Veritasium: The Citicorp Center in New York faced a structural crisis in 1978 due to a design flaw that could have led to its collapse during high winds.
Veritasium - The Most Dangerous Building in Manhattan
In 1978, structural engineer Bill LeMessurier discovered a critical flaw in the Citicorp Center's design, which could cause the building to collapse under certain wind conditions. The building's unique design, with stilts placed at the center of each face rather than at the corners, required innovative engineering solutions, including chevron bracing and a tuned mass damper (TMD) to manage wind and gravity loads. However, a cost-saving decision to use bolts instead of welds for the chevron braces compromised the building's integrity under quartering winds, which were not initially considered in the design calculations. LeMessurier faced a moral dilemma but chose to act, initiating secret repairs to reinforce the structure. The repairs were completed without public panic, and the building's safety was ensured. The incident led to changes in building codes and highlighted the importance of engineering ethics, as LeMessurier was later praised for his transparency and corrective actions.
Key Points:
- The Citicorp Center had a design flaw that made it vulnerable to collapse in high winds due to bolted rather than welded chevron braces.
- Bill LeMessurier discovered the flaw and initiated secret repairs to reinforce the building's structure, avoiding public panic.
- The building's unique design required innovative solutions like chevron bracing and a tuned mass damper to manage loads.
- The incident led to changes in building codes, requiring consideration of quartering winds in structural calculations.
- LeMessurier's actions are now a case study in engineering ethics, demonstrating the importance of addressing and correcting mistakes.
Details:
1. 💡 The Discovery of a Fatal Flaw
- Citicorp Center, a skyscraper in Manhattan, had a fatal structural flaw identified by engineer Bill LeMessurier in 1978.
- The building could collapse under winds as low as 110 kilometers per hour, posing a risk to over 200,000 people in the vicinity.
- Citicorp Center had a 100% probability of total collapse by the end of the century without intervention.
- The unique design constraints were due to the need to incorporate Saint Peter's Church, leading to the use of stilts and chevron braces.
- The stilts were placed at the center of each face of the building, not at the corners, creating an engineering challenge.
- LeMessurier's design involved six layers of diagonal braces, transferring forces to the center of each face and down to the stilts.
- The design was unique and driven by the building's specific conditions, with gravity loads transferred through chevrons to midface columns and stilts.
2. 🏗️ Engineering Innovations: Skyscraper on Stilts
- Diagonal bracing is crucial for reducing deformation in skyscraper frames. It allows beams and columns to carry horizontal loads more effectively due to their strength in compression or tension, rather than bending.
- Chevron bracing transfers wind loads down the building, preventing rotational forces. This design enables skyscrapers to withstand significant wind forces by distributing the load effectively across the structure.
- In skyscrapers, the force on lower floors is greater because they bear the accumulated load of all floors above. This requires careful engineering to ensure stability at lower levels.
- LeMessurier's design for the Citicorp building used chevrons that were almost 40 meters long, which had to be welded on-site due to transportation constraints, showcasing innovative construction techniques.
- The chevron bracing system resulted in a lighter building at 22 pounds per square foot, a significant reduction compared to typical New York skyscrapers, but it introduced sway under wind conditions.
- To address sway, traditional methods would involve adding more structural steel, but LeMessurier proposed an elegant alternative solution.
3. 🎯 The Role of Tuned Mass Dampers in Stability
- Tuned Mass Dampers (TMDs) have been traditionally used in bridges, power lines, and ships, but their application in buildings was pioneered by LeMessurier.
- In the Citicorp building, a TMD consisting of a concrete mass weighing 400 tons was installed on the top floor to reduce sway.
- The TMD works by transferring kinetic energy from the building to a pendulum, which oscillates out of phase with the building, thereby dissipating energy and reducing sway.
- For effectiveness, the TMD's mass should be 1-5% of the building's weight, and the system must be tuned to the building's frequency.
- LeMessurier's TMD reduced the building's sway amplitude by approximately 50%, saving $4 million by eliminating the need for 2,800 additional tons of structural steel.
