How a Design Flaw Nearly Caused the Citicorp Center's Collapse and How it Was Fixed

Veritasium

Summary:

The Citicorp Center, completed in 1977, was built on four central stilts to accommodate Saint Peter's Church, a unique design choice.
Structural engineer Bill LeMessurier designed a system of chevron braces to transfer gravity and wind loads to these central columns.
To reduce the building's sway, a 400-ton Tuned Mass Damper (TMD) was installed on the top floor, the first of its kind in a skyscraper.
In 1978, a student's inquiry prompted LeMessurier to re-evaluate his calculations, revealing a critical flaw: the design was only stable for perpendicular winds, not for quartering winds.
Quartering winds increased stress on certain connections by up to 60%, requiring significantly more bolts than the four originally used.
LeMessurier acted immediately, implementing "Project Serene" to weld steel plates to over 200 joints at night, without public knowledge.
During the repairs, Hurricane Ella threatened New York, prompting a secret 10-block evacuation plan, but the hurricane veered off course.
The repairs were completed in six weeks, enhancing the building's resilience to a 1-in-1000-year storm.
The crisis remained a secret for nearly two decades until a 1995 "New Yorker" article revealed the story, leading to updated building codes and LeMessurier's recognition for ethical conduct.

Citicorp Center's unique design features its main structure elevated on four central stilts, with Saint Peter's Church nestled beneath one of its corners. [ 00:00:00 ]

Citicorp Center's unique design features its main structure elevated on four central stilts, with Saint Peter's Church nestled beneath one of its corners. [ 00:01:40 ]

Why the Citicorp Building is on Stilts [0:00]

The Genesis of a Unique Design [0:05]
In the 1960s, Citicorp sought to build a new headquarters in Manhattan on a city block where Saint Peter's Church was located.
The church refused to sell or relocate, demanding to be an independent part of the new tower, with two-thirds of the space above it remaining open.
LeMessurier's Innovative Solution [0:00]
Architect Hugh Stubbins and structural engineer Bill LeMessurier were hired to design the tower around the church.
LeMessurier proposed elevating the skyscraper on four central stilts, instead of corner columns, to accommodate the church underneath.
This unusual placement presented a significant engineering challenge for both gravity and wind loads.
The Chevron Bracing System [0:3:26]
LeMessurier's solution involved a system of diagonal braces, or chevrons, up each face of the tower.
These chevrons were designed to transfer the building's gravity loads to the middle of each face and down to the stilts.
Half of the gravity load from every eight stories was directed through these chevrons to the mid-face columns.
The system was unique, driven by the specific site conditions and column placement.

How Wind Load Works [4:44]

Understanding Wind Forces on Skyscrapers [4:45]
Wind hitting a building creates horizontal loads.
Traditional corner-column structures deform, but diagonal bracing enhances stability by converting bending loads into compression and tension, which steel is stronger in.
In Citicorp's design, wind load wrapped around the entire building, building up as it traveled down the structure.
The Challenge of Fabrication [6:47]
The chevron braces were almost 40 meters long, too large to transport as single pieces.
They were sent in sections and welded together on-site.
An Unforeseen Consequence: Building Sway [7:11]
The chevron bracing, while efficient for load transfer, resulted in a lighter structure (22 pounds per square foot), making the building more susceptible to swaying in the wind.
This swaying was not a structural failure risk but could cause discomfort for occupants.

Tuned Mass Dampers (TMDs) [7:49]

The Innovative Solution for Sway Reduction [7:50]
LeMessurier introduced a Tuned Mass Damper (TMD), a technology previously used in bridges and ships but never in a skyscraper.
A TMD works by having a mass oscillate out of phase with the main structure, transferring kinetic energy and dissipating it through friction, thus dampening the building's sway.
Citicorp's "Block of Cheese" TMD [9:49]
Citicorp's TMD was a 400-ton concrete block (29 feet square, 8 feet thick) installed on the top floor.
It was designed to reduce swaying amplitude by about 50%.
This saved an estimated $4 million by reducing the need for 2,800 tons of additional structural steel.
The building was deemed structurally sound with both the chevron bracing and the TMD in place.

