The Skyscraper That Almost Toppled: A Story of Engineering and Near Disaster

In the heart of New York City, the Citicorp Center stands as a testament to modern architecture. But behind its sleek facade lies a gripping tale of a design flaw, a determined student, and a secret operation to prevent a catastrophe.

The Discovery

In 1978, Diane Hartley, an architecture student, was deeply immersed in her undergraduate thesis, which focused on the Citicorp Center. After weeks of meticulous examination of the building's plans, she uncovered a potentially fatal flaw that could have led to the collapse of the 59-story skyscraper into one of New York City's most populated areas.

A Unique Design

Completed just two years prior, the Citicorp Center was celebrated as one of the world's tallest buildings. Its sloped roof made it a unique addition to the city's skyline. However, its most distinctive feature was at its base. Due to the presence of St. Peter's Lutheran Church on the construction site, the skyscraper was erected on columns, similar to stilts.

The Challenge of Stilts

While using stilts on a building's corners was not unprecedented, the church's location at the corner of the block necessitated placing these stilts at the center of each side. This unconventional design raised concerns among some of the building's stakeholders.

Engineering Precautions

William LeMessurier, the chief structural engineer, implemented several precautions to ensure the building's stability:

V-Shaped Chevrons: The exterior consisted of v-shaped chevrons, creating a robust exoskeleton to support the skyscraper.
Lighter Structure: This external structure also reduced the building's weight, lessening the overall load.
Tuned Mass Damper: To counter the building's vulnerability to strong winds, a 400-ton counterweight, controlled by computerized sensors, was installed to counteract any swaying.

Calculations indicated that each side of the building could withstand powerful winds, and with all safety concerns addressed, the building opened in 1977.

The Overlooked Threat

However, Hartley's analysis revealed a critical oversight. While the building's faces could withstand powerful winds, the unique base design meant that winds blowing on the building's corners posed a greater threat. Traditional designs did not typically account for corner winds in safety calculations, and it appeared that this threat had been overlooked.

The Alarming Detail

When Hartley contacted LeMessurier's firm, she was initially assured that the building was strong enough to handle these winds. However, upon reexamining the plans, LeMessurier discovered a concerning detail: the exoskeleton's welded joints had been replaced with cheaper and weaker bolted joints without his knowledge.

The Perfect Storm

While this change alone was not enough to cause the tower to topple, it did leave the building vulnerable. If a storm were to knock out the building's power, the counterweight's sensors would deactivate, leaving the building susceptible to winds of just 70 mph (112 kilometers per hour). According to available weather data, a storm of this magnitude had a one-in-sixteen chance of hitting New York City each year.

A Secret Operation

LeMessurier kept Hartley's discovery confidential and worked with architects, Citicorp executives, and city officials to develop a secret plan. Construction crews began working overnight to reinforce the bolted joints without alerting the residents.

Race Against Time

This delicate operation commenced in mid-August 1978 and was only halfway completed when Hurricane Ella approached the city in September. City officials and Citicorp executives devised an emergency evacuation plan for a ten-block radius. Fortunately, the hurricane veered out to sea at the last minute.

Covert Construction

The secret evacuation plans were never implemented, and the reinforcements were completed a month later. The press, preoccupied with a newspaper strike, failed to notice the covert construction. The public remained unaware of how close they had come to disaster until 1995, when a New Yorker article revealed the story.

Acknowledgment

While the article, like LeMessurier, did not initially give Hartley the credit she deserved, she at least knew that her diligence had saved countless lives. The Citicorp Center stands tall today, thanks to a combination of innovative engineering, a student's sharp eye, and a secret operation to correct a potentially catastrophic flaw.