- Different types of TMDs include pendulums, liquid columns, and masses on springs; Citicorp used a large mass on springs.
- The Citicorp Center was the 11th tallest building upon its opening in 1977 and was praised as an architectural feat.
4. ⚠️ Unveiling the Oversight and Calculations
- The contractor's cost-saving measure of using bolts instead of welds for the chevron braces saved $250,000 but compromised structural integrity.
- LeMessurier identified that wind hitting the building's corners increased stress by 40% compared to the originally considered perpendicular wind loads.
- Original calculations failed to account for quartering winds, leading to an underestimation of bolts required from 4 to 14 per joint.
- Dynamic analysis revealed stress during building movement could increase by 60%, exceeding initial expectations.
- The weakest joints were identified on the 30th floor, posing a significant collapse risk if they failed.
- Historical weather data indicated a storm capable of causing collapse occurs every 67 years, equating to a 1 in 16 chance annually without intervention.
- LeMessurier faced a significant moral dilemma, balancing professional risk and public safety, ultimately prioritizing the latter.
5. 🚨 Project Serene: Emergency Measures and the Media
5.1. Initial Response and Project Naming
5.2. Repair Strategy and Evacuation Plan
5.3. Monitoring Efforts
5.4. Communication Strategy
5.5. Media Management and Press Blackout
6. 🌪️ Hurricane Ella's Close Call and the Race Against Time
- Hurricane Ella began in the Caribbean, sparking significant concern due to its high intensity.
- The Citicorp building was undergoing crucial repairs, only halfway completed, and was designed to withstand a one in 200-year storm, but concerns remained about its resilience to the unknown intensity of Hurricane Ella.
- LeMessurier, the structural engineer, was cautious and closely monitored the situation despite the building's robust design.
- As Hurricane Ella approached New York, it brought winds of 200 kilometers per hour, prompting city officials to prepare for the possible evacuation of a 10-block radius around the Citicorp building.
- For 24 hours, Hurricane Ella stalled near North Carolina, causing significant anxiety and uncertainty.
- Ultimately, Hurricane Ella veered into the ocean, intensifying to 225 kilometers per hour and striking Canada, leaving the Citicorp building and New York City unscathed.
7. 🔍 The Secret Revealed: Engineering Ethics in Action
- The Citicorp building was retrofitted to withstand a one in 1000 storm, with repairs costing between $4 and $5 million.
- Original building design approved by Citicorp would have cost $5 to $6 million more.
- The secret repair was initially confined to a small group and only publicized in 1995 by "The New Yorker."
- LeMessurier was commended for addressing his engineering mistake promptly.
- The exposure of the issue led to changes in New York's building codes, including the need for quartering wind calculations.
- Tuned Mass Dampers (TMDs), first used in the Citicorp building, have become standard in skyscrapers worldwide.
- TMDs are crucial for tall buildings, especially in typhoon or earthquake-prone regions, with six of the world's 20 tallest buildings using them.
- Taipei 101 features a 660-ton pendulum TMD to stabilize it against winds up to 200 km/h.
8. 🎓 The Mystery Student and Lasting Legacy
- In 1978, Diane Hartley's senior thesis at Princeton identified potential structural issues with the Citicorp Tower, specifically regarding wind load calculations.
- Despite initial dismissal, her thesis contributed to raising awareness about the building's vulnerabilities, which were later confirmed in media reports.
- In 2003, Hartley was identified as the likely student who raised these concerns, though she never directly contacted the engineer, LeMessurier.
- In 2011, Lee DeCarolis claimed responsibility for alerting officials, aligning with LeMessurier's account, but the student's identity remains disputed.
- LeMessurier's passing in 2007 left the mystery unresolved, but the case became a cornerstone for teaching engineering ethics.
- A 2021 study verified wind pressure concerns but did not address internal structural details of Citicorp.
- The building, now 601 Lexington, was sold in 2001, with repairs remaining undisclosed in official documents, highlighting ongoing transparency issues in engineering.
