The Most Dangerous Building in Manhattan

The Citicorp Center in Manhattan seemed a monumental engineering triumph, but in 1978 a hidden wind flaw threatened collapse. This Veritasium episode follows engineer Bill LeMessurier as he uncovers the danger, navigates competing interests, and orchestrates a clandestine retrofit using chevron braces and a tuned mass damper to keep the skyline safe. A near-miss hurricane, secret repairs, and a bold stance on professional responsibility ultimately reshaped building codes and the practice of engineering ethics.

Introduction and Context

Citicorp Center, a landmark in Manhattan, was hailed as a bold engineering triumph when it opened in the late 1970s. Its radical design featured a tower built around a historic Gothic church, with a central core and a unique system of chevron braces that connected the frame to stilts on the building's faces. The goal was to maximize usable floor area while accommodating the church in the lot, leading to an innovative but untested structural arrangement. The video begins by explaining Bill LeMessurier’s role as the structural engineer who questioned whether the novel design could withstand wind loads in a city famed for tall towers and severe storms. This context sets up the central tension: a leap of engineering faith that could either redefine skyscraper design or unleash a catastrophe in the heart of New York City.

The Structural Solution: Chevron Bracing and the Central Core

To meet the client’s demands, Citicorp Center used stilts at the midpoints of each building face and a central core to carry gravity loads. The architect and structural engineer agreed not to place corner columns, which would complicate the gravity transfer but would increase floor area efficiency. The response to winds—often the critical design driver for tall buildings—was to develop a system that could transfer horizontal loads to the lower portions of the structure without relying on corner columns. LeMessurier sketched a concept on a napkin that would become the heart of the building’s resilience: six diagonal chevrons on each face that would convey forces from the outer frame to the central core and ultimately to the stilts. In addition, braces would be installed to counteract sway and keep the building from twisting under wind loads. The system was unusual and its success depended on precise execution, with braces traveling long spans that were welded (or bolted in later revisions) to transfer loads efficiently. The chevron bracing was a radical departure from conventional skyscraper design and represented a major engineering risk that would demand careful verification and ongoing monitoring. The decision to use mid-face stilts and chevron braces created a complex load path that would require precise connection details, a subject that would become pivotal in the ensuing crisis.

"It's probably the first time in history that an engineer has come to an architect and said let's make our job harder for us." - Henry van Dyck

Welding vs Bolting: A Quick Cost Cut That Had Long-Term Consequences

As construction moved forward, a cost-driven decision altered the integrity of the brace connections: the team swapped welded connections for bolted joints to save money. The transcript shows a tension between cost optimization and structural reliability. While bolts are not inherently worse than welds, the assumption that the bolt connections would behave equivalently under all loading cases proved naïve when directional wind loads were considered. LeMessurier later confronted questions about whether the bolt-based joints would handle quartering winds, a loading condition that had not been emphasized in the initial calculations. The critical point was that the design assumed perpendicular wind and did not account for diagonal wind interactions, leading to an underestimation of stresses in certain braces. The ethical and technical stakes became clear as the structure’s resilience hinged on the integrity of dozens of joints. A pivotal moment in the film is LeMessurier’s realization that the weld-to-bolt substitution could be a fatal oversight under non-perpendicular wind directions. "There is nothing that says a bolt is inherently worse or better than a weld." - Prof. Eric Hines

Wind Load Realities: Static vs Dynamic Analysis

The team’s calculations initially used static analyses that treated wind as a steady force. However, the wind is dynamic; gusts and building movement interact to produce complex stress patterns. LeMessurier, seeking validation, consulted wind engineering experts and conducted real-world verification. The critical discovery emerged when quartering winds—winds hitting the face of the building at an angle—were examined. The forces on several braces increased significantly, and the earlier assumptions about gravity loads did not hold under these conditions. The situation became especially alarming when the 30th floor joints showed the highest vulnerability. The dynamic analysis, which accounts for the building’s movement, suggested up to a 60 percent increase in stresses compared with the initial estimates. "The stresses could increase up to 60% more than originally anticipated." - Henry van Dyck

Discovering the Scale of the Risk and the Turning Point

Armed with this new information, LeMessurier conducted a thorough reassessment, floor by floor, joint by joint, recalculating bolts required per joint under quartering wind loads. The math showed that some braces would need many more bolts than originally planned. The gravity loads and the wind loads had combined to create a worst-case scenario that threatened the building’s overall stability. Alarm bells sounded; this was the moment of truth for LeMessurier’s professional integrity. He traveled to Canada for independent wind-tunnel testing with Alan Davenport to confirm the dynamic behaviors observed in the New York City scenario. The results confirmed the danger: the previously unconsidered wind directions could trigger failure in the brace joints if not reinforced. The reality was stark: a potential collapse could occur in the event of a storm powerful enough to cause critical stress, which made urgent action non-negotiable. The stakes were astronomical, with thousands of lives at risk in New York City and the surrounding area.

