The building's structural skeleton was designed by an engineer named William J. LeMessurier (pronounced "La Measure") in the early 1970s. Making room for the St. Peter's church was a difficult problem, but LeMessurier was a highly capable and creative engineer. His design called for the building to sit atop nine-story-tall stilts, one centered on each side with a specific geometry in the structure's framing to take maximum advantage of the oddly placed support columns. It also had a single, narrower column in the center which housed the building's elevator banks and provided additional strength to the framing. This design made room for the church under the building's northwest corner, and gave the giant structure a graceful, almost levitating effect.
The concept as delivered by LeMessurier was quite sound, in fact it was elegant and technically brilliant. At only 25,000 tons, the steel superstructure of the building was remarkably light compared to other skyscrapers, such as the Empire State Building's 60,000 ton skeleton. Because of the stilted design and low weight, his plans also included a tuned mass damper, a 410-ton block of concrete housed in the upper floors of the building, floating on a thick film of oil and controlled by an automatic system. This substantial piece of stabilizing equipment was intended to cut the building's sway in half by converting the kinetic energy of swaying into friction.
But the conversation got him thinking, and he started doing some calculations on just how much diagonal wind the structure could withstand. He was particularly interested in the effects of an engineering change made during construction which had seemed benign at the time: numerous joints were secured with bolts rather than welds. Normally such a change was acceptable, but the Citicorp Center's design was unusually sensitive to diagonal winds, which the builders hadn't realized. The results of his calculations were troubling.
The force of wind upon a building's flat surfaces is enormous, measured in thousands or millions of pounds. Wind pushing against a tall building has a great deal of leverage against its base, but gravity does much of the work in holding a building together via compression. This makes a building secure against wind so long as the joints are strong enough to resist whatever wind force is not countered by gravity. LeMessurier worried that the bolts in the Citicorp Center's joints were insufficiently strong for the task.
He took his calculations to fellow engineer Alan Davenport, who was an expert on the behavior of buildings in high-wind conditions. Davenport found that seventy-mile-per-hour gusts would be sufficient to break the bolts holding the joints, resulting in structural failure. Such winds were not unknown in New York, indeed storms with such strength occurred about once every sixteen years on average. Hurricane season was fast approaching, and now only two men in the world knew that Citicorp's new $175 million tower and its occupants were vulnerable to destruction by catastrophic collapse.
Together they flew to New York City to confront the executive officers of Citicorp with the dilemma. "I have a real problem for you, sir," LeMessurier said to Citicorp's executive vice-president, John S. Reed. The two men outlined the design flaw and described their proposed solution: to systematically reinforce all 200+ bolted joints by welding two-inch-thick steel plates over them.
Work began immediately, and continued around the clock for three months. Welders worked all night, and carpenters labored during the day. In case of imminent disaster, an evacuation plan was put in place for the surrounding area, but the general public knew nothing of the circumstances... the press was on strike at that time, so news of the repairs did not disseminate to the populace. About halfway into the repairs Hurricane Ella formed, and it appeared to be on a collision course with Manhattan, but fortunately the storm veered out to sea rather than testing the limits of the half-repaired building. The reinforcements were completed in September of 1978, and the entire structure was re-evaluated for safety. Following the repairs, the building was found to be one of the most sturdy skyscrapers in the world. Despite the success, the crisis was kept hidden from the public for almost twenty years, until an article appeared in the New Yorker in 1995.
Unfortunately, the amount of damage caused to New York City by a falling skyscraper is now well known, but in 1978 the idea was still unthinkable. Imagine, however, if such an imposing structure had been blown over by nature itself, from a gust of wind in a storm. Had this engineering gaffe not been identified and resolved, such a catastrophic event might have become a horrifying reality.
Update 18 Feb 2014: We discovered the engineering student's name and revised the article text to credit her for her role in preventing catastrophe.
via the frequently-fascinating Cynical-C