As a history buff, you can probably tell I greatly prefer old buildings to modern ones. But there’s something special about 601 Lexington Avenue, also known as the Citigroup Center. From a distance, the 59-story skyscraper’s diagonal slash of a rooftop (originally intended to support solar panels) provides a dash of drama to Midtown East. But the real story is on the ground, where the full weight of a skyscraper rests on four stilts, almost in defiance of gravity.
Today, 601 Lexington Avenue is solid as a rock, but in the 1970s, a stiff breeze could have destroyed the tower, and with it, half of midtown Manhattan.
The story of 601 Lexington starts with the church underneath. Since its founding in a feed and grocery loft in 1862, the congregation of St. Peter’s Lutheran Church had been hunting for a suitable place of permanent worship. In the early 1900s, Pastor Edward Friedrich Moldenke bought the block on Lexington and 54th and built a perfunctory parish church.
Yet by 1969, the neighborhood had changed from a far-off suburb to a bustling center of high finance. Banks were eyeing the aging church…and its valuable land holdings. None more than Citicorp (now Citigroup), desperate to expand their office space and make their mark on the Manhattan skyline. St. Peter’s agreed to sell their land on two conditions. One, Citicorp would build them a new place of worship. Two, the skyscraper’s support structure would not pass through the church.
Citicorp readily agreed to the first, hiring Emory Roth and Sons to build a sunken house of worship with a steep diagonal rooftop of Caledonia granite. The second was a harder nut to crack. Most skyscrapers distribute weight horizontally first, then down through steel beams on the outside of the building’s skeleton. But Citicorp couldn’t do that without either driving support beams through the church (not an option) or sacrificing valuable office space (also not an option.)
Lead architect Hugh Stubbins and engineer William LeMessurier pondered the conundrum for weeks. One night, LeMessurier sat in a Greek restaurant in Cambridge doodling on the back of a napkin when inspiration struck. What if, instead of placing the building’s strongest vertical supports at the corners, he placed them in the center? He sketched out a crude draft of a skyscraper placed on stilts – the embryo of 601 Lexington’s final design.
The result was the $175-million, 59-story Citicorp Center. Rather than distribute weight out, LeMessurier and Stubbins used a chevron pattern that drove pressure inward to four cement columns and a central shaft. To control the swaying of the building in high winds, LeMessurier installed what’s called a tuned mass damper, a 400-ton block of concrete gliding on a thin sheen of oil, held in place by computer-controlled pistons. If, say, a strong wind blew towards the west, the pistons would push the damper east and mitigate the effects. (You can learn more than you ever thought you wanted to know about tuned mass dampers here.)
Critics hailed Citicorp Center as a masterpiece of engineering and space management…
…until 1979, when LeMessurier got a call from a Princeton engineering student named Diane Hartley. After discussing the new skyscraper with her professor, she called to notify him of a fatal design flaw in Citicorp. “Listen, I want you to tell your teacher that he doesn’t know what the hell he’s talking about,” LeMessurier said, “because he doesn’t know the problem that had to be solved.” The crotchety engineer promptly hung up.
Then he went back to the plans.
The typical skyscraper is highly susceptible to perpendicular winds, or winds that hit the flat sides of a building, and LeMessurier had designed Citicorp to withstand powerful perpendicular winds. Then there are quartering winds, winds that hit the building at the corners. It’s rarely an issue for normal skyscrapers, but Citigroup was no normal skyscraper. Only a system of thick welded joints would keep the building together. LeMessurier called Stubbins to confirm that the joints had been properly welded.
Stubbins had no idea what he was talking about.
In a cost-cutting effort, Citicorp had opted for cheaper but substantially weaker bolted joints. After crunching the numbers, LeMessurier and Stubbins discovered that, in optimal conditions, the bolted joints would feel a 40% increase in stress. In reality, the stress could reach up to 160% as a random pattern of winds could set the building quivering like a jello mold. The odds of catastrophe were even higher in the event of a blackout, where the out-of-control tuned mass damper would add to the stress of the building rather than alleviate it. Best case scenario, a storm strong enough to knock the building over came along an average of once every 50 years. With random winds and a power outage, that average changed to 10 years. And hurricane season was approaching.
The Citicorp team rushed into action. In the dead of night, construction workers welded 2-inch steel plates over the skyscraper’s 200 bolted joints, blocked by curtains during the day to prevent panic. LeMessurier named it Project SERENE, which, no joke, stands for “Special Engineering Review of Events Nobody Envisioned.” Their cover was almost blown in early September, as Hurricane Ella battered Cape Hatteras with winds strong enough to knock over Citicorp Tower like a house of cards.
Fortunately, Ella shifted course into the Atlantic and repairs were completed without incident. Today, the renamed Citigroup Tower stands straight and sturdy – the American Institute of Architects even uses it is in case studies of corporate responsibility. Visitors can rest assured that if Midtown East is crushed, it’s more likely to come from Godzilla or Mothra than 601 Lexington Avenue.