Citicorp Center engineering crisis

In July 1978, a possible structural flaw was discovered in Citicorp Center, a skyscraper that had recently been completed in New York City. Workers surreptitiously made repairs over the next few months. The building, now known as Citigroup Center, occupied an entire block and was to be the headquarters of Citibank. Its structure, designed by William LeMessurier, had several unusual design features, including a raised base supported by four offset stilts and a column in the center, diagonal bracing which absorbed wind loads from upper stories, and a tuned mass damper with a 400-ton concrete weight floating on oil^[1] to counteract oscillation movements. It was the first building that used active mechanical elements (the tuned mass damper) for stabilization.^[1][2] Concerned about "quartering winds" directed diagonally toward the corners of the building, Princeton University undergraduate student Diane Hartley investigated the structural integrity of the building and found it wanting. However, it is not clear whether her study ever came to the attention of LeMessurier, the chief structural engineer of the building.

At around the same time as Hartley was studying the question, an architecture student at New Jersey Institute of Technology (NJIT) named Lee DeCarolis chose the building as the topic for a report assignment in his freshman class on the basic concepts of structural engineering.^[3] A Professor Zoldos of NJIT expressed reservations to DeCarolis about the building's structure, and DeCarolis contacted LeMessurier, relaying what his professor had said. LeMessurier had also become aware that during the construction of the building, changes had been made to his design without his approval, and he reviewed the calculations of the building's stress parameters and the results of wind tunnel experiments.^[2] He concluded there was a problem. Worried that a high wind could cause the building to collapse, LeMessurier directed that the building be reinforced.

The reinforcements were made stealthily at night while the offices in the building were open for regular operation during the day. The concern was for the integrity of the building structure in high wind conditions. Estimates at the time suggested that if the mass damper was disabled by a power failure, the building could be toppled by a 70-mile-per-hour (110 km/h) quartering wind, with possibly many people killed as a result. The reinforcement effort was kept secret until 1995. The tuned mass damper has a major effect on the stability of the structure, so an emergency backup generator was installed and extra staff was assigned to ensure that it would keep working reliably during the structural reinforcement.

The city had plans to evacuate the Citicorp Center and other surrounding buildings if high winds did occur.^[2] Hurricane Ella did threaten New York during the retrofitting, but it changed course before arriving. Ultimately, the retrofitting may not have been necessary. A NIST reassessment using modern technology later determined that the quartering wind loads were not the threat that LeMessurier and Hartley had thought. They recommended a reevaluation of the original building design to determine if the retrofitting had really been warranted.^[4] It is not clear whether the NIST-recommended reevaluation was ever conducted, although the question is only an academic one, since the reinforcement had been done.