From: veritasium
The Citicorp Center, located in Manhattan, was a cutting-edge skyscraper that opened in 1977. Less than a year after its completion, its structural engineer, Bill LeMessurier, discovered a fatal flaw: winds of just 110 kilometers per hour could cause the building to collapse, potentially endangering thousands of lives in the surrounding area during hurricane season [00:00:00]. LeMessurier faced a critical choice: remain silent or risk professional ruin by trying to fix the issue [00:00:37]. The building had a 100% probability of total collapse by the end of the century if not addressed [00:00:47].
Unique Architectural Constraints
In the 1960s, Citicorp aimed to build a new headquarters in Manhattan on a city block that included Saint Peter’s Church [00:01:05]. The church pastor, Ralph Peterson, insisted that the new tower must integrate the church, requiring it to be physically distinct and independent, with two-thirds of the space above it remaining open and clear [00:01:26].
To accommodate these demands, architect Hugh Stubbins and structural engineer Bill LeMessurier designed the tower to be built around the church [00:02:00]. LeMessurier proposed constructing the skyscraper on “stilts” by notching out all four corners of the tower [00:02:22]. This unusual placement meant the stilts were at the center of each face, rather than the corners, creating an inherent engineering challenge that seemed unstable [00:02:56].
Innovative Design Solutions
Chevron Bracing System
LeMessurier’s solution involved designing six layers of diagonal braces, or chevrons, up each face of the tower [00:03:26]. These chevrons would transfer forces to the middle of each face and down to the stilts [00:03:31]. To ensure gravity loads were channeled into the braces, columns at the top and middle of each chevron were removed, making every tier act as a separate unit connected only to the braces and the central core [00:04:06]. This unique system was driven by the specific conditions of the building [00:04:28].
The chevron system also managed wind loads, which build up as one goes down a tall building [00:04:45]. Unlike typical buildings where corner columns deform under wind, Citicorp’s chevrons pulled down in tension and pushed up in compression to resist rotation, causing the wind load to wrap around the entire building [00:05:49]. These massive braces, almost 40 meters long, were fabricated in pieces and welded together on site [00:06:44].
Tuned Mass Damper (TMD)
While the chevron bracing saved significant money and weight, resulting in a remarkably light structure (22 pounds per square foot), it also made the building susceptible to swaying in the wind [00:07:11]. To address this and prevent discomfort for occupants, LeMessurier implemented a tuned mass damper (TMD) – a solution previously used in bridges, power lines, and ships, but never before in a building [00:07:41].
The TMD in Citicorp was a 400-ton concrete block, 29 feet square and 8 feet thick, installed on the top floor [00:09:52]. As the building swayed, the block would move in the same direction, transferring kinetic energy from the building to the block and dissipating it through viscous dampers [00:10:07]. The block oscillates out of phase to the building’s motion, significantly damping its sway [00:10:17]. LeMessurier estimated it would reduce swaying amplitude by 50% and saved $4 million by avoiding additional structural steel [00:10:24].
Discovery of the Fatal Flaw
The first hint of trouble emerged in May 1978. LeMessurier discovered that the chevron brace connections, originally designed for welds, had been swapped for bolts by the contractor to save money [00:11:18]. Although bolts are not inherently inferior to welds, this change was a surprise given the building’s cutting-edge design [00:12:04].
A month later, a student’s inquiry about the building’s design prompted LeMessurier to re-examine the calculations, specifically considering wind loads from all directions [00:13:40]. This led to a critical realization: the worst-case scenario was not the diagonal wind (which engineers often consider), but rather the “ordinary” wind pushing straight on the building’s corners, known as quartering winds [00:14:00].
When LeMessurier analyzed the impact of quartering winds, he found that the stresses in some diagonals doubled, leading to forces 40% higher than his initial perpendicular wind load calculations [00:14:24]. This increase, combined with the bolted connections and an incorrect factor of safety used by his firm (considering the braces minor structural elements), revealed the severity of the flaw [00:14:57].
For instance, a brace around halfway down the tower, originally calculated to need four bolts, actually required around 10 bolts for quartering winds and, factoring in the safety margin, a total of 14 bolts [00:16:30]. The weakest joints were found on the building’s 30th floor, where failure would cause the entire structure to fall [00:18:32]. Furthermore, wind tunnel tests revealed that dynamic analysis, accounting for building movement, could increase stresses by up to 60% beyond static conditions [00:18:09].
The Looming Disaster
LeMessurier determined that a storm strong enough to tear the building apart occurred, on average, every 67 years [00:18:49]. However, if a storm caused a power outage, even 110 km/h winds blowing for just five minutes would lead to collapse, with a 1 in 16 chance of such a storm in any given year [00:18:57]. Hurricane Belle, with similar wind gusts, had passed through New York City just a year before Citicorp was completed [00:19:11].
LeMessurier acted swiftly, informing Citicorp’s chairman, Walter Wriston [00:20:31]. Emergency generators were acquired for the TMD, which now became a critical stability device [00:20:42]. A confidential repair plan, “Project Serene,” was initiated [00:20:55].
Repair and Legacy
Welders secretly worked at night, removing sheetrock, and welding two five-centimeter thick, two-meter long steel plates onto each of the over 200 affected joints, starting with the most critical ones on the 30th floor [00:21:10]. Citicorp and the Red Cross developed a 10-block evacuation plan, acknowledging the building’s collapse could trigger a catastrophic chain reaction [00:21:41]. Despite the risks, the repairs were kept secret from the public to avoid mass panic [00:22:25]. Strain gauges were installed to monitor the skyscraper’s movements, providing warning if conditions worsened [00:23:31].
The repairs were completed in October 1978, just six weeks after the crisis was revealed to Citicorp [00:26:59]. The building could then withstand a one in 1000-year storm [00:27:07].
The secret was kept for almost two decades until “The New Yorker” broke the story in 1995 [00:27:24]. Far from being condemned, LeMessurier was lauded for his ethical conduct in owning his mistake and swiftly correcting it [00:27:35].
The Citicorp case had a significant impact on building regulations. New York updated its building code to require quartering wind calculations [00:27:42]. Furthermore, the tuned mass damper, pioneered in Citicorp as “mechanical help” for structural stability, has since spread globally, enabling taller and slimmer skyscraper designs, particularly in typhoon or earthquake-prone regions [00:27:47]. Six of the world’s 20 tallest buildings now incorporate TMDs [00:28:14].
The identity of the student who called LeMessurier remained a mystery for years, with Diane Hartley, a Princeton undergraduate, initially believed to be the caller due to her thesis on the Citicorp Tower’s quartering wind analysis [00:28:43]. However, Lee DeCarolis from the New Jersey Institute of Technology later came forward in 2011, claiming to have made the call that aligned with LeMessurier’s own account [00:30:45]. The full details surrounding the initial discovery remain a sensitive topic for those involved [00:31:16].
The Citicorp case is widely taught in engineering ethics courses as an example of a professional’s responsibility beyond themselves, especially when faced with a “social risk” that could lead to widespread harm [00:32:27]. It highlights the profound emotional pressure structural engineers face, knowing that their calculations dictate whether a building stands [00:32:31]. The initial counterintuitive engineering concepts (stilts at mid-face, the impact of quartering winds) and the subsequent ethical response define its enduring legacy.