How did a student's question make an engineer fear his skyscraper could topple?

It made him check a case he had never calculated. Less than a year after Citicorp Center was completed in Manhattan in 1977, its own structural engineer concluded it was vulnerable to collapse in high winds, and the person who set him on that path was an engineering student. Built for Citibank, the 915-foot, 59-story tower, designed by architect Hugh Stubbins and structural engineer William LeMessurier, stood on four giant stilts placed at the middle of each wall rather than at the corners, a choice made because the northwest corner's stilt would otherwise have cut into St. Peter's Lutheran Church next door, which insisted its sanctuary stay structurally separate from the tower above it. Centered stilts were supposed to handle "quartering" winds hitting the building's corners better than corner stilts would, so LeMessurier, following standard building code, had only calculated the straightforward case of wind hitting the building head-on.

Then, in June 1978, engineering student Diane Hartley asked LeMessurier a question about the design that pushed him to actually run the numbers for quartering winds. He found they would hit the building's diagonal braces far harder than he'd assumed—and that assumption mattered enormously, because those braces, originally meant to be welded, had been changed to cheaper bolted joints by the steel contractor in 1974, a substitution LeMessurier's own office had accepted without him knowing. Running the safety numbers with the real joints and the real wind pattern, he calculated that a storm capable of toppling the tower had roughly a 1-in-55 chance of striking in any given year, worse still, a 1-in-16 chance if the giant damper weight on the roof ever lost power.

LeMessurier's fix was to weld steel plates over the bolted joints. Starting in August 1978, crews did the work at night, finishing that October; afterward, LeMessurier put the odds of a toppling wind at just 1-in-700 per year. Because New York's newspapers happened to be on strike at the time and almost no one outside the project was told, the public had no idea how close the building had come to disaster—the full story only came out in a lengthy New Yorker article in 1995, long after the repairs were complete. And in 2019, a reassessment by the National Institute of Standards and Technology found that the quartering wind loads had not been as severe as LeMessurier and Hartley had thought.