Historic blunders

12th March 2004 at 00:00
Anyone who doubts that necessity is the mother of invention should study the Citicorp Center tower in New York. With a church taking up one corner of their site, the designers were faced with a unique challenge: how to build a 59-storey skyscraper, one of whose corners was not permitted to touch the ground.

Their answer was to move the supports away from the angles and into the middle of each side, leaving all four corners hanging 70 feet in the air - a daring solution, and one that leaves the most jaded passer-by asking: how does it stay up? Which, as it turns out, is a very good question, and one that so bothered a New Jersey student that he took the trouble to call the project's chief structural engineer, William LeMessurier, and ask him directly.

It was 1978, and the building had already been standing for a year. So LeMessurier explained that a cunning system of welded V-girders braced the structure against strong winds, and hung up.

But the call unnerved him. For one thing, building codes had not required him to calculate the effect of winds striking the tower obliquely. And for another, he had recently learned that those steel braces were not welded, as specified, but bolted together. Frantic calculations and a series of wind-tunnel tests convinced him that a hurricane blowing at an angle had a 50:50 chance of toppling the building, killing tens of thousands of people.

With winds that strong battering New York every 16 years, and the hurricane season fast approaching, LeMessurier realised that the frame must be modified, and quickly.

For two and a half months, an army of welders strengthened the braces, working at night so as not to cause widespread panic. A secret evacuation strategy was devised to empty 156 city blocks in the event of a storm, and it was almost implemented when it looked as if Hurricane Ella was blowing their way. It never came to that, of course, and the Citicorp Tower, with its characteristic triangular roof, is now guaranteed wind-proof. But if that student hadn't made his callI

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