Citigroup Center

Did the wind almost topple the Citigroup Center? Here at 601 Lexington Avenue, the Citigroup Center stands tall, a slanted roof cutting a distinctive silhouette against the Manhattan skyline. This architectural marvel, once known as Citicorp Center, opened in 1977, designed by Hugh Stubbins with a unique feature: four massive stilts supporting the tower mid-block. This unusual design arose from a unique challenge. The northwest corner of the block belonged to St. Peter’s Lutheran Church. The church agreed to allow the skyscraper only if their new sanctuary remained on the same spot, structurally independent of the tower. Hence, the stilts. The Citigroup Center’s design included internal chevron-shaped braces to transfer wind loads from the upper stories down to those stilts. Initially, these chevrons were designed with welded joints. However, a cost-cutting measure led to bolted joints being used instead, a change approved by LeMessurier’s firm, though not by LeMessurier himself. Less than a year after opening, a discovery was made that nearly led to disaster. A Princeton engineering student, Diane Hartley, analyzing the building for her thesis, discovered a potential flaw. She calculated the wind loads from quartering winds, which hit the building at its corners diagonally, and found they were significantly higher than anticipated. Around the same time, another student, Lee DeCarolis from NJIT, contacted LeMessurier, relaying his professor’s concerns about the structure. These inquiries prompted LeMessurier to re-examine his calculations. He realized the change to bolted joints, combined with the quartering wind loads, made the Citigroup Center dangerously vulnerable in high winds. If the tuned mass damper, designed to counteract swaying, failed during a powerful storm, the building could collapse. The situation was dire. A wind capable of toppling the Citigroup Center was estimated to have a one-in-sixteen chance of occurring each year if the damper failed. A secret, nighttime operation ensued, dubbed Project SERENE. For weeks, while the city remained unaware, welders reinforced the bolted joints with steel plates. Emergency generators were installed to ensure the tuned mass damper remained operational, and evacuation plans were drawn up, ready to be implemented at a moment’s notice. Hurricane Ella even threatened the city during the repairs, luckily veering out to sea before disaster struck. The Citigroup Center stood strong, its secret safe. This story reminds us of the precarious dance between innovation and risk in architecture. The Citigroup Center, with its striking design and hidden drama, continues to capture our imaginations and reminds us that even the most impressive structures can have hidden vulnerabilities.