O Prédio Mais Perigoso de Manhattan

City Corp Center: A Structural Engineering Dilemma

The Discovery of a Fatal Flaw

• In the summer of 1978, Bill Myier, the structural engineer for City Corp Center, discovered a critical flaw that could lead to its collapse under winds as low as 110 km/h.

• The potential disaster posed a significant risk to over 200,000 people living and working in Manhattan, especially with hurricane season approaching.

The Ethical Dilemma

• Myier faced a moral choice: remain silent about the defect or attempt to fix it at the risk of professional ruin and public panic.

• Henry Van Dijke was brought in for further investigation into how such a dangerous oversight occurred during construction.

Construction Challenges

• City Corp aimed to build their headquarters on a block that included St. Peter's Church, which refused to relocate without specific conditions being met.

• Agreements were made to replace the old church with a new structure while ensuring it remained distinct from the tower above.

Innovative Architectural Solutions

• Architect Hugo Stubbins and engineer Bill Lee Messurier were tasked with designing both the tower and church while adhering to strict spatial requirements.

• To maximize floor area, they proposed cutting corners off the tower; this led to an unconventional design where support columns would not be placed at corners but rather at midpoints.

Engineering Breakthrough: Chevron Supports

• Messurier conceptualized using diagonal supports (chevrons), which would transfer gravitational forces downwards effectively through each face of the building.

• This innovative approach allowed for load distribution without relying solely on traditional column placements.

Addressing Wind Forces

• Lemesurier focused on wind resistance after confirming that chevrons could handle gravitational loads. He explored ways to strengthen joints against lateral forces caused by wind.

• By adding diagonal reinforcements, he enhanced structural integrity against horizontal pressures exerted by strong winds.

Complex Load Management

• Each chevron worked collectively to manage wind loads throughout different levels of the building, ensuring stability despite unconventional column placements.

Understanding the Structural Design of Citycorp Center

Shear Forces and Structural Challenges

• The concept of shear forces in buildings is introduced, highlighting that these forces increase as one descends the structure. For instance, the force on the 10th floor is less than that on the 60th floor, but it must support all weight above it.

• Lemesurier's design incorporated massive chevron supports, which were too large to transport whole through Manhattan; thus, they were sent in pieces for assembly on-site.

• While chevrons effectively addressed wind and gravitational load challenges, they resulted in a lighter building (22 pounds per square foot), leading to oscillation issues during high winds.

Innovative Solutions to Oscillation

• To mitigate discomfort from oscillation without adding excessive structural steel, Lemesurier proposed using a Tuned Mass Damper (TMD), a technique previously utilized in bridges and ships but novel for buildings.

• A demonstration at Stark Laboratories illustrates how a TMD works: as the building sways, energy is transferred to a pendulum that moves out of phase with the building's motion, dissipating energy through friction.

Engineering Adjustments for Effectiveness

• The effectiveness of a TMD requires careful tuning; its mass should be 1-5% of the building's weight. Engineers adjust frequency by changing pendulum length and damping by tightening or loosening screws.

• Various types of TMD exist; Lemesurier opted for a concrete mass weighing 400 tons installed at Citycorp’s top floor. This mass helps reduce sway by moving opposite to the building’s motion.

Cost Efficiency and Structural Integrity

• The TMD was expected to cut sway amplitude by about 50%, saving approximately $4 million by eliminating the need for an additional 2800 tons of structural steel.

• With chevron supports directing forces efficiently and TMD reducing sway, Lemesurier felt confident about Citycorp Center's structural integrity upon its completion in 1977.

Unforeseen Issues Post-Inauguration

• Initially celebrated as an architectural marvel, problems arose within a year when concerns about construction methods surfaced regarding bolted versus welded connections used in chevron supports.

• An architect raised questions about whether bolted connections could withstand necessary loads compared to welded ones. Despite initial confidence in calculations supporting this method, doubts began to emerge regarding their adequacy under stress conditions.

Academic Scrutiny and Design Validity

• A student inquiry revealed further skepticism about design choices made during construction—specifically regarding column placements—which prompted Lemesurier to reassess his team's decisions amidst growing scrutiny from academia.

Analysis of Wind Load on the Back Bay Hilton

Understanding Structural Calculations

• Bill Lemesurier reassures a student about the placement of structural pillars while considering wind loads from all directions, emphasizing the complexity of analyzing forces acting on a triangular building design.

• During a critical phone call, Lemesurier identifies that the worst-case scenario for wind load is not diagonal but rather direct winds impacting the building's face.

• He further explores how wind hitting corners (diagonal winds) affects structural integrity by breaking down wind forces into perpendicular components.

Discovering Critical Insights

• Lemesurier observes that tensions in half of the diagonals diminish while they double in others, leading to an increase in calculated wind forces by 40% compared to perpendicular loads.

