The Most Dangerous Building in Manhattan

Key Concepts:

• Citicorp Center structural flaw: Vulnerability to collapse under 110 km/h winds due to bolted chevron braces and underestimated quartering wind loads.
• Chevron bracing: Diagonal braces on the building's faces designed to transfer wind and gravity loads to the stilts.
• Stilts: Columns at the center of each face of the building, instead of the corners, supporting the structure.
• Tuned Mass Damper (TMD): A large mass on the top floor designed to counteract swaying caused by wind.
• Quartering winds: Winds hitting the building at a 45-degree angle, creating higher stress on the structure than perpendicular winds.
• Project Serene: The secret repair operation to reinforce the bolted connections with welded steel plates.
• Factor of safety: An engineering design principle where the maximum stress a material can withstand is significantly higher than the expected stress.

1. The Discovery of the Flaw

• In 1978, Bill LeMessurier, the structural engineer for Citicorp Center, discovered a critical flaw less than a year after its opening.
• The building was vulnerable to collapse under winds of just 110 kilometers per hour, a risk exacerbated by the approaching hurricane season.
• LeMessurier realized that the bolted connections of the chevron braces were insufficient to withstand quartering winds, a load case not adequately considered in the original design.
• "Here I am, the only man in the world who knew this. This thing is in real trouble." - Bill LeMessurier

2. The Design and its Innovations

• Citicorp sought to build its headquarters on a site occupied by St. Peter's Church, leading to a unique design where the tower was built on stilts to accommodate the church.
• Architect Hugh Stubbins and engineer Bill LeMessurier collaborated to create a design where the building was supported by stilts at the center of each face, rather than at the corners.
• LeMessurier's innovative chevron bracing system was designed to transfer both gravity and wind loads to these stilts.
• The chevron bracing system allowed for a lighter structure (22 pounds a square foot), but also made the building more prone to swaying.
• To counteract the sway, LeMessurier implemented a tuned mass damper (TMD), a 400-ton concrete block on the top floor, which reduced the amplitude of swaying by roughly 50%.

3. The Bolted Connection Substitution

• The original design specified welded connections for the chevron braces, but the contractor substituted bolts to save $250,000.
• LeMessurier's firm initially approved the substitution, believing the gravity load would keep the braces in compression.
• However, the bolted connections were not strong enough to withstand the tension forces induced by quartering winds, especially when combined with an inadequate factor of safety.

4. The Quartering Wind Calculation Error

• A phone call from a student prompted LeMessurier to re-evaluate the building's response to quartering winds.
• He discovered that the forces on the braces were 40% higher under quartering winds than under perpendicular winds.
• Further analysis, including wind tunnel testing by Alan Davenport, revealed that dynamic wind conditions could increase stresses by up to 60% more than originally anticipated.
• The combination of higher stresses and insufficient bolt capacity created a significant risk of structural failure.

5. The Risk Assessment and Decision

• LeMessurier calculated that a storm strong enough to topple the building occurred on average every 67 years if the TMD was working.
• However, if a power outage disabled the TMD, winds of just 110 kilometers per hour could cause collapse.
• The chance of such a storm occurring in any given year was 1 in 16.
• Faced with the potential for catastrophic loss of life, LeMessurier decided to inform Citicorp and initiate repairs, despite the risk of professional ruin.

6. Project Serene: The Repair Operation

• LeMessurier, Stubbins, and Citicorp Chairman Walter Wriston developed a confidential repair plan called Project Serene.
• Welders worked nightly to reinforce the bolted connections with steel plates, prioritizing the weakest joints on the 30th floor.
• Emergency generators were acquired to ensure the TMD remained operational.
• Citicorp collaborated with the Red Cross to create a 10-block evacuation plan.
• Strain gauges were installed to monitor the building's structural integrity.

7. The Hurricane Threat and Resolution

• Hurricane Ella threatened New York City in late August, forcing officials to prepare for a potential evacuation.
• The repairs were only halfway complete when Ella approached.
• Fortunately, Ella veered off into the sea at the last minute, sparing Citicorp and New York City.
• The repairs were completed in October, six weeks after LeMessurier informed Citicorp, at a cost of $4 to $5 million.

8. The Aftermath and Legacy

• The New Yorker published an article in 1995, revealing the crisis to the public.
• LeMessurier was praised for his ethical conduct and his decisive action to correct the flaw.
• New York City updated its building code to require quartering wind calculations.
• Tuned mass dampers became widely adopted in skyscraper design worldwide.
• The identity of the student who alerted LeMessurier to the issue remains a subject of debate, with Diane Hartley and Lee DeCarolis both being potential candidates.
• A 2021 study from the National Institute of Standards and Technology (NIST) examined quartering winds but did not include internal structure specific to Citicorp.
• The Citicorp case is now taught in engineering ethics courses as an example of responsible engineering practice.
• "If you see something that is a social risk, good heavens, this thing would kill thousands, you must do something, you must do something." - Bill LeMessurier

9. Tuned Mass Damper (TMD) Explained

• A TMD is a device mounted in structures to reduce the amplitude of mechanical vibrations.
• It consists of a mass attached to a structure with springs and dampers.
• The frequency of the TMD is tuned to be close to the resonant frequency of the structure.
• When the structure vibrates, the TMD oscillates out of phase, dissipating energy and reducing the overall vibration.
• The mass of the TMD needs to be at least 1-5% of the building's weight to be effective.

10. Conclusion

The Citicorp Center story is a compelling case study in engineering ethics, risk management, and the importance of thorough analysis. Bill LeMessurier's discovery of the structural flaw, his decision to act despite the personal and professional risks, and the successful repair operation highlight the critical role of engineers in ensuring public safety. The incident led to significant changes in building codes and design practices, and the widespread adoption of tuned mass dampers demonstrates the lasting impact of this near-disaster.