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Citigroup Center: The Skyscraper that Could Have Fallen Over

Mention New York and disaster and most peoples’ minds will jump to the unforgettable events of September 11, 2001. The images and stories of two of New York’s largest buildings collapsing and strewing debris and wreckage for blocks around are burned into the U.S. national memory. As the saying goes, “ask any American and they can tell you where they were on 9/11.” 

Standing on the sidewalk and looking up at a skyscraper can strike awe into anyone willing to consider its sheer size, not to mention the amount of material that went into its building. Seeing one of these behemoths collapse is a terrible and scary sight, enough to strike fear into any sane person.  

Well, now we can add fear of the unknown into the mix. Two decades before 9/11, “the greatest disaster never told” struck fear into the hearts of architects, engineers, and corporate executives, and—decades later—the public. It sparked a debate over ethics and professionalism in the construction and architecture industry and how those in power handle information sharing during a crisis. The tale of the Citigroup Center, though kept secret for years, found its own immovable place in the psyche of New Yorkers.  

The Background

The Citigroup Center, originally the Citicorp Center and now 601 Lexington, was first built in 1977. Upon its completion, it became the seventh-tallest building in the world. But its height is not what makes it memorable. 

The building’s design is unique, standing out among Manhattan’s skyline, even today. At first glance, many people notice The Citigroup Center’s slanted roof. Upon drawing closer, however, pedestrians soon see what makes this skyscraper truly inimitable: its stilted base. When I say stilted, I mean literal stilts supporting a skyscraper. The lower nine floors of the Citigroup Center comprise four columns that support the upper fifty floors. 

Citigroup Center base
Citigroup Center’s “stilted base”

The brains behind this architectural oddity are the architect Hugh Stubbins and the structural engineer William LeMessurier; the idea for the design going to the former while the rest of the credit lies with the latter. They devised this design when they encountered the challenge of building around the existing St. Paul’s Lutheran Church. Many people claim that church is their foundation, but few can claim to have actually built fifty-nine floors around one. However, it’s not its looks that make this building memorable. 

The Design

St Peter Lutheran Church under Citigroup center building

After meetings with representatives of the church, Le Messurier devised the stilt design to meet the requirements of leaving the existing structure of the church undisturbed. The placement of the lower support columns makes this building even more distinctive. While most buildings’ support is located in their corners, the stilts of the Citigroup Center are under each straight side, giving it a precarious look. Given its slanted roof and its minimalist base, the Citigroup Center has the iconic look of a structure built in Minecraft.

Unlike in Minecraft, though, this tower had to have supports that would keep it standing when met with real life physics. As a way of keeping the building lightweight and cost-effective, the tower was built to just meet the minimum safety requirements. Le Messurier made a system of V-shaped braces that would redirect the gravitational forces acting on the building to the middle support structure, which then would transfer them to the lower, aforementioned, stilts, which would transfer them to the ground. These Chevron-like braces would also provide resistance against the forces of the wind. Le Messurier’s firm used bolts to affix the chevron supports in place of welding. 

This support system made the upper bulk of the building less of a load for its bare base to handle, but also made it more susceptible to swaying in the wind. To ensure the building wouldn’t blow over like a stack of cards, Le Messurier took advantage of the cutting-edge technology of the day, the tuned mass damper. 

A tuned mass damper is a 350-ton mass of concrete, metal, springs, and cables that helps balance out the vibrations and frequencies acting on a building. In this case, the tuned mass damper helped the Citigroup Center better withstand the wind. However, it’s not even the building’s vanguard technological support system that makes it memorable.  

The Problem

When construction was completed and the building opened in 1977, its eye-catching design made it famous with tourists, and its ingenious use of alternative systems made it a much-discussed topic in architectural circles. Sight-seers came to ogle its looks, and students and professionals studied its design and innovative technology. One such study turned out to be the Citigroup Center’s lifebuoy, pulling it from the brink of disaster. 

In 1978, Le Messurier received a call from an undergraduate student. According to this student, her calculations revealed a massive oversight. The support system could resist the force of winds blowing against one side of the building (perpendicular winds), but not a wind blowing against the building’s corners (quartering winds). This wind could cause the building to collapse. 

Le Messurier double-checked the calculations and found that, in installing the buildings supports, his firm had only accounted for the force of perpendicular winds and not quartering winds. His check revealed that quartering winds above 75 miles per hour could easily place too much stress on bolted joints. He also found that a storm strong enough to produce such strong winds could easily cause a power outage, rendering the tuned mass damper inoperable. Without that support and the safety net of technology, the building would be under further stress, and the joints and the chevron supports could fail making collapse imminent. New York encounters such conditions every sixteen to fifty-five years. To add one last dash of drama to the mix, hurricane season was right around the corner. 

Le Messurier and his firm and Citigroup executives scrambled to make a plan. First responders from the NYPD and thousands of Red Cross volunteers made a ten-block-wide evacuation plan and stayed on standby, workers worked for months to reinforce joints, and weather services kept a close watch on forecasts for any sign of windstorms. 

They managed to keep the situation hidden from the building’s occupants, neighbors, and even the media. They disclosed information only to those who ‘needed to know.’ To keep it a secret, the repairs could only start in the evening and workers would work straight through the night welding and affixing steel plates over the joints. 

They finally completed the repairs with minimal time to spare. But, as luck would have it, no hurricane made landfall in New York that year. Less than a year later, the church that had spurred the need for such a unique and, in the end, stressful design was demolished. To this day, the nine-story space under the Citigroup tower remains simply an architectural curiosity. 

The Aftermath

The story stayed hidden for over a decade. It wasn’t until 1995, when the journalist Joe Morgenstern overheard the story at a party, interviewed Le Messurier, and published his piece in The New Yorker, that the information went public. 

The story was so sensational that the BBC made a television special on the ordeal, spreading the story to wider audiences. It just so happened that a former undergrad student of architecture watched said special. 

Diane Hartley remembered her undergraduate thesis at Princeton, where she studied the groundbreaking Citigroup Center. She recalled the phone call she had made to Le Messurier’s firm. It wasn’t until watching the BBC special that she realized her actions had made any difference. She had assumed her call had only reached some junior intern and had not realized that her work had most likely saved Manhattan from a catastrophe. 

The story of the Citigroup Tower raised heated debates about a myriad of topics. Architects and engineers reviewed the practice and safety measures they used. The media scrutinized corporate tendencies to cut costs and save face in the subsequent scandal. Even policies of those in power to maintain secrecy to avoid panic instead of notifying the public of a threat were thrown into question. 

To this day, the Citigroup Center, now 601 Lexington after Citigroup left and sold the space to Boston Properties, still stands as an eye-catching addition to the Manhattan skyline. The only other renovations made to the building were done shortly after the 9/11 attacks. Blast resistant shielding and additional steel bracing were added as a safeguard against terrorist attacks. 

In the end, what made this building memorable is how it could have collapsed, and how the story of this near-disaster almost never fell on public ears. 

Sources

https://www.archdaily.com/564014/ad-classics-citigroup-center-hugh-stubbins-william-le-messurier

https://www.designingbuildings.co.uk/wiki/Citigroup_Center

http://www.slate.com/blogs/the_eye/2014/04/17/the_citicorp_tower_design_flaw_that_could_have_wiped_out_the_skyscraper.html

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