Citicorp Center engineering crisis

The Citicorp Center engineering crisis was the discovery, in 1978, of a significant structural flaw in Citicorp Center, then a recently completed skyscraper in New York City, and the subsequent effort to quietly make 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 diagonal bracing which absorbed wind loads from upper stories.

Citigroup building with sketch of internal framework superimposed on one side. The same design is used on all four sides and transmits wind and gravity loads to the four support stilts.

In the original design, potential wind loads for the building were calculated incorrectly. The flaw was discovered by Diane Hartley, an undergraduate student at Princeton University who was writing a thesis on the building which was communicated to the firm responsible for the structural design. However, LeMessurier asserted that he spoke with a young male student of architecture whose questioning prompted him to recalculate the wind loads[1] - now identified as Lee DeCarolis.[2] Concerned about the quartering wind loads should the power to the mass dampers be lost, he investigated the structural integrity of the building. Worried that a high wind could cause the building collapse, reinforcements were made stealthily at night while the offices were open for regular operation during the day. Estimates at the time suggested that the building could be toppled by a 70-mile-per-hour (110 km/h) wind, with possibly many people killed as a result. The crisis was kept secret until 1995 and Hartley had no knowledge of the significance of her work until after that time.

No lives were at risk. The triggering event was known to be high winds and NYC had plans to evacuate Citicorp and the surrounding buildings if high winds did occur.[3] A hurricane did threaten NYC during the retrofitting, but it diverted before arriving.

A NIST re-assessment using modern technology has determined that the quartering wind loads were not the threat that LeMessurier and Hartley had thought. They recommend a reevaluation of the original building design to determine if the retrofitting was warranted.[4]

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