In 1977, the Citicorp Center in New York City (NYC) was completed. It was distinctive and remarkable because it was built on stilts, due to the fact that it had to be built over several tenants who only sold Citicorp the air rights over their properties.

To meet the restrictions of the building site, the structural engineer, William LeMessurier, designed the building to be elevated on stilts nine stories over the streets and the existing tenants. One magazine gushed, “the raising of [Citicorp’s] tower on stilts was one of the most daring designs to be completed in the city’s history.” In 1978, largely on the strength of this achievement, LeMessurier was elected to the prestigious National Academy of Engineering and was asked to teach a structural engineering class to architecture students at Harvard.

Yet in June that year, LeMessurier received a call from a female engineering student in New Jersey named Diane Hartley. She was writing a paper on the Citicorp Building and had calculated that the famous building was vulnerable to a quartering wind (wind not perpendicular to the side of the building). The student’s professor had reviewed Hartley’s calculations and concurred. LeMessurier responded, “Listen, I want you to tell your teacher that he doesn’t know what the hell he’s talking about, because he doesn’t know the problem that had to be solved.”

But as it turned out, Hartley was right: the Citicorp building was in imminent danger of total collapse when exposed to high quartering winds.

Recognizing the Building Defect

Because of the constraints of the building site, the load of the Citicorp Building had to be supported by five columns: the single central column containing the building’s elevators and a column on each of the building’s four sides (the corners of the building site were occupied by other tenants and could not be used for supports). The corners of each floor cantilevered 72 feet over each corner of the city block.

LeMessurier accordingly designed a system of braces in the shape of the letter “V,” directing the building load down toward the point of the “V” shape, which was supported by the columns. LeMessurier had originally intended to display these braces on the exterior of the building, but the lead architect decided to conceal them beneath the skin of the building.

LeMessurier also added a tuned mass damper to prevent building sway–the first time, one had been used in a skyscraper in the U.S. This was a 400-ton weight suspended from the top of the building that sensed building sway due to wind and “dampened” the sway by absorbing the energy. The pendulum construction was tuned to match the frequency of the building.

NYC code did not require the calculation of quartering wind forces, so they were not part of LeMessurier’s original calculations. But after Hartley’s call, he decided to calculate the effect of such winds on his design as a case study for the class he taught. He was stunned to discover that quartering winds would increase the stress on the “V” braces by as much as 40%! Yet even that would not have jeopardized the building, with the safety factors he had built in–except for a significant change in LeMessurier’s design that was made during construction.

LeMessurier originally specified that the “V” braces be manufactured out of welded steel joints. But the builder of the braces, Bethlehem Steel, considered welded steel to be labor-intensive, expensive, and unnecessarily strong. Bethlehem Steel proposed using bolted joints instead, and LeMessurier’s staff approved the proposal at the start of construction without his knowledge. He had only recently learned of this change, but calculated that the “V” braces with bolted joints would experience as much as 160% more stress in a quartering wind.

LeMessurier learned that his staff had not accounted for quartering winds when approving the change in joint construction. He then calculated that the most vulnerable joints in the building were on the 30th floor, and that if they failed, the entire building would collapse. He reviewed the history of storms in NYC and calculated that storms with wind sufficient to topple the building occurred roughly every 16 years. And though the tuned mass damper he installed might increase stability a little, it wouldn’t function in a power outage.

His innovative Citicorp Building was a disaster waiting to happen.

Remedying the Building Defect

LeMessurier was in a predicament. To admit the safety defects in the Citicorp Building might ruin his career. He might face litigation and bankruptcy, and his reputation would be destroyed. He considered his options: suicide, remaining silent, or informing the client of the issue.

He chose the latter. LeMessurier developed a plan to remedy the Citicorp Building’s vulnerability and, with that in hand, contacted Citicorp about the building’s weakness.

Citicorp responded immediately, forming a crisis team to both reduce the risk and manage the repairs. The team promptly installed two emergency generators to operate the tuned mass damper in case of a power outage and contracted the manufacturer of the tuned mass damper for 24-hour technical support. These steps ensured that the tuned mass damper would help stabilize the building even in a storm.

The crisis team also notified NYC building authorities of the issue and showed them LeMessurier’s remediation plan. The authorities agreed to it and enlisted the help of the New York Police Department (NYPD) to develop a crisis management plan for evacuating the building and the surrounding area in the event of a wind alert. The team also collaborated with the Red Cross to plan emergency food and shelter for potential evacuees.

Meanwhile, within the Citicorp building, workers welded two-inch-thick steel plates over the bolts of the over 200 joints in the “V” braces, converting them from bolted joints to welded joints with a significant additional safety factor. When the remediation was completed in October 1978, the building was capable of withstanding a “once in 700 years” storm and was called “one of the safest structures ever built–and rebuilt–by the hand of man.”

The Aftermath

Remarkably, Citicorp completed the work to remedy the defects of their building in almost complete secrecy. LeMessurier and Citicorp, and later the NYC building authority and NYPD, agreed that notifying the public would cause unnecessary fear.

Citicorp negotiated with the architect and LeMessurier to recover some of the estimated $8m that the remediation cost, settling for $2m from LeMessurier’s firm. This was agreed upon without litigation to prevent public scrutiny.

Twenty years after the repairs, the history of the building’s construction and repair was uncovered by a New York Times reporter. The response was positive–LeMessurier was hailed for his honesty and his efforts in making the building safe, and Citicorp was praised for its quick action to minimize the risk, then repair the building.

The authorities’ response to LeMessurier’s admission was also praised for achieving a quick solution: one city engineer said, “It wasn’t a case of ‘We caught you, you skunk.’ It started with a guy who stood up and said, ‘I got a problem, I made the problem, let’s fix the problem.’ If you’re gonna kill a guy like LeMessurier, why should anybody ever talk?”