Citigroup Center Stilts in New York, New York

In October of 1976, the construction of a 59-story midtown Manhattan skyscraper was completed. This $175 million project, originally called Citicorp Center (now Citigroup Center), was considered an architectural marvel for its unique design, which used four massive stilts to lift the tower over an existing structure. But a design flaw that went unnoticed for several years held catastrophic potential.

While designing the building, lead architect Hugh Stubbins and lead structural engineer William LeMessurier were faced with the challenge of accommodating a church located in the corner of the proposed building lot. Saint Peter's Church, which was built in 1862, allowed for the use of the airspace above the church on the condition that a new church building would be constructed in the same location, and that the northwest corner of the lot not be touched. The 112-foot (34-meter) stilts that support the tower are located in the center of each side of the building, rather than the corners. 

In 1978, two years after Citicorp Center was completed, a Princeton engineering student named Diane Hartley was completing her undergraduate thesis and was interested in the newly constructed skyscraper. She contacted LeMessurier's engineering firm, who provided the architectural plans and engineering calculations for the building. After going through the plans and calculations, Hartley uncovered a fatal flaw in the building's design: It did not take into account quartering winds, winds that approach the building at an angle and blowing across two sides instead of one.

Hartley brought her findings to a professor, who agreed with her analysis. Hartley then called LeMessurier himself to inform him of the risk. LeMessurier dismissed Hartley's findings, saying that she "didn't know what the hell she was talking about." But after hanging up the phone, LeMessurier went to double-check the building plans and found that he was, in fact, wrong.

Once LeMessurier realized that the building was not structurally secure, he brought the issue to his firm and Citigroup. A solution was quickly figured out: additional steel plates would be welded in place at several key points, to better brace the building against quartering winds. To avoid causing disturbance or panic to people working in the building, this work was completed at night, seven days a week, for two months straight. During the renovations, Hurricane Ella was coming up the eastern seaboard, potentially bringing winds strong enough to topple the building if it reached New York City. Luckily, Ella never made landfall and the reinforcements were completed.

The emergency repairs were kept secret until the 1990s. They might have been held longer, but writer Joe Morgenstern overheard someone discussing the work at a party and started digging. He spoke to LeMessurier and other people who had worked on the skyscraper and detailed the story for the New Yorker in 1995.

Citigroup Center still stands today, with one corner of the building towering over Saint Peter's Church.