Citicorp Center engineering crisis facts for kids
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The Citicorp Center engineering crisis was a big problem discovered in 1978 with a newly built skyscraper in New York City. This building, now called Citigroup Center, had a serious hidden flaw in its structure. Over several months, quiet repairs were made to fix it. The building was meant to be the main office for Citibank. Its structure was designed by William LeMessurier. It had some unusual features, like being supported by four stilts in the middle of each side, and special diagonal braces to handle wind.
The original design didn't correctly calculate how strong the wind could be. A student named Diane Hartley from Princeton University noticed this problem while working on a school project. She told the engineering firm about it. LeMessurier was later praised for admitting his mistake and organizing the successful repairs. At the time, experts thought a strong wind (around 70 miles per hour) could make the building fall over, putting many lives at risk. This whole problem was kept secret until 1995. Diane Hartley didn't even know how important her work was until then.
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About the Citicorp Building
The Citigroup Center is a very tall, 59-story skyscraper. It's located in the Midtown Manhattan area of New York City. It was designed by architect Hugh Stubbins to be the main office for Citibank. LeMessurier Associates were the structural engineers. The building was officially opened on October 12, 1977.
During the building's construction, a new church, St. Peter's Lutheran Church, was built next to it. Because of an agreement, the church had to be separate from the main tower. To avoid building on top of the church, the skyscraper is held up by four large stilts. These stilts are placed in the middle of each side of the tower, instead of at the corners.
To make this design work, Bill LeMessurier added special diagonal braces inside each face of the building. These braces look like upside-down "V" shapes. They are designed to carry the weight and force from wind down to the stilts.
The long diagonal braces were made in sections and put together on site. Each brace needed five connecting points. LeMessurier's first plan used welded joints for these connections. Welding means melting metal together to join pieces. But to save money, the steel company suggested using bolted joints instead. Bolted joints use strong bolts to hold pieces together. LeMessurier's office agreed to this change, but he didn't know about it until later.
For his original design, LeMessurier only calculated how the building would handle wind blowing straight at its sides. This was all that New York City building rules required. Usually, if a building can handle straight-on winds, it can handle winds from other angles too. So, LeMessurier didn't specifically calculate the effects of winds blowing diagonally (at a "quartering" angle).
How the Problem Was Discovered
In June 1978, a Princeton University engineering student named Diane Hartley was writing her final school project. Her professor suggested she study the Citicorp Center's design. As part of her work, she looked at the building's structure. She calculated the stress (the force on parts of the building) from diagonal winds. She found that these forces were higher than what LeMessurier's firm had told her to expect.
Her professor also questioned the numbers from the firm. Diane asked her contact at the firm about this difference. She was told the building was safe, so she didn't pursue it further. Her concerns were written in her project paper.
Separately, LeMessurier was designing a similar building. A builder for that project questioned the cost of using welded joints. This made LeMessurier ask his office about the Citicorp building. That's when he learned that bolts had been used instead of welds.
Diane Hartley's questions eventually reached LeMessurier. On July 24, 1978, LeMessurier did his own calculations for the Citicorp Center. He found that for some of the diagonal braces, diagonal winds would increase the force by 40 percent. At the bolted joints, the force would increase by a huge 160 percent!
If the building only had bolted joints OR only had higher loads from diagonal winds, it might have been okay. But the combination of both problems was serious. LeMessurier realized that while the original welded design could handle all winds, a strong diagonal wind (like a 70 mph hurricane wind) would be too much for the bolted joints. He also found that his firm had used the wrong safety factor for the braces.
On July 26, LeMessurier visited a wind expert, Alan Garnett Davenport. Davenport's team confirmed LeMessurier's findings. They even found that in a real storm, the forces could be even higher. LeMessurier then went to his summer home to think about the problem. He estimated that a wind strong enough to make the building collapse would happen about once every 55 years. If the building's special "tuned mass damper" (a device that helps the building sway less in wind) didn't work due to a power cut, a dangerous wind could happen every 16 years.
Fixing the Problem
LeMessurier was very worried about what to do. If he told the public, his career could be ruined. It could also cause panic for people living and working near the building. For a moment, he thought about keeping it a secret.
But LeMessurier decided to do the right thing. He contacted his lawyer and his insurance company. Then he told Citicorp's lawyers, who hired another expert to advise them. Citicorp agreed to LeMessurier's plan: weld strong steel plates over the bolted joints to make them stronger. A company called Karl Koch Erecting was hired for the welding work. Very few people knew about the problem, only Citicorp leaders, the mayor, and a few officials.
Construction crews started welding the steel plates at night in August 1978. Officials didn't tell the public about any structural problems. Luckily, New York City's major newspapers were on strike at the time, so the news didn't spread widely. Officials described the work as routine maintenance. Citicorp claimed the building could already handle very strong winds and had no problems.
As a safety measure, emergency power generators were installed for the mass damper. Special sensors were placed on important beams to check their strength. Weather forecasters were also hired to watch for storms. Citicorp and city officials even made secret plans to evacuate the neighborhood if a big storm hit. These plans were not made public.
Six weeks into the repair work, a major storm (Hurricane Ella) was heading towards New York. The building was only half-repaired. Luckily, Ella changed direction and moved out to sea, missing New York City.
The repairs were finished in October 1978, before the newspapers started publishing again. LeMessurier later said that a wind strong enough to damage the repaired building would only happen about once every 700 years. LeMessurier and the architect covered all the repair costs, which were millions of dollars.
The Story Comes Out
Since the building didn't collapse, the story of the crisis was kept secret for many years. It was finally made public in a long article in The New Yorker magazine in 1995. The article mentioned a "young man" who had called LeMessurier about the building. However, it seems the student never actually spoke directly with LeMessurier.
Years later, in 2011, Diane Hartley identified herself as the engineering student. She said she had spoken with Joel S. Weinstein, who worked in LeMessurier's office.
Important Lessons Learned
Many people see LeMessurier's actions as very brave and ethical. Many engineering schools now use his story as an example of how engineers should act responsibly. However, some people have also criticized LeMessurier. They point out his initial mistakes in oversight. They also question the secrecy around the repairs, arguing that the public, architects, and other engineers should have been told.
Architect Eugene Kremer discussed several important ethical questions from this case:
- Wind Load Analysis: LeMessurier initially only checked the wind calculations required by building rules, not all possible calculations.
- Design Changes: The builders changed from welded to bolted joints without fully consulting LeMessurier.
- Professional Responsibility: Before telling Citicorp, LeMessurier briefly thought about keeping the problems a secret.
- Public Statements: During the repairs, officials didn't tell the full truth about the problems.
- Public Safety: When Hurricane Ella threatened, secret evacuation plans were made for the area.
- Sharing Knowledge: Keeping this problem secret for almost 20 years meant that other engineers couldn't learn from it sooner.
