Citigroup Center

© Štěpán Vrzala, 2007

The Citigroup Center building (formerly the Citicorp Center) is one of the tallest buildings in New York. It has made an indelible mark on the Manhattan skyline due to its distinctive design. The crown of the skyscraper was cut off at a 45° angle to create space for the installation of solar panels, even though the slope does not face directly towards the sun.
From the outset, the construction of the skyscraper posed an engineering challenge for Citigroup. A Lutheran church, St. Peter's Church, stood on the northwest corner. The church allowed the company to demolish the church, provided that a new church would be built on the same site, disconnected from the skyscraper and protected from the encroachment of the skyscraper's supporting structure into the church's interior. The church's request seemed impossible to fulfill, and doubts began to arise about the realization of the new construction nearby.
Structural engineer William LeMessurier placed the 59-story mass on four massive columns. These rise to a height of 35 meters and are not located at the corners of the building, but at the midpoint of its side facades. This solution allowed the building to be cantilevered (extending 22 meters) over the northwest corner, where Citigroup built the new church. Individual floors are supported by V-shaped beams, concentrating the load into the columns.
The daring structure soon began to show defects shortly after completion. LeMessurier's static design for the connections of the steel structure was replaced due to cost savings with a system of locking joints. Poorly executed joints began to collapse at wind speeds exceeding 113 km/h. An extreme wind situation arose when a hurricane exerted pressure on the walls of the skyscraper at a 45° angle, causing the entire structure to begin to twist.
LeMessurier's remedy involved thoroughly reinforcing the structural joints. A crew of welders carried out the necessary work unnoticed. A two-inch thick steel plate was welded over each joint during nighttime over the course of three months. Halfway through the work, reports emerged of Hurricane Ella heading towards New York. Only a few hours remained before the evacuation of the area around the skyscraper. Fortunately, the hurricane veered eastward, and the workers were able to successfully complete their work in the following weeks. However, the public was not informed of this event for a long 20 years. After the scandal was reported in the New York Times, a wave of criticism fell upon the engineer, and his name was negatively recorded in textbooks.
The Citigroup Center was the first place on the American continent to utilize a system of stabilization dampers that mitigate the effects of wind. A 350-ton weight eliminates the wind effect by up to 50%.