"If you see something that is a social risk, good heavens, this thing would kill thousands. You must do something." - Bill LeMessurier

Project Serene: A Bold, Secret Retrofit

Facing a looming hurricane season and the escalating risk, LeMessurier and his colleagues devised a rapid, clandestine retrofit plan. They launched Project Serene, a program that involved welding 25-centimeter thick steel plates on more than 200 joint connections to reinforce the brace-work and connections. The repairs were carried out under cover of night to avoid causing public panic and to prevent a mass exodus from offices, which could have disrupted the city’s daily life. Emergency measures included procurement of power generators for the tuned mass damper to ensure it remained functional during a storm. The team coordinated a meticulous, seven-week schedule to complete the most critical repairs on the 30th floor first, recognizing the vulnerability of the weakest joints. At the same time, City Corp coordinated an evacuation plan with the Red Cross to mitigate the risk to thousands of residents and workers around the building.

Public disclosure was avoided, not to hide the truth but to prevent panic, a decision that would become a subject of ethical debate decades later. The team also installed new communications infrastructure to monitor the building's structural performance during a storm, including line updates to connect to monitoring centers.

Hurricane Ella and the Final Gate: The Near-Disaster Averted

In early September 1978, Hurricane Ella threatened the East Coast and brought with it the possibility of a 200-km/h wind event through New York. City officials prepared for mass evacuations, and the world watched as engineers waited for the storm’s track. The storm ultimately veered away, but Citicorp Center had already undergone the critical repairs. LeMessurier later described the day after the relief as the “most beautiful day that the world’s ever seen” in New York, a sentiment borne from the knowledge that thousands of lives had been saved by a combination of engineering science, ethical decision-making, and rapid, discreet action. The repairs cost about $4 million, a fraction of the cost of the alternative scenario. Analysts note that Citicorp Center’s approach would influence future design approaches and lead to more widespread use of tuned mass dampers in skyscrapers around the world; the technology has since become a standard feature in some of the tallest and most slender towers, particularly in seismically active or typhoon-prone regions.

Legacy: Ethics, Codes, and the Global Impact

By the time the New Yorker finally highlighted Project Serene in 1995, LeMessurier’s actions were celebrated as a case of ethical engineering and courage. Diane Hartley, an undergraduate student at Princeton, had contemporaneously raised questions about quartering winds in City Corp’s design, though her role remained largely uncredited for years. The 1990s and 2000s saw renewed scrutiny on the Citicorp Center episode, with the BBC producing a documentary and the National Institute of Standards and Technology revisiting quartering wind effects in tall buildings. The consensus in the engineering community is that LeMessurier’s decisions, while fraught with personal risk and professional danger, represented the gold standard of engineering ethics: transparency, accountability, and a willingness to revise designs in the face of new data to protect public safety. The Citicorp Center incident has since been included in curricula around the world, serving as a cautionary tale about the perils of overconfidence and the necessity of re-evaluating assumptions under real-world loading conditions. "Among the 20 tallest buildings in the world, 6 include a tuned mass damper, and they're especially critical in typhoon or earthquake prone regions." - Derek Muller

Conclusion: A Landmark in Engineering Ethics

The Citicorp Center saga stands as a testament to the responsibility engineers bear to the public when confronted with potentially catastrophic risks. LeMessurier’s insistence on a transparent, data-driven response, despite the potential for professional ruin, demonstrated that safeguarding lives must come first. The project reshaped building codes by embedding quartering-wind considerations into standard practice, and it popularized tuned mass dampers as a practical solution for tall, slender buildings facing demanding wind and seismic environments. As the video notes, the legacy of Citicorp Center lives on in modern skyscraper design, including iconic structures like Taipei 101, which uses a massive pendulum damper to stabilize its own sway. The episode ultimately celebrates the fusion of engineering ingenuity with moral courage, underscoring that the pursuit of knowledge and safety is an ongoing, social enterprise that extends beyond any single building or project.