The year was 1977, and the Citicorp Center had just been unveiled. A skyscraper at the corner of Lexington Avenue and 53rd Street, it stood at 57 storiesthe
second seventh-tallest tower in the world. From the top, it's distinct because of its asymmetrical top, which slopes downward at a 45-degree angle; from the bottom, it's distinct because the first nine stories are mere stilts, supporting the mass above. Almost 40 years later, Slate and design radio show 99% Invisible recall the construction of the Citicorp Center, now known as the Citigroup Center or 601 Lexington Avenue, and how its chief engineer had to scramble to fix a major flaw in the construction that left the building seriously at risk of collapse due to wind.
Here are the basics: Engineering guru William LeMessurier had designed the building on those stilts in order to preserve an existing church on one corner of the skyscraper's lot. In order to give the top-heavy structure support, he used a chevron bracing mechanism along the facades as well as installed a tuned mass damper in the middle of the building, a central 400-ton weight that serves to prevent swaying and other movements.
To make a long story short, an undergrad studying architecture rang up LeMessurier one year after the building opened and informed him that the building was vulnerable: not to perpendicular winds, which hit the building straight on and which LeMessurier had accounted for. But quartering winds, which blow at its corners, could fell the whole thing. Each year, LeMessurier calculated, there was a 1 in 16 chance that it would collapse if a storm hit. Workers immediately started reinforcing joints at the building's corners, and prepared themselves for any hurricane or other emergency. In 1978, the tower was shored up out of the public eye, but a reporter broke the story in 1995, after which the undergrad who saved Midtown from a potential collapse revealed herself. Cool story, right?
· The Design Flaw That Almost Wiped Out an NYC Skyscraper [Slate]
· Episode 110: Structural Integrity [99% Invisible]
· Podcast Structural Integrity [SoundCloud]