The situation was more or less like this. For two decades, hundreds of thousands of people entered and left the doors of one of the largest skyscraper in New York City. These people, many of them workers, went up and down in the elevator completely unaware of the critical failure that the building had, terrifying in architectural terms, and that no one took into account. Rarely in the history of urban planning in large cities has there been a similar situation.
The story dates back to the beginning of the 20th centurywhen the Lutheran church of Saint Peter was located on land of 53rd Streetbetween Lexington Avenue and Third Avenue, in Midtown Manhattan. By 1960, the church community was experiencing serious financial problems, which led the city council to sell the land. The negotiations were not easy and lasted years. Mainly, because the church demanded the creation of a new building separate from the apartment block in which it could continue its activities.
In the end the project was given the green light. The developer accepted the conditions, and Citi Bank commissioned Hugh Stubbins & Associates to design the skyscraper. William LeMessurier was in charge of engineering. The final project consisted of a skyscraper, a church, a public space below street level and landscaping.
The most important element was, of course, the skyscraper. The plan marked 46 floors that were to be distinguished from the rest of the city by the polished and anodized aluminum of the façade. In addition, between the panels there were rows of windows. It didn’t really look complicated, at least not like the roof and base of the building.
The happy roof
Thus, in 1977 the skyscraper was completed. By then it had grown larger, with 59 floors and a total height of 279 meters. An architectural work that dazzled at first glance on the city skyline, a colossal tower where its 45-degree inclined top stood out.
The top of the roof resembles an isosceles triangle. The original plan was to build terraces and apartments, but over time the architects decided to install huge solar panels. LeMessurier, a professor and graduate of the Massachusetts Institute of Technology, carried out a series of tests to verify their efficiency. It turned out that the energy converted by the installation was insufficient. Eventually, the idea of a small solar plant was abandoned.
However, nothing like the base on which the building stood. Some “stilts,” as LeMessurier himself described, among which the then seventh largest skyscraper on the planet seemed to float. We are referring, of course, to those four gigantic pillars (34 meters each) that are located in the center of each side (rather than in the corners) of the base.
It also had a single column in the center, in this case narrower, which housed the building’s elevator banks and provided additional strength to the frames. This design made room for the church under the northwest corner of the building, and gave the giant structure a brutal effectalmost as if he were levitating. In fact, it was exceptionally “light”, of only 25,000 tons (for reference, the Empire State Building was 60,000).
The famous pillars
The base became an architectural icon, as it made the space in the corners empty. LeMessurier had the weight of the skyscraper distributed to the exterior skeleton. Specifically, in a grid of triangular-shaped frames hidden under the façade. Interestingly, this structure was visible from the inside. The elements were not completely welded, but only fixed with bolted joints.
Apparently, the steel frame designed in this way was intended to withstand perpendicular winds. According to the engineers, other types of wind should not pose a threat. Furthermore, municipal regulations did not require other air gusts to be taken into account in the design.
The truth is that the architecture hid an important mechanism on the upper floors. The Citigroup Center had one of the first tuned mass dampers (TDM). It is a 360-ton concrete sphere embedded in oil. When vibrations from the ground or wind moved the building, the mechanism would oscillate in the opposite direction to the tilt of the building.
The problems begin
This swing was in turn balanced by hydraulic arms that support the sphere. With this solution, the skyscraper was able to “maintain balance.” As LeMessurier explained at the time, this piece was key, since its function was to cut the sway of the building in half by converting the kinetic energy of sway into friction.
Once completed, the building was praised, but also the first doubts arrived. New York is not a major hurricane state, but it does have them from time to time. What would happen if, once every 50 years, the winds blew over 100 km/h? These winds can blow from different directions.
The Citigroup Center opened in 1977 under the name Citicorp Center (which changed to Citigroup Center in 1998 following the merger of Citicorp and Travelers Group). But only a year after its inauguration it became clear that it could have a very serious defect structural.
A year later, LeMessurier receives the call that no architect expects in life. It was Diane Hartleyan architecture student at the prestigious Princeton University who had studied the construction of the skyscraper for her thesis. The first of the calls was to ask him several technical questions about the design. Hartley’s professor had expressed doubts to him regarding the strength of a tilted skyscraper where the supporting columns were not at the corners.
Hartley did some calculations of the building’s wind load. He then compared them with LeMessurier’s calculations and discovered that the construction engineers’ figures were incorrect. The student asked to be sent exact load calculations for different types of wind. Only received data related to perpendicular winds and guarantees on the solidity of the structure.
What’s more, LeMessurier told him that the professor had no idea and that everything was in order. The geometry of the building’s frame worked perfectly with the pillars in such positions, allowing it to resist very strong winds, even from a diagonal angle.
Shortly after, the engineer receives a second wake-up call. Another student, this time from the architecture department at the New Jersey Institute of Technology in Newark. It was Lee DeCarolis, and he convinces LeMessurier to make a new calculation.
The man start to doubt for the first time of his project. When he finishes the new calculation, a cold sweat runs through his head. Now the maximum load on the steel triangles seemed to exceed by 40% when the winds blow diagonally. If so, the bolts connecting the structures were even more overloaded, along with an increase of up to 160% load on all connection joints.
LeMessurier was known to be interested in the effects of an engineering change that was made during construction and that had seemed correct at the time: the numerous joints were not welded (as they were in the original design), and were secured with bolts (screws). Normally, this change may be acceptable, but the Citicorp hub design was sensitive to diagonal winds. Hence the results of their calculations were more than worrying.
Discovering the cake
To give us an idea of what the engineer had just discovered, let’s think that the force of the wind on the flat surfaces of a building is enormous. The wind that pushes against tall architecture such as skyscrapers has a great influence against his basealthough gravity does much of the work in holding the entire building together through compression.
This makes a building safe from wind, as long as the joints are strong enough to resist any force that is not counteracted by gravity. In the case at hand, LeMessurier feared that the screws were not too strong for the task.
After a few days without leaving home, the engineer contacts lawyers and other specialists to agree on a process to rectify his mistake. They confirm that gusts at more than 100 kilometers per hour would be enough to break the bolts that hold the bases of the building, resulting in a “very serious” structural failure.
Shortly after, workers begin repair work at night, there is no time to lose in the event of a possible catastrophe with unpredictable consequences. Meanwhile, life continued to function “normally” inside the skyscraper. The engineer’s plan: reinforce the 200 bolted joints welding 5.1cm thick steel plates to cover the bolts.
Furthermore, the integrity of the columns and the entire skeleton was constantly being checked, they could not allow even the slightest failure. The concrete ball on the roof was insured regarding access to energy sources. That said, Manhattan had a plan in case of collapse, one that they never made public so that no one would panic.
The truth is that the reinforcement plan ended at the end of 1978, a year after the structural failure was known, but no one said anything. The case was uncovered in 1995 with a New Yorker article describing what happened almost twenty years ago, bringing to light, now, the historic failure with which the skyscraper was built.
However, neither LeMessurier, nor the architects and engineers of the Citigroup Center, had to face legal consequences for correcting their errors. Apparently, the cost of the modifications made amounted to several million dollars, an amount that was covered by the company’s insurance.
Today, according to new calculations, winds occur every several hundred years that can seriously damage a building. We will never know what would have happened if Citicorp had not been fixed, but we do know the name of the heroine who, perhaps, saved thousands of lives: Diane Hartley.
Image | Andrew Moore, Elisa.rolle, Johan Burati, Trxr4kds, Max Hermus, Amar.raavi
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*An earlier version of this article was published in July 2024
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