Greg Neville, Citicorp building, 601 Lexington Ave, corner of 53rd St. New York.

Citicorp is one of the ten tallest buildings in Manhattan and it’s at the centre of a great architectural tale. Errors made during design and construction led to it becoming a structurally unsound building, 59 storey high, in crowded mid-town Manhattan. Here is what happened:

A student’s question made engineers realise that wind loads at a 45-degree angle to the tower had not been considered during design. A hurricane plus a failure of the electrically powered mass-dampener could combine to shear off the bolts that join the building’s girders together. While the original design stipulated welded joints this had changed to bolted joints during construction, a weaker outcome. Statistically, this outcome was possible once every 16 years, a serious threat.

For three months a construction crew welded two-inch-thick steel plates over each of the skyscraper’s 200 bolted joints during the night, just as the the hurricane season brought a major storm, Hurricane Ella, toward New York.

These alarming facts were kept from public knowledge and did not emerge for twenty years. No buildings below were evacuated in case the truth came out. The story has since become an architectural legend and is taught as an ethical and engineering case study in architectural courses.

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