AD Classics: Citigroup Center / Hugh Stubbins + William Le Messurier

Written by David Langdon

This article was originally published on November 5, 2014. To read the stories behind other celebrated architecture projects, visit our AD Classics section.

In a city of skyscrapers of nearly every shape and size, the Citigroup Center on Lexington Avenue is one of New York’s most unique. Resting on four stilts perfectly centered on each side, it cantilevers seventy-two feet over the sidewalk and features a trademark 45-degree sloping crown at its summit. The original structure responsible for these striking features also contained a grave oversight that nearly resulted in structural catastrophe, giving the tower the moniker of “the greatest disaster never told” when the story finally was told in 1995. The incredible tale—now legendary among structural engineers—adds a fascinating back-story to one of the most iconic fixtures of the Manhattan skyline.

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Before the Citigroup Center was started in 1970, the lot was occupied by the Victorian-style St. Peter’s Evangelical Church, which gave permission to Citigroup to build a skyscraper on the condition that it wouldn’t affect the existing structure. This meant that the entire building would need to hover above the church, nine stories above street level, presumably by placing it on piers. More importantly, preserving the church meant that none of the supports could be located at the corners of the lot, resulting in a design that moved the columns to the middle of each profile. Although the church was demolished within the year, fifty stories of office space were erected over the spot where it once stood, reaching a soaring height of 915 feet. Upon its completion in 1977, the Citigroup Center became the seventh tallest building in the world.

AD Classics: Citigroup Center / Hugh Stubbins + William Le Messurier - Facade — © Flickr user Jeff Stvan

Looking up at the building from street level, there is something unnervingly precarious about the way it rests on its stilts. Like a table with badly positioned legs, it appears unstable, as if pushing too hard on one corner of the building would send the whole thing toppling over. Yet, it has a successful interface with the sidewalk, creating a captivating overhang with a sleek, gridded face that draws the eye. The details under the cantilever are thoughtfully articulated, giving a clean and hyper-ordered veneer to the rarely seen underbelly of a mammoth skyscraper.

Starting at the tenth floor, the building assumes a form much more typical of 1970s high-rise architecture. The curtain wall system affixes striations of metal cladding and strip windows, wrapping the sheer walls of the building with a horizontal banding that visually thickens its presence. Structural devices are pulled back from the façade, leaving a clean finish that works well with the formal clarity of the building. On the north profile, the tallest and least dynamic of the four sides, the crown is visually differentiated through an indentation in the façade that is illuminated at night, as if to offer a new take on the age-old typology of the architectural beacon.

While the architect Hugh Stubbins created the iconic form of the building, most of the credit for the project is generally given to its structural engineer, William Le Messurier. Working on an enormous scale with an unprecedented massing configuration, the task of inventing a structural system to support this unusual design fell squarely on his shoulders. Le Messurier devised a V-shaped system of braces that spanned the building in eight-story units, resulting in five sets of inverted chevron-like supports. In textbook diagrammatic form, these braces redirect the downward forces of the building toward its center where the load-bearing piers transfer them to the ground, all the while providing resistance to the horizontal forces generated by the wind.

To keep it hyper-efficient and low on mass, the finished structure was built to a minimal safety factor, a decision that would eventually come to haunt the engineers. As a result, the building's unusual lightness made it subject to large sways in the wind. Le Messurier turned to the relatively novel invention of the tuned mass damper to help compensate for the lack of weight, placing a four hundred-ton block of concrete in the pinnacle of the building just beneath the sharply angled roof. In order to reduce building sway, the electricity-powered damper was programmed to move in sync with the building while slowing down the speed of its movements. The first tuned mass damper of its kind in the United States, it has worked to perfection, offsetting the tower’s unusual design and allowing it to behave like its more conventionally configured neighbors.

The engineering marvels of the building brought it a widespread notoriety that ended up being its saving grace. Shortly after construction was finished, Diane Hartley, an undergraduate architecture student at Princeton, attempted to mathematically replicate the performance figures of the building. Just like the engineers before her, when she calculated the force of wind applied directly to the side of the tower, the structure had no problem offsetting it. However, she went a critical step further, calculating the effect of quartering winds—that is, winds that strike the building from two sides simultaneously—and found that above a certain windspeed, the joints of the building would buckle and catastrophic failure was imminent. She phoned Le Messurier’s office to find out where the figures had gone off, received the offhand assurances of a project engineer, and didn’t hear another word about it for nearly twenty years.

AD Classics: Citigroup Center / Hugh Stubbins + William Le Messurier - Lighting, Cityscape, Facade — © Flickr user Steven Severing-Haus

Meanwhile, in a reflection of admirable humility, Le Messurier decided to look into the student’s questions and realized that Hartley’s figures were correct. A late-stage cost-cutting measure that substituted fully welded joints in the spliced diagonal braces for bolted joints resulted in insufficient strength to withstand the quartering winds, and given the wrong combination of forces, the building could eventually collapse. Le Messurier quickly handed the figures to several meteorological experts to determine how frequently the wind speeds necessary for structural failure occurred in New York. The answer was sickening: every fifty-four years, they said, unless a storm cut off power to the tuned mass damper, which would reduce the frequency of potentially catastrophic storms to a mere sixteen years.

Le Messurier hurriedly contacted the executives at Citigroup to let them know that their brand new, $175 million icon could easily become the greatest disaster in New York's history. Together, they hatched a plan to superimpose fully welded steel plates over the weak joints on the braces, and began fortifying the building around the clock while keeping a watchful eye on the weather forecast. In conjunction with high-up government officials and the American Red Cross, they also prepared a 12-block evacuation plan for the midtown area in case collapse became imminent. But all of this was unremarkable compared to the fact that the entire operation was conducted in total secrecy. Not a single worker in the Citigroup building or any of its neighbors was made aware of the threat looming above them, and the hundreds of officials, welders and forgers that were brought into the fold successfully kept the affair away from the media. Incredibly, the story remained buried until 1995, when “The Fifty-Nine Story Crisis” was finally published in The New Yorker Magazine. BBC subsequently televised a special program about the near miss, watched by none other than the fiancé of an incredulous Diane Hartley.

For architects, the project prompts a series of provocative ethical and professional questions that cast a more sinister shadow over the otherwise heroic efforts of Le Messurier and his team. The duty to inform the public of potentially catastrophic threats posed by failing architecture, if not during repairs then immediately after, is counteracted by legitimate interests in avoiding mass panic and less-legitimate concerns over corporate and professional reputation. Furthermore, while certainly not implying that more active oversight by the architect could have avoided the structural failures, this story begs a reconsideration of how collaborative efforts on complex projects are coordinated. If a series of unfortunate events were required to bring about the near miss—low structural safety margins, late-stage budgetary substitutions, the failure to account for necessary environmental conditions—an equally varied array of counter fixes could have been proposed by individuals with a more comprehensive and holistic perspective on the project. There are countless ways that architects and engineers can learn from the mistakes and achievements of the Citigroup Building, and it is perhaps the greatest shame of all that they were kept hidden from the public for so long.