How a Student's Question Saved This NYC Skyscraper

Structural engineer Bill LeMessurier discovered that Citicorp Center, open for less than a year, had a fatal flaw that could cause it to collapse in winds of just 110 kilometers per hour.

• Over 200,000 people lived and worked in the surrounding area • Hurricane season was only weeks away • LeMessurier was the only person in the world who knew about the danger

LeMessurier faced a choice between staying silent and hoping for the best, or attempting to fix it and risking professional ruin and mass panic.

Citicorp Center had a 100% probability of total collapse by the end of the century, making the question of how to save New York from a near certain disaster urgent.

Citicorp wanted to build headquarters on a city block, but the land was partially occupied by Saint Peter's Church, whose pastor refused to leave and demanded the church have its own separate identity.

• Replace the old gothic church with a brand new one • Make the church physically distinct from the new tower • Keep two-thirds of the space above the church free and open

Engineer LeMessurier proposed an unconventional solution: notch out all four corners of the tower and construct the skyscraper on stilts, which was probably the first time an engineer asked an architect to make their job harder.

The stilts would support at least half the building's gravity load while a larger central column held the rest. The stilts would be positioned at the center of each face, not at the corners, creating an engineering challenge.

LeMessurier sketched out an idea on a napkin: six layers of diagonal braces up each face of the tower that would transfer forces to the middle of each face and down to the stilts.

• Gravity loads come down the column and are forced into the braces by removing columns at strategic points • Every eight stories, half the gravity load is forced through the chevrons to the midface columns • This system was entirely unique, driven by the placement of the columns and building conditions

The chevron bracing transfers wind loads that accumulate as they travel down the building. Unlike ordinary structures with corner columns, this design channels horizontal forces through the diagonal braces in a wrapping pattern.

The chevron braces were massive, almost 40 meters long end to end. They were fabricated in pieces and welded together on site since they could not be transported through Manhattan as single units.

The lightweight chevron bracing system made the building swayable in the wind. While not necessarily a structural problem, it could be uncomfortable for occupants.

LeMessurier adopted a tuned mass damper (TMD), a system previously used only in bridges, power lines, and ships. He used a 400-ton concrete mass on the top floor, affectionately known as 'the great block of cheese.'

• As the building sways to one side, the concrete mass moves in the same direction • Energy is dissipated through viscous dampers as the mass oscillates out of phase with the building • The system reduces swaying amplitude by roughly 50%

The damper saved approximately $4 million by eliminating the need for an additional 2,800 tons of structural steel while maintaining the elegant design.

In May 1978, while discussing a similar project, a contractor and architect asked LeMessurier how the welded chevron braces worked out. LeMessurier called his New York office and discovered the braces had been bolted instead of welded.

The contractor had suggested saving a quarter million dollars by using bolts instead of welds. LeMessurier's firm agreed without reviewing the critical implications.

Since gravity loads compress the braces, some chevrons only go into tension under very high winds. LeMessurier trusted his team's calculations that bolts were adequate for these lower-tension scenarios.

About a month later, a student called LeMessurier with questions about the building. A professor claimed LeMessurier's column placement was wrong. This conversation sparked LeMessurier to reconsider wind loads from all directions.

While working on a triangular hotel building, LeMessurier began analyzing what happens when wind hits a corner instead of straight on the face. These quartering winds revealed a critical oversight.

• Wind is split into perpendicular components hitting each side • Each side receives force divided by the square root of two • The diagonal braces experience double the force compared to perpendicular wind calculations • Total stress increase of 40% over original calculations

The original bolted connections were calculated assuming wind hitting straight on the building face. The firm had not considered quartering winds, which was critical since bolts cannot handle the increased tension that welded connections could manage.

Looking at the actual joint requirements, braces that needed only 4 bolts now required 10 bolts. With incorrect safety factors applied, the actual requirement was 14 bolts, but only 4 had been installed.

LeMessurier flew to Canada to verify his calculations with Alan Davenport at the Boundary Layer Wind Tunnel. The results showed the situation was even worse than his static analysis indicated.

• Static calculations showed 40% increase in stress • Dynamic analysis revealed stresses could increase up to 60% when the building sways • The weakest joints were at the 30th floor where failure would cause total collapse

On average, a storm strong enough to tear the building apart occurred every 67 years if the tuned mass damper was working. If a storm knocked out power, even 110 km/h winds for five minutes would collapse the building, with a 1 in 16 annual probability.

Hurricane Belle had passed through New York just one year before, with wind gusts of 110 kilometers per hour. A similar storm hitting during the upcoming hurricane season could be catastrophic.

LeMessurier considered disappearing or driving intentionally off the Maine Turnpike into a bridge abutment. In a later interview he admitted these dark thoughts during the crisis.

With a 1 in 16 chance of collapse that fall and thousands of lives at risk, LeMessurier recognized there was never any choice but to act.

• LeMessurier consulted with lawyers and engineering experts • He informed architect Hugh Stubbins • Together they told Citicorp chairman Walter Wriston • Emergency generators were acquired for the tuned mass damper within hours

LeMessurier called the repair plan Project Serene (Special Engineering Review of Events Nobody Envisioned) to avoid the ominous sound of Project Pandora.

• Each night, welders would enter after workers left • They removed sheet rock around chevron beams • Welded two 5-centimeter thick, 2-meter long steel plates on each joint as reinforcement • Replaced walls and cleaned up before morning workers arrived

Over 200 joints needed welding. LeMessurier ranked them by importance, starting with the 30th floor joints, the most critical to prevent collapse.

Repairs would not be completed before hurricane season. Citicorp worked with the Red Cross to develop a 10-block evacuation plan affecting thousands of people in and around the building.

• Decision made not to tell the public or office workers • Strain gauges installed on structural members monitored from 8 blocks away • AT&T president personally installed emergency telephone lines overnight for communication

When The New York Times tried to reach LeMessurier for comment, he was about to call them when they announced a strike at 6 p.m. All New York newspapers went on strike until October, providing perfect cover for the repairs.

• Hurricane Ella began brewing in the Caribbean in late August • By Friday, September 1st, Ella approached with winds reaching 200 km/h • City officials prepared 10-block evacuation with door-to-door police deployment • Repairs were only halfway complete

For 24 tense hours, Hurricane Ella stalled near North Carolina. LeMessurier and the team were 'sweating blood.' At the last minute, the hurricane veered off into the sea, intensified, and hit Canada with 225 km/h winds.

Repairs were completed in October, just six weeks after LeMessurier told Citicorp about the crisis. The building could now withstand a one in 1000-year storm.

For nearly two decades, Project Serene remained secret. In 1995, The New Yorker published the full story, bringing LeMessurier praise rather than vilification for owning up and fixing the mistake immediately.

• New York updated building codes to require quartering wind calculations • Tuned mass dampers have spread globally across buildings • Six of the 20 tallest buildings in the world now use TMDs • Taipei 101 uses a 660-ton pendulum damper for typhoon and earthquake resistance

• Initially believed to be Diane Hartley, a Princeton engineering student studying Citicorp for her thesis • In 2011, Lee DeCarolis from the New Jersey Institute of Technology came forward claiming to be the caller • Sadly, LeMessurier died in 2007 before confirming the student's identity

• LeMessurier Associates refuses to discuss the repairs or acknowledge mistakes • Boston Properties (current owner, renamed 601 Lexington) refused filming access or comment • A 2021 NIST study revisited quartering winds analysis without complete internal structural details • The case remains controversial despite being taught as a model of engineering ethics worldwide