Long Summary
In 1978, less than a year after the new Citicorp Center skyscraper opened in Manhattan, its structural engineer Bill LeMessurier discovered a fatal flaw in the building's design that could cause it to collapse under winds of 110 kilometers per hour. The tower was built on stilts with columns placed at the center of each face rather than the corners, a unique and risky architectural solution to accommodate a church on the same plot. This design, combined with bolted rather than welded joints on the crucial diagonal brace systems, created vulnerabilities under certain wind loads.
The building's distinctive structure used six layers of large diagonal braces called chevrons to channel gravity and wind forces to the stilts. While the gravity loads were accounted for, the design did not initially consider the effect of quartering winds—wind striking the building diagonally—which increased the stress on some braces by about 40%, and later wind tunnel tests indicated forces could be up to 60% higher dynamically. Unfortunately, the bolted connections in the braces were insufficient to handle these loads, with as few as four bolts where up to fourteen were needed, raising the risk of catastrophic failure.
LeMessurier faced an ethical dilemma: to keep silent risking mass casualties or to disclose the flaw and risk his career and public panic. Choosing responsibility, he devised a secret emergency plan, Project Serene, involving nighttime weld repairs of over 200 critical connections to restore the structural integrity without alarming the public. This was done under extreme time pressure as hurricane season approached, and an evacuation contingency was prepared for thousands in the vicinity.
To mitigate wind-induced swaying, the building also employed a then-novel tuned mass damper (TMD)—a massive concrete block suspended at the top that oscillates out of phase with the building to reduce motion. Originally intended for comfort, the TMD became essential in preventing collapse. LeMessurier’s proactive approach and engineering ingenuity ultimately saved the building and many lives, though the full story remained confidential for nearly two decades.
Following revelations in the 1990s, LeMessurier was praised for his integrity, and new engineering standards were adopted requiring quartering wind calculations in skyscraper designs. The Citicorp Center's challenges also popularized the use of tuned mass dampers worldwide, influencing modern skyscraper engineering in seismic and typhoon-prone areas. Despite this, controversy remains over the identity of the student who raised initial concerns and the reluctance of LeMessurier’s firm to openly discuss the issue, highlighting the enduring complexity of engineering ethics and accountability.
The Citicorp case serves as a powerful lesson about the responsibility engineers bear for public safety, the importance of rigorous design and testing, and the courage needed to act when lives are at stake. It remains a seminal example in engineering ethics courses globally, inspiring professionals to prioritize safety and honesty above reputation or profit.
