A secret structural flaw nearly toppled a Manhattan skyscraper just months after its completion.
In short: When the Citigroup Center opened in 1977, it was a marvel of modern engineering. Yet, a student's inquiry soon revealed that the building's unique design made it dangerously vulnerable to high winds. For months, crews worked in secret to reinforce the tower, preventing a potential catastrophe that remained hidden for decades.
Designed by architect Hugh Stubbins and structural engineer William LeMessurier, the Citigroup Center was a bold addition to Midtown Manhattan. Completed in 1977, the 915-foot tower was distinguished by its sloped roof and four giant stilts placed mid-wall rather than at the corners. However, the building's structural integrity was compromised by a construction change: to save money, Bethlehem Steel replaced the original welded joints in the load-bearing chevron braces with bolted ones. LeMessurier, who had only calculated wind loads for perpendicular winds as required by code, was unaware of this modification.
The crisis emerged in June 1978 after engineering student Diane Hartley asked about the building's wind resistance. Upon recalculating the loads for quartering winds, LeMessurier discovered that the bolted joints were significantly weaker than the original design. He determined that a severe windstorm could potentially topple the tower, a risk that was even higher if the roof's tuned mass damper was inactive. In a desperate race against time, LeMessurier coordinated a covert operation to weld steel plates over the joints, working nightly to avoid public scrutiny during a citywide newspaper strike.
The structural vulnerability was not publicly disclosed until a 1995 article in The New Yorker. While LeMessurier believed the building was at high risk, a 2019 reassessment by the National Institute of Standards and Technology suggested that the threat posed by quartering winds was less extreme than initially feared. Today, the building stands as a landmark of both architectural ambition and the hidden complexities of engineering, having survived its precarious infancy to remain a prominent feature of the New York City skyline.
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- Summary by Intellect · source: Citigroup Center
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