How an Engineering Student’s Question Prevented a NYC Skyscraper from Falling Down
This article is a compilation of various sources, including videos and a Wikipedia article about how a Princeton engineering student caught a mistake in the engineering of the Citigroup center that could have led to the skyscraper falling down in a strong wind.
Citigroup Center
Engineering crisis of 1978 (from Wikipedia)
Due to material changes during construction, the building as initially completed was structurally unsound. William LeMessurier‘s original design for the chevron load braces used welded joints. To save money, Bethlehem Steel changed the plans in 1974 to use bolted joints, which was accepted by LeMessurier’s office but not known to the engineer himself.[22] Furthermore, according to The New Yorker, LeMessurier originally only needed to calculate wind loads from perpendicular winds under the building code; in typical buildings, loads from quartering winds at the corners would be less.[22][159] In June 1978, after an inquiry from Princeton University engineering student Diane Hartley, LeMessurier recalculated the wind loads on the building with quartering winds.[159][f] He found that, for four of the eight tiers of chevrons, such winds would create a 40 percent increase in wind loads and a 160 percent increase in load at the bolted joints.[22]
Citicorp Center’s use of bolted joints and the increased loads from quartering winds would not have caused concern if these issues had been isolated from each other. However, the combination of the two findings prompted LeMessurier to run tests on the structural safety.[103] The original welded-joint design could withstand the load from straight-on and quartering winds, but a 75-mile-per-hour (121 km/h) hurricane force quartering wind would exceed the strength of the bolted-joint chevrons.[99] With the tuned mass damper active, LeMessurier estimated that a wind capable of toppling the building would occur on average once every 55 years.[162][161] If the tuned mass damper could not function due to a power outage, a wind strong enough to cause the building’s collapse would occur once every 16 years on average.[162] LeMessurier also discovered that his firm had used New York City’s truss safety factor of 1:1 instead of the column safety factor of 1:2.[99]
LeMessurier debated how to address the issue before ultimately contacting Stubbins’s lawyer. LeMessurier then contacted Citicorp’s lawyers, the latter of which hired Leslie E. Robertson as an expert adviser.[163] Citicorp accepted LeMessurier’s proposal to weld steel plates over the bolted joints, and Karl Koch Erecting was hired for the welding process.[104] Very few people were made aware of the issue, besides Citicorp leadership, mayor Ed Koch, acting buildings commissioner Irving E. Minkin, and the head of the welder’s union.[99][104] Starting in August 1978, construction crews installed the welded panels at night. Officials made no public mention of any possible structural issues, and the city’s three major newspapers had gone on strike.[88][104] The work continued despite the threat of Hurricane Ella several weeks after the repairs started.[99][164] Repairs were completed in October 1978, before the media resumed publishing. LeMessurier claimed a wind strong enough to topple the building would only occur once every 700 years.[88][165] Stubbins and LeMessurier covered all of the repair costs, which were estimated to be several million dollars.[165] Since no structural failure occurred, the work was only publicized in a lengthy article in The New Yorker in 1995.[88][160]
William LeMessurier-The Fifty-Nine-Story Crisis: A Lesson in Professional Behavior
Citicorp Center | NYC skyscraper saved by a student’s question
The Citicorp Center repair is a classic engineering case study of how mistakes must be avoided in engineering and construction of public works. A skyscraper in New York City needed a unique structural system. While reviewing the design a student (named Diane Hartly) asked a question that made the engineer realize that a mistake had been made. There is a daring race to make the repairs for the building collapses. The video gives the details and then discusses how the engineer handled the situation.
Princeton Engineering Student
Diane Hartley
Principal at Hartley LLC
Washington, District of Columbia, United States Contact info
Twitter: @DianeHarley