“One dark night, when we were all in bed,
“Mrs. O’Leary lit a lantern in the shed,
“And when the cow kicked it over, she winked her eye and said,
“‘There’ll be a hot time in the old town tonight. Fire! Fire! Fire!'”
This famous bit of verse doesn’t mention it, and somehow I doubt either Mrs. O’Leary or her cow (one of the most famous cows in history — actually, just about the only famous cows in history) intended it, but on that fateful night of October 8, 1871, they not only started the Great Chicago Fire, they also helped invent the skyscraper.
With a third of Chicago in ruins, architects had the rare opportunity of completely redesigning the commercial centre of a major city (Chicago already had 300,000 people). Two recent developments enabled them to make more efficient use of the space by building upward instead of outward: the invention of the passenger elevator in 1857 by Elisha Otis (before that, buildings were rarely taller than six stories) and the idea of using an internal metal framework to hold up a building, instead of its walls.
The importance of that notion is best demonstrated by the tallest masonry building ever erected, Chicago’s Monadnock Building. It’s only 16 stories, two stories shorter than Regina’s City Hall, but its walls are four metres thick at the bottom and two metres thick at the top. Walls that thick make windows almost useless. If the walls don’t have to support anything, though, you can just hang them like curtains and make them out of anything: brick, granite, or even glass.
Construction on the first skyscraper, the 10-story Home Insurance Building, began in Chicago in 1883. Engineer-architect William Le Baron Jenney’s design called for cast-iron columns and wrought-iron beams, but the supplier requested and received permission to substitute rolled steel beams instead, the first use of structural steel. Steel soon replaced cast-iron columns, as well, because cast iron is brittle when stretched out.
This steel framework can be erected amazingly quickly; the steel for New York’s famous 102-story, 381-metre Empire State Building went up in just 23 weeks in 1930.
Before you can start building up, you have to build down; down to a firm foundation, like the extremely hard bedrock under Manhattan Island. For a really tall building the foundation may have to bear loads in excess of 500 tonnes per square metre.
Also bearing that load are the concrete footings into which the beams are set. Concrete is a marvelous material that is extremely solid when compressed. It can’t be twisted, however, so it’s reinforced with steel, to which it bonds very firmly as it dries. By an important coincidence, concrete and steel expand and contract at the same rate with changes of temperature. (If they didn’t, steel-reinforced concrete would tear itself apart.)
Floors follow the soaring steel. Metal decking is covered with still more concrete, which puts so much weight on the beams supporting it that, like bookshelves with too many books on them, they sag in the middle. To keep tenants’ chairs from rolling toward the building’s centre, the beams are manufactured with a slight upward bend in them.
The steel frame must also resist the wind, which tries to twist the building. Some of New York’s tallest buildings sway as much as a metre from side to side in the wind, although engineers generally try to make them stiffer than that to prevent cracked walls, stuck elevators and seasick tenants. It’s possible to make a tall building that doesn’t sway at all, but the cost of the necessary reinforcement generally isn’t worth it.
If you find it hard to believe that mere wind could move a giant building like the world’s tallest, the 110-story, 443-metre Sears Tower in Chicago, consider that a rare but possible 160-kilometre-per-hour wind exerts a force of 370 kilograms per square metre. Even a light wind exerts a great deal of total force on something as big as a skyscraper.
For several years all new skyscrapers were nothing but “glass boxes,” but recent engineering advancements and computer-assisted design have made it possible for architects to use just about any shape, and outer materials, they want. As far as height goes, it’s already perfectly feasible, one engineer told a recent conference on tall buildings, to build a building a half-mile high: almost twice as tall as the Sears Tower.
That’s not “scraping” the sky, that’s downright stabbing it!
