If you’re not familiar with New York’s Citicorp building, I suggest you read about this Potentially Disastrous Design Error:
With its distinctive forty-five degree diagonal crown, the Citicorp building is one of the most recognizable skyscrapers on the New York City skyline. At fifty-nine stories, it’s the third tallest building in midtown Manhattan, and at the time of its completion it was the seventh-tallest building in the world. At ground level, the huge skyscraper almost seems to hover above Lexington Avenue, held aloft by four massive, 114-foot-tall stilts which are located at the center of each side rather than on the corners. This unusual architecture was one of necessity – the structure had to be built around St. Peter’s Church – but the design left room for a serious engineering flaw which went completely unnoticed during its construction and initial use. Had the weakness not been accidentally discovered and secretly fixed, the mighty skyscraper could have been toppled by a stiff gust of wind without any warning.
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LeMessurier first became aware of the building’s weakness in 1978, about a year after its completion. An engineering student contacted him to ask some technical questions about the design, which he was delighted to address. The student’s professor had expressed doubts regarding the strength of a stilted skyscraper where the support columns were not on the corners. “Listen, I want you to tell your teacher that he doesn’t know what the hell he’s talking about,” LeMessurier told the student, “because he doesn’t know the problem that had to be solved.” He went on to explain how the building’s framing geometry worked perfectly with the stilts in such positions, allowing it to withstand very forceful winds, even from a diagonal angle.But the conversation got him thinking, and he started doing some calculations on just how much diagonal wind the structure could withstand. He was particularly interested in the effects of an engineering change made during construction which had seemed benign at the time: numerous joints were secured with bolts rather than welds. Normally such a change was acceptable, but the Citicorp Center’s design was unusually sensitive to diagonal winds, which the builders hadn’t realized. The results of his calculations were troubling.