The Citigroup Center office building was to be an engineering innovation in the high-rise market. The 279 m high skyscraper was one of the first in the city to have a vibration eliminator located on the roof. The building’s support on four columns impressed the occupants. Unfortunately, their arrangement and an error in calculation almost led to disaster. The tragedy was avoided thanks to the vigilance of two architecture and engineering students.
Church negotiations
Back in the early 20th century, St Peter’s Lutheran Church stood on a plot of land on Lexington Avenue. By the 1960s, the church community was struggling financially, which prompted the council to sell the plot. Negotiations over this took several years. The church demanded the creation of a new building, separate from the tower block, in which it could continue its activities. This condition was met by the developer, but the current church bears little resemblance to the old one.
Citi Bank commissioned Hugh Stubbins & Associates to design the skyscraper. In turn, the engineering issues were entrusted to William LeMessurier. The project consists of a skyscraper, a church, a public space below street level and landscaping. The most important element is, of course, the skyscraper. the 46 floors of the building are mainly distinguished by polished and anodised aluminium. Between the panels are rows of windows. The façade is not complicated. The same cannot be said for the roof and the base of the building.
A colossus on legs of clay
Visually, the structure is based on four pillars spread out in the middle of each side of the square. This means that the space at the corners is empty. The weight of the skyscraper has been distributed to the external skeleton. Specifically, on the triangular-shaped grid of frames hidden under the façade. Interestingly, the structure is visible from the inside. The elements were not fully welded, but only fixed with bolted joints. The steel frame designed in this way was intended to withstand perpendicular winds. According to the engineers, other types of winds should not pose a threat. Besides, city regulations did not require other gusts of air to be taken into account in the design.
The top of the truncated roof resembles an isosceles triangle. The original plan was to build terraces and flats on the roof, but over time the architects decided to install large solar panels. LeMessurier, who is a professor and graduate of the Massachusetts Institute of Technology, conducted a series of tests to check the efficiency of the panels. It turned out that the energy converted by the installation was insufficient. Ultimately, this small solar power plant was abandoned.
The interesting shape of the roof stands out from other skyscrapers. The 49-metre high aluminium wall conceals an important mechanism. The Citigroup Center has one of the first tuned mass damper. It is a concrete sphere weighing 360 tonnes and embedded in oil. During ground vibrations or winds moving the building, the mechanism swings in the opposite direction to the building’s tilt. Balancing the sway is done using hydraulic arms holding the sphere. With this solution, the skyscraper is able to ‘keep its balance’.
The plot has room for a small St Peter’s Church and a public area with benches and greenery. The church resembles a large boulder with a glass recess. Behind the pillars hides the lobby space. The skyscraper is connected to the church via a glass bridge.
Engineering crisis
New York is a state where hurricanes, the likes of those in New Orleans, are a rather rare occurrence. But what if, once every 55 years, the winds in NYC blew at 110 km/h? Not only does such a phenomenon occur in nature, but such winds can blow from different directions. The Citigroup Center opened in 1977, and only a year later it became apparent that the skyscraper might have a serious structural flaw.
It was in June 1978 that an engineering student at the prestigious Princeton University studied the Citigroup Center’s construction for her thesis. Diane Hartley made some calculations of the building’s wind load. She then cross-referenced these with LeMessurier’s calculations and found that the building engineers’ figures were wrong. The student asked to be sent the exact load calculations for different wind types. She only received data related to perpendicular winds. Despite further enquiries, Hartley was only met with assurances from the company about the robustness of the structure.
Later that month, LeMessurier received a call from another student, this time from the architecture department at the New Jersey Institute of Technology in Newark. Lee DeCarolis convinced LeMessurier to carry out a recalculation. Quite quickly, it became apparent that the maximum load on the steel triangles is exceeded by 40 per cent when the winds blow diagonally. The bolts connecting the structures can be said to be even more overloaded. Further tests carried out at a university in Ontario, Canada, showed that oblique winds can cause even greater overloads than expected. In addition, a tuned mass damper would not be able to control such a high force. All the more so because it needed a constant current supply to be fully operational. The terrified engineer returned home and sank into depression.
“Routine repairs”
After a long while, LeMessurier shook off his dark thoughts and took action. He contacted lawyers and other specialists to agree on a process to rectify his mistake. As early as August, the workers started welding. The task was to weld 5.1 cm thick steel plates to cover the bolts. The integrity of the columns and the entire skeleton was constantly checked. In addition, the concrete ball on the roof was secured in terms of access to energy sources.
Office workers may not even have been aware of the danger. The work was carried out in the evening, as part of ‘routine repairs’. Admittedly, the city had prepared an evacuation plan for this part of Manhattan in case of a threat of collapse, but it was not announced. In the middle of the work, Hurricane Ella began to approach the city. The storm bypassed the city and repairs to the building were completed in October.
The whole incident was not reported in the press. However, it was not LeMessurier’s intrigue, but a strike by journalists from three of New York’s largest editorial offices. It was not until 1995 that an article for The New Yorker outlined the case from almost twenty years ago. However, the Citigroup Center’s architects and engineers did not face legal consequences because of the correction of their mistakes. The cost of the changes carried out amounted to several million dollars. This amount was covered by company insurance. According to new calculations, the wind that can seriously damage a building occurs every 700 years.
Hero or lightweight?
It was only in 2011 that the identities of the two students who expressed their doubts about the project were revealed. LeMessurier is considered a hero for admitting his mistake and moving to rectify it. On the other hand, the engineer’s oversights were due to his overconfidence. Such an innovative and risky design was not thoroughly checked from every angle. On the one hand, making the plans for evacuating the area secret helped to offset the panic of the residents. On the other hand, secrecy limited the possibility of implementing the plan. This story is an important example of the indefensible laws of physics and nature that do not tolerate human inattention.
Source: Dlubal
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