In 1977 the skyscraper Citicorp Center – one of the most innovative engineering feats of the 1970s – fit into the already dense Manhattan skyline. However, one year after the inauguration, an alarming fact emerged: a sufficiently intense storm could have caused New York’s iconic skyscraper to collapse, putting thousands of people at risk. This is where the story of began William LeMessurierstructural engineer responsible for the project, who identified the critical issue and contributed to averting a possible catastrophe by initiating a review of the regulatory standards and bringing the theme ofprofessional ethics in the work of the civil engineer.
What Citicorp Center looks like
Designed to house the headquarters of the giant Citibank, the Citicorp Center – also known as Citigroup Center and today 601 Lexington Ave – it’s a skyscraper 59 floors which reaches i 279 meters high. Built in three years, from 1974 to 1977, it presented a design paradigm innovative and unusual at the same time for the time. In fact, observing it, you immediately notice something singular: the building does not rest on the ground either along its perimeter or at the edges, but rises on four vertical elements 35 meters high (9 floors), located in the center of each facade. The reason is linked to the lot: the land on which Citicorp was supposed to be built was in fact located St. Peter’s Lutheran Churchwho agreed to the construction of the skyscraper only on the condition of not being touched. To respect this constraint, the designer had to move the “legs” of the building to the center, leaving the church free under the large overhang of the upper floors. LeMessurier thus designed an innovative structural skeleton perimeter: a series of V-shaped braces that transferred the gravitational loads, and the lateral ones due to the wind, from the external facades towards the large central columns, allowing the corners to remain suspended in the void with a notable weight saving of the designed structural part.
The structural problem
In 1978, while studying for her thesis, the student at Princeton University Diane Hartley noticed a strange fact: that the Citicorp Center’s V-bracing system seemed optimized to resist winds perpendicular to the facades, but appeared more vulnerable when the wind hit the building diagonally (at 45°), then pushing on the corners without columns. The regulations of the time did not require checking this condition, which was considered less critical; However, these were criteria based on traditional structural schemes, therefore on “standard” pillar systems, very different from the one adopted by the designer to overcome the presence of the church. Although LeMessurier had initially reassured her about the doubts raised by the student, he nevertheless decided to review his calculations: he thus discovered that, due to its specific “V” structure, the diagonal wind increased the internal stresses in the braces by 40% compared to what he had considered in the calculations.
The wind problem alone would not have been enough to compromise the building, but it led to the discovery of a second important detail: during construction, the company had proposed changing the method of joining diagonals (too long to be made in a single piece), and thus move from fully penetrating welded joints to bolted jointscheaper and quicker to assemble. The modification, although respectful of the design actions, drastically reduced the safety margin guaranteed by the welded unions – originally conceived and much more resistant – completely canceling it in the extreme case described above. According to calculations, a wind that occurs on average every 16 years would have been sufficient to undermine the bolted joints, leading to the collapse of the building.
The secret resolution plan
LeMessurier, demonstrating a strong sense of responsibility as well as great professional and ethical ethics, communicated the problem to the top management of Citicorp. An emergency action plan was therefore organised: the SERENE project. For three months, teams of welders worked every night, from 8:00 pm to 4:00 am, so as not to interrupt the activities that took place daily in the building and to avoid alarmism among the occupants. The solution implemented included the welding of steel plates 5 cm thick to reinforce the weak bolted joints: real “plasters” applied to the structure. Fortunately, there was a strike going on in New York at the time among the major newspapers, which helped keep the news from getting out.
The danger of Hurricane Ella
In the same year that the repairs were gradually being carried out, Hurricane She was of great concern: the police had in fact prepared a secret evacuation plan for a radius of 10 blocks. In those critical days, the only element still capable of guaranteeing a minimum safety margin for the building was the Tuned Mass Damper (TMD)the first installed in New York: a 400-ton concrete block, placed at the top of the skyscraper, which counterbalanced the oscillation of the wind by sliding on a layer of oil. LeMessurier made sure that the device was powered by dedicated generators, so as to avoid any possible deactivation or operational problems. Against every standard engineering principle, the TMD – normally used to improve the comfort of occupants by reducing annoying oscillations – became in those weeks the main structural component for the statics of the skyscraper: the only protection against collapse in the event of strong winds. Fortunately, Hurricane Ella turned toward the ocean a few hours before impact, allowing for a safe conclusion the consolidation intervention of the structure.
The history of Citicorp remained secret until 1995, when an article by New Yorker revealed what happened. LeMessurier was praised for his professional integrity and ethical couragedemonstrating that error is human, but that at the same time it is necessary to recognize it and take responsibility for trying to remedy it.