• The original design did not account for this increase due to changes from welded connections to bolted ones, raising concerns about whether sufficient bolts were used based on these new calculations.

Analyzing Structural Integrity

• Concern arises as Lemesurier realizes his team likely only considered direct winds in their calculations and did not factor in diagonal winds when determining bolt requirements.

• He examines diagrams and acknowledges that despite higher wind speeds at tower tops, shear forces increase lower down, complicating load distribution across supports.

Reassessing Bolt Requirements

• At mid-tower supports, perpendicular wind loads reach 454 tons; however, gravitational loads also contribute significantly to stress levels on these supports.

• Upon recalculating with diagonal winds included, it becomes evident that some supports require more than four bolts—potentially up to ten—to handle increased tension effectively.

Confirming Calculations and Risks

• A miscalculation regarding support classification leads to underestimating necessary safety factors for bolt counts; initial assumptions proved inadequate under dynamic conditions.

• After further testing at a boundary layer wind tunnel reveals even greater stress increases (up to 60%), Lemesurier faces alarming implications for structural safety during high-wind events.

Evaluating Storm Risk Factors

• Historical weather data indicates severe storms capable of damaging the structure occur approximately every 67 years; however, power outages could exacerbate risks during such events.

The Crisis at Citycorp: A Structural Dilemma

The Unexpected Storm and Decision-Making

• Lemesurier faced a critical moment when he realized the potential danger posed by structural issues in the Citycorp building, which could lead to catastrophic consequences.

• He contemplated drastic measures, including self-harm, due to the overwhelming pressure of the situation but ultimately recognized that action was necessary to protect lives.

• After consulting with legal and engineering experts, Lemesurier informed Citycorp's president about the urgent need for repairs to ensure safety.

Implementation of Repairs

• The repair plan was initially dubbed "Project Pandora," but it was later renamed "Serene Project" to avoid alarming stakeholders.

• Workers conducted covert repairs at night, reinforcing over 200 joints in the building while ensuring minimal disruption during business hours.

• The evacuation plan involved collaboration with local authorities and aimed to mitigate risks for thousands of people potentially affected by a structural failure.

Risk Management Strategies

• Despite recognizing the gravity of the situation, Citycorp opted not to inform employees or the public to prevent mass panic.

• Tension meters were installed on key structural components to monitor movements remotely; however, installation delays posed challenges.

• Urgent communication with AT&T led to expedited installation of necessary phone lines for monitoring purposes.

Media Relations and Public Perception

• As media inquiries increased regarding ongoing repairs, Lemesurier felt compelled to provide vague explanations without revealing full details about potential dangers.

• A strike by New York newspapers inadvertently shielded Citycorp from media scrutiny during a critical period of repair work.

Facing Hurricane Threat

• As hurricane season approached, concerns heightened about whether ongoing repairs would withstand severe weather conditions; preparations were made for possible evacuations.

• On September 1st, an intense hurricane threatened New York but ultimately changed course at the last minute, sparing the city from disaster.

Successful Resolution and Legacy

• By October, repairs were completed successfully; Lemesurier claimed that the building could now withstand storms with a probability of one in a thousand years.

The Legacy of the Citycorp Building

The Role of Mass Dampers in Skyscrapers

• The Citycorp building is recognized as the first skyscraper built with mechanical assistance, allowing for taller and thinner structures. This innovation has been widely adopted in Japan.

• Among the world's 20 tallest buildings, six utilize tuned mass dampers, which are crucial in areas prone to typhoons or earthquakes. Taipei 101 features a massive 660-ton pendulum that stabilizes it against winds up to 200 km/h and earthquakes of magnitude 6.8.

Controversy Surrounding Engineering Contributions

• The legacy of the Citycorp building is marred by controversy regarding its engineering origins, particularly concerning a student named Diane Hartley from Princeton who conducted significant research on it.

• Hartley's thesis focused on the new City Corp tower and included original engineering calculations. She questioned whether diagonal winds were considered in the design but was reassured by engineers that they were not a factor.

Miscommunication and Recognition Issues

• After her graduation, Hartley learned about a documentary referencing her work but assumed she had no direct involvement with Bill Lemesurier, an engineer associated with the project.

• In 2003, it was revealed that Hartley was indeed the only researcher studying Citycorp at that time; however, there was confusion over who made critical connections regarding wind considerations.

New Insights into Historical Claims

• A man named L Carolis later claimed he made contact with Lemesurier about these concerns but did not seek fame or recognition; he simply wanted to clarify his role.

• Unfortunately, Lemesurier passed away before confirming any details about this communication, leaving some questions unanswered.

Ethical Considerations in Engineering

• The ongoing sensitivity surrounding this topic indicates unresolved feelings among those involved; many prefer not to discuss their roles openly.

• The case of Citycorp serves as an ethical lesson globally within engineering education, emphasizing responsibility and integrity when asserting structural stability claims.

Emotional Weight on Engineers