to hold the concrete foundation above in place.
The nextobstacle in erecting a superskyscraper, and perhaps the biggestone, is wind. Tall buildings actually sway in the breeze, in much thesame way that a diving board bends under the weight of a diver.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 ofskyscraper should never drift more than 1/400 of its height at awind velocity of 150 km/h.
Older buildings, like the Empire StateBuilding, were built so that their core withstood all bendingstresses. But structural engineers have since found that by shiftingthe bracing and support to the perimeter of a building, it can betterresist high winds. The most advanced buildings are constructed likea hollow tube, with thin, outer columns spaced tightly together andwelded to broad horizontal beams. Toronto’s First Canadian Placeand New York’s World Trade Center towers are all giant, framedtubes. A superskyscraper would undoubtedly need extra rigidity,which you could add by bracing its framework with giant diagonalbeams. You’ll see this at Chicago’s John Hancock Center wherethe architect has incorporated diagonal braces right into the look ofthe 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. Acontrol mechanism at the end of each cable would act like a fishingreel, drawing in the cable whenever the sway of the buildingcaused it to slacken. Tall buildings also encounter the problem ofvortex shedding, a phenomenon that occurs as the wind swirlsaround the front corners of the building, forming a series of eddiesor vortices. At certain wind speeds, these vortices vibrate thebuilding, threatening to shake it apart.
In New York City’s CiticorpCenter, engineers have tackled vortex shedding with a 400-tonneconcrete block that slides around in a special room on one of theupper stories. Connected to a large spring and a shock absorber,and riding on a thin slick of oil, the big block responds to oscillationsof the building by moving in the opposite direction. Other ways todisrupt vortex shedding include making several large portals in theupper part of the tower, through which the wind passes freely. InNew York City’s World Trade Center, vibrations are dampenedwith special spongelike pads sandwiched in its structure.
The pricetag on a superskyscraper is going to be enormous, but one way to