The Value of a Second Opinion

This 59-story Citicorp (n/k/a Citigroup) Tower, completed in 1977 for $175 million, was designed by LeMessurier Consultants (called LeMessurier Associates at the time). It was the 7th tallest building in the world at the time of its opening. It is known for its sloped roof and for sitting on four 114-ft-tall columns positioned at the center of each side of the building rather than at the corners.

The genesis of this unusual design was the savvy negotiation of the leadership of St. Peter’s Lutheran Church, which owned title to the northwest-corner lot at Lexington Avenue’s east side between 53^rd and 54^th Streets. They had been congregating there in a decaying, 1905 Gothic structure, and were not interested in selling the lot. The parties agreed that Citicorp would demolish the old church and build a new one as a free-standing part of Citicorp Center. To effectuate this, architect Hugh Stubbins and engineer Bill LeMessurier set the tower on the four massive, 9-story-high stilts, and cantilevered the tower 72 feet out over the newly-constructed church. This design would also cantilever over a plaza on the southwest with columns projecting the impression of a weightless-seeming monolith hovering above the street.

To help stabilize the building, the design team installed at the top of the building’s mechanical space a 400-ton large-scale, tuned mass damper, which is generally a set of concrete or steel weights that move on pendulums or fluids. The damper served to counteract swaying motions due to the effect of wind on the building and reduce by as much as 50% the building’s movement due to wind. Citigroup Center was the first skyscraper in the United States to feature a tuned mass damper.

The building was supported by 48 braces, in six tiers of eight, arrayed like giant chevrons within the building's aluminum and glass envelope. These chevrons served to pull weight from the corners of the building and redistribute it towards the stilts centered in the middle of the façade. LeMessurier had long since established the strength of those braces in perpendicular winds--the only calculation required by New York City’s then Building Code.

Within a year of the building’s opening, at least one university student (in some accounts it was two separate students) reviewed the building’s design drawings and manually performed independent calculations, comparing them to the actual, structural design’s calculations. From this review revealed a discrepancy: the building’s design was particularly vulnerable to quartering winds (which were relatively unknown at the time of the building’s design in 1971). Quartering winds hit a building from a diagonal of 45 degrees and, by flowing across two sides of a building at once, increase the forces on both. In contrast, perpendicular winds strike one flat face of a building. And usually, a building’s corners are one of the strongest parts of the building – but that was not the case here.

Initially, LeMessurier was not concerned, as he thought that with the several structural safeguards in place, the building remained safe. Upon further review, however, he discovered that the firm’s steel construction team had authorized a late design change, from welded to bolted joints. Welded joints were labor-intensive and therefore expensive, but, in the project’s steel contractor’s opinion, can be needlessly strong, such that in most cases, bolted joints are more practical and equally safe. LeMessurier also learned that the engineering team performed their wind analysis by defining the diagonal wind braces not as columns but as trusses.

LeMessurier’s new calculations found that, with welded joints, in four of the eight chevrons in each tier, a quartering wind would increased the strain by 40%. With the substitution of bolted joints, however, the 40% increase in tension produced by a quartering wind became a 160% increase. Further, in the event of a power failure – as periodically occurs during a hurricane -- the mass damper would be disabled, rendering the building vulnerable to toppling over, potentially set off a domino-like chain reaction in a city full of skyscrapers.

After contacting his lawyers, his firm’s E&O policy’s carrier and the Citigroup Bank,

LeMessurier spent the next several months working with the local authorities in their plan to secretly, during non-work hours, open up drywall and weld two-in.-thick steel plates over the more than two-hundred bolted joints throughout the building. Crews installed emergency generators for the mass damper and placed strain gauges on critical beams. Three different weather services were employed 24-7 to keep an eye on potential windstorms heading towards the city. Meanwhile, the New York Police Department surreptitiously surveyed the area within a 10-block radius for residents including those with disabilities, and prepared an evacuation plan. The Red Cross put 2,500 volunteers on standby – although they were not advised why. And all had the fortune of an unusually-minimal amount of news coverage, as the city's three major newspapers had gone on strike.

Since the episode became public in 1995, it has been the subject of many ethics’ discussions in engineering classes. A legal examination, however, would likely have a different focus. Were Citicorp to have become combative versus collaborative, its construction lawyers may have questioned the process by which the engineering firm’s steel construction team authorized the substitution of bolted joints from welded joints, including to what extent they re-ran the structural calculations to accommodate such a change, and whether it was within the applicable standard of care that the team did not inform the lead structural designer of this change. In the end, it is a blessing that a student (or two students) happened to independently perform the wind calculations – and spoke up – about what can be bluntly defined as a serious design error, preventing a horrific disaster.

CREDITS

The World's Best Construction Podcast: “The Secret Race to Save a New York Skyscraper From Collapse”, Episode #143, May 29, 2025

Richard Korman, “Author Details Citicorp Tower Design Error and the Race to Fix It”, Book

Review of The Great Miscalculation: The Race to Save New York City's Citicorp Tower (NYU Press: $27.95), ENR, April 25, 2025

99 Percent Invisible Podcast: “Structural Integrity”, Episode #110, April 15, 2014

“People to Know: Delayed Reaction”, MODERN STEEL CONSTRUCTION, October 2012

Joe Morgenstern, “The Fifty-Nine-Story Crisis”, NEW YORKER, May 29, 1995