400-tonne concrete block that slides around in aspecial room on one of the upper stories. Connected to a large springand a shock absorber, and riding on a thin slick of oil, the big blockresponds to oscillations of the building by moving in the oppositedirection.Other ways to disrupt vortex shedding include making several largeportals in the upper part of the tower, through which the wind passesfreely. In New York City’s World Trade Center, vibrations are dampenedwith special spongelike pads sandwiched in its structure. The price tag on a superskyscraper is going to be enormous, butone way to cut costs is with high-strength concretes. Ordinary concreteis much cheaper than steel, but lacks steel’s rigidity, and could notwithstand the huge burdens in a superskyscraper. But recentexperiments with chemical additives, called superplasticizers, havewhipped up double and triple-strength concretes that could makesuperskyscrapers an economic reality.Once you’ve built your superskyscraper, there still remains thejob of servicing it — providing water, electricity, fire protection,
ventilation and cooling. Servicing also means controlling stack effect. If you’ve ever been up in a skyscraper and heard the wind moaning andwhistling by the elevator — that’s stack effect. In any tall building,the difference in temperature and air pressure between the outside andinside the structure pushes air up the stairwells and elevators, likesmoke up a chimney. Strong, cold drafts blowing up the building createheating problems and make it difficult to open doors into stairwells. To control stack effect, buildings must be as airtight as possible,with ventilation ducts extending only part way up the building, andrevolving doors at ground level.The one invention that, above all, has enabled buildings to climbhigher is the elevator. As skyscraper populations have grown, elevatormanufacturers have handled larger loads with double-decker cars — onecar piggybacking another, with each one stopping at alternative floors. Another innovation is the sky lobby system, in which passengers takeone car to a floor part way up the building, and then transfer the nextflight up to another car in the same elevator shaft for the rest of thejourney.Elevators will probably never move any faster than they do today,since the human ear can only endure a descent speed of 600 m perminute. So, an elevator ride in a superskyscraper might be comparableto a subway trip, with several transfer points and extended waitsbetween cars. Which brings designers to the inevitable question: Will officestaffs and tenants stand for such long rides? Indeed, will theytolerate all the other shortcomings of skyscrapers — the feelings ofentrapment inside them, the dark, windy canyons between them, and thecongested traffic below — made worse by higher heights.Developers now claim they’ve worked most design bugs out of thenew megastructures Whether or not people will actually want to occupythem should prove if the sky is really the limit. Don Valley — loose deposits of sand and silt overlying deepdeposits of cllay. Soft deposits. — or is sand cover on top of clay. terms: loose sand, loose silt, soft clay. Increase surface area ofpiles. Perhaps the most critical servicing job is protecting thebuilding’s occupants from fire and smoke. Today’s skyscrapers areequipped with ultra-sophistated fire-control systems: automaticsprinklers help douse the fire while exhaust fans suck out the smokefrom burning areas, preventing it from escaping into other floors andstairwells. Feeding the sprinkler systems are huge water storage tanks that siton the top floor or roof. These are the same tanks that Paul Newmanblew up to douse the rampaging fire in “The Towering Inferno”. Exploding tanks undoubtedly made for exciting climax, but they couldnever contain that much water to put out a skyscraper fire. Built in the early Seventies by I.M. Pei, one of America’s foremostarchitects, the “John Hancock” towers majestically over the Back Bayarea of Boston. Over time, it developed the bad habit of letting its windows fall outon windy days. This problem grew so serious, that police had to cordonoff the leeward side of the skyscraper to keep unsuspectingpedestrians from getting beaned by falling glass. In fact, thesituation became so dangerous that doormen were escorting workers inand out of the building during the daily invasion and exodus, keeping awet finger to the wind and an eye peeled for falling glass. And what was the foundation of this perplexing and disturbingwindow-popping habit? As it turned out, the foundation was to blame; itand what is known as Bernoulli’s Principle, ( which states that thepressure of a gas falls as its velocity increases.)What happens is this: a light wind comes along and has to get around alarge slab of building. It pushes against the front of the tower, andthen speeds up to get to the edges of the building so it can keep upwith the rest of the wind, (this is why the areas around tall buildingsand groups of tall buildings become very windy). The back side of theskyscraper, because of all the fast air on its sides, develops an areaof low pressure, as predicted by Bernoulli’s Principle, and because theair pressure inside the wall is suddenly higher than that outside,there is the potential for windows blowing outThis is obviously what was happening to Mr. Pei’s building; but why wasit happening with such frequency? After all, this building was becominga lethal weapon! The search for the solution would have to start fromthe ground up, and the design team began with the history of thesite… As is the case with many cities built beside a body of water, Boston’sdowntown area expanded rapidly during the last century, and its baywas filled in to provide more building space. Because this land wasbuilt on more or less right away, it didn’t have the chance to compactand provide as much support as land that had been settling forthousands of years.The design of the “John Hancock” took into consideration the conditionof the soil on which it was built, and the engineers did their best toallow for settling. What they couldn’t accurately predict was how thebuilding would settle, so they planned for a uniform settling of thebuilding. Instead, they found that the building had settled unevenly!The result of this settling caused an unequal surface tension on thecurtain wall, which, as all curtain walls are, had been designed onlyto serve as an envelope for the building, and to support no weightother than its own. This meant that it was nearing its maximum strengthlimit even without any wind blowing on it. The suction of the lowpressure area on the leeward side of the building caused the wall tobillow out and pop windows like buttons.The mechanical engineers, realizing that the negative air pressure wastoo much for the wall, decided to fight that negative pressure withnegative air pressure of their own. Using the fact that allskyscrapers are completely sealed, the perimeter air supply system ofthe whole building was monitored with regards to the exterior airpressure, and then air was supplied or removed to balance the tensionon the curtain wall. Quite literally, they would make the buildingsuck in its billowing stomach to keep from popping buttons. This tale ends with a moral and with a warning: the moral of the storyis to look up when you’re around tall buildings on very windy days ;the warning (for local folks) is that all the land south of FrontStreet is infill!