Remarkable Engineering - Near Disaster - Citicorp Tower, NYC / by Bob Ybarra

 http://www.601lexington.com

http://www.601lexington.com

Location: 153 East 53rd StreetManhattan,  10022, United States

Construction started: 1974

Completed & Opened : 1977

Cost: $195 million (USD)
(in adjusted inflation $758,904,448)

Height: Architectura l915 ft (279 m)  Floor count 59

ArchitectHugh Stubbins / KlingStubbinsEmery Roth & Sons

Structural engineer: Le Messurier Consultants, James Ruderman

Ever see a building and ask, "How did they do that?"  The Citicorp Building in NYC, now called the Citigroup Center, is one of those.  Most recognized by its 45 degree top that stands out over the NYC skyline its base is what prompts the question.

As an architectural work the building has little to say but as an engineering work it has lots to say.  The structural engineer was the foremost William LeMessurier.  Due to a requirement by the sellers of the property, St. Peter's Lutheran Church, a new church building was required to be built along side the new tower.  This resulted in a design where the typical corner columns would instead be centered with the sides of the building with chevron bracing (diagonal bracing) to the corners.  

chevron bracing CitiCorp.jpg

But then the intrigue of 1979 has added to the engineering story.  After the the completion of the building a Princeton engineering student, named Diane Hartley, chose the CitiCorp Building to write her thesis on.  In her research she contacted the office of William LeMessurier and spoke to an associate engineer.  With her evaluation of the materials she was given she asked about the building being evaluated for quarterly winds.  These are winds that travel diagonally against the building pushing against two faces at the same time.  She never learned more about her inquiry or what resulted because of it until she watched the documentary linked below and a meet up with her old professor.

The issue was that the connections between diagonal members and the corner members of the building were changed from welding to bolts while under construction.  This was suggested by the contractor to save money and recalculated by Mr. LeMessurier's office.  However, they did fail to consider the quarterly wind reactions. The inquiry by Diane Hartley triggered a recheck of the calculations and discovery of a potentially catostrophic failure of the bolted connections in a 70 mph plus wind.  In NYC this had a 1 in 16 chance of happing and storm season was approaching. The response was an immediate action to weld those connections at night when the offices were closed.  The added intrigue was that the tenants were not told of the potential danger nor the character of the repairs.  Meanwhile the city put in place an emergency evacuation plan with the RedCross in case there was an imminent threat of 70 mph winds. Fortunately, it never was needed.  

As the story was later told by Mr. LeMessurier, he was called by a young man that was an engineering student who revealed to him the error of the calculations. It wasn't until Diana Hartley later spoke to her retired professor that she realized she was that student.  This makes the story all the more remarkable and maybe even a miracle. So today the CitiCorp Building remains standing with its neighboring buildings.