How did a student's question save Citicorp Center? 1978 Engineering Crisis Explained

In the late 1960s, the Citicorp Center (now 601 Lexington) project in Manhattan faced a seemingly impossible constraint. To acquire the land, Citicorp had to agree to preserve St. Peter's Church, allowing it to remain on its original corner. The solution proposed by architect Hugh Stubbins and structural engineer Bill LeMessurier was a feat of high-wire engineering: a skyscraper elevated on four massive stilts. Unlike traditional corner-column designs, these stilts were positioned at the midpoint of each face, allowing the building to cantilever over the new church structure. This decision turned the skyscraper into an acrobatic marvel, but it also fundamentally altered how the structure would distribute gravity and wind loads.

To manage the gravity load, Bill LeMessurier designed a series of eight-story chevron braces that funneled weight toward the central core and the mid-face columns. This innovative system was remarkably efficient, allowing for a lighter building that used significantly less steel than traditional towers of the era. However, this efficiency came with a trade-off: the building was more flexible and prone to swaying. To counteract this, Bill LeMessurier installed one of the first Tuned Mass Dampers (TMD) in a commercial skyscraper—a 400-ton concrete block designed to oscillate out of phase with the building's movement to reduce sway.

While the design was celebrated as a 'tour de force' by the American Institute of Architects, a hidden danger was lurking within the connections of the chevron braces. During construction, the contractor suggested a cost-saving measure: replacing the specified welds with bolted joints. Believing the substitution was safe for a building whose braces were primarily under compression from gravity, Bill LeMessurier’s firm approved the change. This decision, seemingly minor at the time, would nearly lead to one of the greatest engineering catastrophes in American history.

In 1978, a year after the building opened, Bill LeMessurier received a phone call from a student—later identified as either Diane Hartley or Lee DeCarolus—who questioned the stability of the mid-face column design. The student’s professor had argued that columns should always be at the corners. While Bill LeMessurier initially dismissed the critique, the inquiry prompted him to re-evaluate his calculations, specifically regarding quartering winds—winds that strike the building's corner at a 45-degree angle rather than straight on one of the faces.

Standard building codes at the time did not mandate calculations for diagonal wind loads for rectangular buildings, as it was assumed that perpendicular wind loads were always the most stressful. However, for a building with Citicorp's unique mid-face column geometry, Bill LeMessurier discovered that quartering winds actually increased the stress on certain chevron braces by 40% to 60%. When combined with the fact that the joints were now bolted rather than welded, the math revealed a terrifying reality: the tension in the braces was double what the bolts could handle.