The Anonymous Student and the Discovery of the Flaw [11:17]

A Seemingly Minor Change [11:47]
In May 1978, LeMessurier learned that his firm had agreed to use bolts instead of welds for the chevron connections, saving a quarter of a million dollars.
He initially deemed this acceptable, as bolts are not inherently inferior to welds and the tension forces under perpendicular winds were thought to be low.
The Call from the Student [12:43]
A month later, LeMessurier received a call from an engineering student, identified as Lee DeCarolis (later debated as Diane Hartley, a Princeton student), who questioned the structural integrity of the columns in the middle of each face.
While explaining his design, LeMessurier considered how the building would perform under wind loads from all directions, not just perpendicular ones.
The Critical Re-evaluation: Quartering Winds [14:00]
LeMessurier decided to double-check the building's response to quartering winds (hitting a corner).
He calculated the forces by splitting the wind into perpendicular components.
He discovered that some diagonal braces experienced double the stress compared to perpendicular wind scenarios, resulting in forces 40% higher than originally calculated.
The Compounding Error [15:51]
This increased stress was problematic because the bolts, designed for lower loads, were insufficient.
The original calculations for the bolts had only considered straight-on wind loads, not quartering winds.
Furthermore, LeMessurier's firm treated the braces as "minor structural elements," applying an incorrect factor of safety by underestimating the gravity load.
This meant that instead of the four bolts installed, some critical joints (especially around the 30th floor) actually needed 10 to 14 bolts to safely withstand quartering winds.

The Odds of Collapse and the Race to Fix It [18:38]

A High Probability of Disaster [18:39]
LeMessurier's calculations showed that if the TMD failed during a storm, 110 km/h winds blowing for just five minutes could collapse the building.
The historical weather data indicated a 1-in-16 chance of such a storm hitting New York City in any given year. Hurricane Belle, with similar winds, had passed through just a year before.
The weakest joints were identified on the 30th floor; their failure would lead to total building collapse.
Project Serene: The Secret Repair Plan [20:31]
LeMessurier disclosed the flaw to the architect and Citicorp's chairman, Walter Wriston.
Emergency generators were secured for the TMD, which now became crucial for the building's stability.
A confidential repair plan, named "Project Serene" (originally "Project Pandora"), was initiated.
Welders worked secretly at night, removing drywall, welding two 5-centimeter thick, 2-meter long steel plates to each of over 200 critical joints, and then restoring the walls before morning.
A 10-block radius evacuation plan was developed with the Red Cross, ready for implementation without public knowledge to prevent mass panic.
Strain gauges were installed on structural members, with real-time monitoring from a command center eight blocks away. AT&T quickly installed necessary phone lines at Wriston's request.
Hurricane Ella's Near Miss [25:40]
In late August, Hurricane Ella, a Category 4 storm with 200 km/h winds, brewed in the Caribbean and headed towards New York.
City officials prepared for evacuation, but Ella miraculously veered off into the sea at the last minute, intensifying and hitting Canada instead.
The repairs were completed in October 1978, just six weeks after LeMessurier informed Citicorp, making the building capable of withstanding a 1-in-1000-year storm.

TMDs Take Over The World & Controversies [27:00]

LeMessurier's Legacy [27:47]
The Citicorp case led to updated New York building codes, requiring quartering wind calculations.
Tuned Mass Dampers became ubiquitous in skyscraper design worldwide, especially in earthquake or typhoon-prone regions.
Six of the 20 tallest buildings in the world now incorporate TMDs, like Taipei 101 with its 660-ton pendulum.
LeMessurier, initially praised, later confirmed his initial account of the "anonymous student" conversation.
Unresolved Mysteries and Lingering Feelings [28:36]
The exact identity of the "anonymous student" remains debated, with both Diane Hartley (a Princeton undergraduate) and Lee DeCarolis (a New Jersey Institute of Technology student) having compelling claims.
Despite the heroic resolution, many involved still feel raw about the incident.
LeMessurier Associates and Boston Properties (the building's current owner, renamed 601 Lexington) refuse to discuss the repairs.
A 2021 NIST study confirmed the greater demands of quartering winds on buildings like Citicorp, though it did not include Citicorp's specific internal structure in its analysis.
LeMessurier's actions are now a global case study in engineering ethics, highlighting the profound responsibility engineers bear for public safety.