The Citigroup Center is one of the tallest skyscrapers in Manhattan. The 59-floor building is distinguished by its 45° angled roof. Another unique feature is that it was built on a stilt base to avoid neighboring St. Joseph's church. It was also the first building in the United States to have a tuned mass damper. The 400 ton damper in the roof is designed to counter-act winds and reduce sway.
Citigroup Center went into service in 1977. In 1978, prompted by a seemingly innocent question from a Princeton University engineering student, the building's structural engineer LeMessurier realized a error in the building's wind load calculations.
Wind loads had been calculated for winds that hit the building straight-on, but not for quartering wind loads (winds that hit the building at a 45 degree angle). Initially LeMessurier was not worried because buildings are typically built with a padded safety factor. But after some investigation he realize that the original design calculations were based on welded joints. However, a cost-saving change during construction changed the joints to bolted construction.
When he performed the calculations again with the bolted construction and including quartering wind loads LeMessurier realized that high winds could topple the building. Using a wind tunnel, the necessary wind speeds were predicted to occur every 16 years. The building was in critical danger.
LeMessurier notified Citicorp directly and they agreed to hire welders to weld two-inch-thick steel plates over each of the skyscraper's 200 bolted joints during the night, without informing the public of any dangers.
Six weeks into the work, Hurricane Ella was heading for New York. And New York City considered emergency evacuation. Luckily, the storm veered out to sea and the work was completed without incident.
The crisis was kept hidden for 17 years until an expose was published by The New Yorker magazine in 1995.