Brief Summary
This video explores the near-collapse of the Citicorp Center in 1978 due to a critical design flaw discovered by its structural engineer, Bill LeMessurier. The video details the architectural constraints that led to the building's unique "stilts" design, the innovative use of chevron bracing and a tuned mass damper (TMD), and the terrifying realization that a change in the bracing connections, combined with a miscalculation regarding quartering winds, could lead to catastrophic failure. It further recounts the covert operation to reinforce the building, the threat of Hurricane Ella, and the eventual public revelation of the crisis, highlighting LeMessurier's ethical decision to address the problem despite potential personal and professional repercussions.
- The Citicorp Center's design, with its stilts and chevron bracing, was a unique engineering solution to architectural constraints.
- A critical flaw in the building's design, exacerbated by a change in construction and a miscalculation of wind loads, threatened its collapse.
- The ethical response of the structural engineer, Bill LeMessurier, averted a potential disaster and has become a case study in engineering ethics.
Why is the citicorp building on stilts?
In the summer of 1978, the Citicorp Center, a cutting-edge skyscraper in Manhattan, faced a potentially catastrophic flaw. Structural engineer Bill LeMessurier discovered that winds of 110 kilometers per hour could cause the building to collapse, threatening thousands of lives. The building's unique design was influenced by the presence of St. Peter's Church, which occupied a corner of the intended building site. Citicorp agreed to rebuild the church and ensure it remained independent from the new tower, requiring two-thirds of the space above the church to remain open. Architect Hugh Stubbins and engineer Bill LeMessurier collaborated to design the tower around the church, leading to the innovative idea of constructing the skyscraper on stilts to maximize floor area. The stilts, positioned at the center of each face rather than the corners, would support half of the building's gravity load and withstand high winds, presenting a unique engineering challenge.
How wind load works
LeMessurier devised a system of diagonal braces, or chevrons, to transfer forces to the middle of each face and down to the stilts. By removing columns at the top and middle of each chevron, each tier acted as a separate unit connected only to the braces and the central core, forcing half of the gravity load through the chevrons to the midface columns every eight stories. When wind hits a normal building, the entire frame deforms. Diagonal bracing can carry the horizontal load, with the beams acting like springs that compress and push on the joints or stretch and pull inwards. In Citicorp Center, the chevron bracing system transfers the wind load to the section below, wrapping around the entire building. The shear in the building increases as you go down, with the total force at the 10th floor being much bigger than at the 60th floor.
Tuned Mass Dampers
The chevron bracing system saved money and weight, but it made the building more susceptible to swaying in the wind. To address this, LeMessurier implemented a tuned mass damper (TMD), a device commonly used in bridges, power lines, and ships, but never before in a building. A TMD is a system that transfers kinetic energy from a swaying building to a pendulum or mass, which then dissipates the energy through friction. The mass needs to be at least 1-5% of the building's weight and the frequency of the TMD is tuned by adjusting the length of the pendulum. LeMessurier used a 400-ton concrete block on springs as the TMD in Citicorp, installed on the top floor. The damper was expected to reduce the amplitude of swaying by roughly 50%, saving around $4 million by not needing an additional 2,800 tons of structural steel.
The Anonymous Student
Citicorp Center opened in 1977 and was initially praised as an architectural marvel. However, trouble arose in May 1978 when LeMessurier learned that the chevron braces were bolted instead of welded, a change made by the contractor to save money. While bolts aren't inherently worse than welds, the substitution raised concerns. A month later, LeMessurier received a phone call from a student who questioned the placement of the columns. The student's professor believed the columns should have been placed in the corners.
Quartering Winds
LeMessurier decided to double-check the building's response to wind hitting a corner, known as quartering winds. He split the wind into perpendicular components and computed the forces, discovering that the stresses in half of the diagonals vanished, while the other half doubled. This meant the forces were 40% higher than his original calculations for perpendicular wind loads. This increase wouldn't have mattered with fully welded connections, but the braces were bolted.
What were the odds of collapse?
LeMessurier requested the building diagrams and reviewed the connections, confirming that his firm had only considered straight-on wind, not diagonal wind. He found that the original calculations used just four bolts per joint, which was sufficient for perpendicular wind loads but insufficient for quartering winds. The firm had also used an incorrect factor of safety, further underestimating the number of bolts needed. LeMessurier checked his calculations with Alan Davenport at the Boundary Layer Wind Tunnel, who found that the stresses could increase up to 60% more than originally anticipated under dynamic conditions. The weakest joints were at the building's 30th floor, and if those failed, the entire building would fall. A storm strong enough to tear the building apart occurred every 67 years on average, but if the tuned mass damper was not working due to a power outage, even 110 kilometer per hour winds could collapse the building. The chance of such a storm happening in any given year was one in 16.
How was the citicorp building fixed?
LeMessurier informed architect Stubbins and Citicorp's chairman, Walter Wriston, about the issue. Emergency generators were acquired for the tuned mass damper, which became critical for the tower's stability. The confidential repair plan, initially called Project Pandora, was renamed Project Serene. Each night, welders reinforced the joints by welding steel plates onto them, working secretly to avoid causing panic. Citicorp worked with the Red Cross to develop a 10-block evacuation plan. Strain gauges were fitted on important structural members to monitor the skyscraper's movements from a command center.
Hurricane Ella
In late August, Hurricane Ella threatened New York City. By September 1st, Ella was heading towards New York with winds reaching 200 kilometers per hour. City officials prepared to start the evacuation. However, Ella veered off into the sea at the last minute and eventually hit Canada. The repairs were completed in October, and the building could then withstand a one in 1000 year storm.
TMDs Take Over The World
The secret was confined to a small inner circle for almost two decades. In 1995, "The New Yorker" revealed Project Serene to the public. LeMessurier was praised for owning up to his mistake and fixing the issue. New York updated the building code to require quartering wind calculations. Since the first damper in Citicorp, TMDs have spread across the globe, allowing architects to build taller and slimmer skyscrapers. Six of the 20 tallest buildings in the world include tuned mass dampers, which are especially critical in typhoon or earthquake-prone regions.
Conspiracies and Cover Ups
The identity of the student who initially contacted LeMessurier remains a subject of debate. Diane Hartley, a student at Princeton, studied the Citicorp Tower for her senior thesis and noticed the lack of quartering wind calculations. Lee DeCarolis later came forward, claiming to be the student who spoke with LeMessurier. LeMessurier passed away in 2007 before he could confirm the student's identity. LeMessurier Associates refused to comment on the matter, and the building's current owner, Boston Properties, also declined to provide information. A 2021 study from the National Institute of Standards and Technology concluded that perpendicular winds were more demanding for a building like Citicorp, but their analysis didn't include any internal structure specific to Citicorp. Despite the controversy, LeMessurier's actions are still regarded as upstanding in the engineering field, and the Citicorp case is taught as a case of good engineering ethics.
