Structural Integrity

This is 99% Invisible. I’m Roman Mars.

William LeMessurier: I’m going to tell you a story first about how a building got designed, a very unusual building in New York, the Citicorp Tower.

Roman: When it was built in 1977, Citicorp Center was, at 59 stories, the 7th tallest building in the world.

Joel Werner: You know, I can still remember the first time I saw the Citicorp Center. As you exit the local subway station at Lexington and 53rd in Midtown, Manhattan you’re confronted by a sheer 915-foot building face shooting up right from your feet.

Roman: That’s reporter Joel Werner.

Joel: With its gleaming facade of glass and steel and striking 45-degree angle crown, Citicorp Center is truly one of the most spectacular skyscrapers on a skyline known for its spectacular skyscrapers. But really it’s the base of the building that takes your breath away. This massive 59 story skyscraper levitates midair, hovering 115 feet above the street corner.

Roman: At first glance, the building seems to violate the basic laws of physics.

Joel: When you look up, you look directly at the underside of the building corner. It overhangs the sidewalk. I mean we’d expect the column on even the grounds. Instead, there’s nothing, just thin air.

Roman: Of course the building isn’t really levitating. It’s propped up on stilts which join the building at the midpoint of each side. It does not look sturdy. But it’s got to be sturdy. It’s got to be safe for they wouldn’t have built it this way, right?

Joel: Yes, well that’s the secret about this building. It was literally a secret for nearly 2 decades after it was built. The Citicorp Center could very well have blown over in the wind.

William: But I wonder now was when was this building going to fall down?

Roman: The Architect of the Citigroup Center was Hugh Stubbins but most of the credit for this building is given to the Chief Structural Engineer, William LeMessurier.

William: You’ll laugh at me but I did conceive it on a napkin at a Greek restaurant in Cambridge.

Joel: LeMessurier passed away in 2007. This is him giving a talk at MIT in 1995.

William: The whole problem was to build a building on a site that has a church at one corner.

Joel: Saint Peter’s Lutheran Church.

William: Honey-old building but this is the lowest point of Victorian architecture.

Roman: I don’t think it was that bad.

Joel: Saint Peter sat at the corner of the lot. When the church sold their land to Citicorp, it was on the condition that they build a new church in the exact same location. Beyond that, the company was free to build their skyscraper around the church and in the airspace above it. So a plan was hatched to float the building over the church. Hence the stilts and because the church was in the corner of the lot, LeMessurier put the stilts in kind of a weird place.

William: You couldn’t have columns in the corners where they ought to be on a conventional building so they were moved to the middle.

Joel: Placing the columns at the midpoint of each side rather in the corners made the building less stable. So for additional support, LeMessurier designed a chevron bracing structure, essentially rows and rows of V’s, each 8 stories high, pointing down the middle of each building face to where the supporting columns had been moved.

William: Actually the architect wanted to keep the building simple at its skin because there was so much drama going on at the bottom and at the top anyway. He said, “No, if we’re going to have any diagonals I don’t ever want to see them.”

Joel: So the chevron bracing structure was slipped under the skin of the building. It formed the Citicorp Center’s skeleton. Now, one of the amazing things about this chevron bracing structure is that it’s so lightweight that when the wind blows–

William: It’s more dynamically excitable than a building that weighed twice as much.

Joel: In other words, it moves with the wind.

Roman: Which most people find really unsettling.

Joel: Understandably, right? So, LeMessurier added something called a tuned mass damper.

William: The tuned mass damper is a mass of concrete which is 29 feet square and about 8 feet thick. It weighs 400 tons. It is floated on pressurized oil bearings.

Joel: The device sits at the very top of the building and acts to steady the oscillations. It offsets the wind gusts.

Roman: So to recap, we have a 59 story tower floating on 115-foot stilts.

Joel: Stilts which are not in the corner where you would expect them but rather in the middle of each side of the base.

Roman: With a skeleton of beams shaped like V’s which make it super light for a skyscraper.

Joel: And to stop it from swaying in the wind, there’s a 400-ton block of concrete on the roof, totally ingenious, really cutting edge design. And LeMessurier is a pro, so in accordance with the New York City building code, he runs all kinds of mathematical models to make sure the building is going to hold up.

Roman: And everything seemed just fine until, as LeMessurier tells it, he got a phone call.

William: June 1978, I’m in my office. I got a call from a student at some– I do not know the school. I wish he would call me for the past he never has. But anyway, he was a real student from New Jersey. I think he was an architectural student and his teacher had given him this building to study and report on because it’s unusual.

Joel: As LeMessurier tells it, these precocious college senior calls him up to ask how his building structure is going to cope with quartering winds.

Roman: An undergraduate student calling out the head of a world-renowned engineering firm on one of the most innovative skyscraper designs of its day.

Joel: Quartering winds, the winds that caught the student’s attention, are those that hit the building on the corner, exerting pressure on 2 faces at once. LeMessurier’s calculations on the perpendicular winds, winds that hit the face of the building square on, seemed to be fine. But as the story goes, the student raised the alarm about Citicorp Center being particularly vulnerable at its corners.

Roman: It turns out LeMessurier had never run the numbers on what quartering winds would do to the building because normally buildings are strongest at their corners.

Joel: But this is no normal building. Remember what’s going on here. Supporting columns shifted to the midpoint of each building face, the chevron bracing skeleton, and the unusually light mass of the building. Taken alone, no one of these design quirks should have presented a problem. But as the student told LeMessurier taken together, they left Citicorp Center particularly vulnerable to wind striking the structure at its corners.

Roman: And when LeMessurier started to investigate the student’s claims, he realized that the root of the problem had to do with a change that had been made to the original plans. It had to do with how the chevron bracing structure was fastened together. LeMessurier’s plan originally called for the joints to be welded.

William: With full penetration welds.

Roman: But the contractor had suggested using bolts instead.

William: Bethlehem Steel actually designed beautiful bolts and connections for the forces that my people gave them.

Roman: It was a cost-saving measure. LeMessurier says that this change would have happened without his knowledge.

Joel: Given the wind load calculations LeMessurier’s team had supplied, welding these V’s together was overkill. Run the math and it looked like bolts would do just fine.

Roman: And bolts would have done just fine on pretty much any other building except Citicorp Center had an Achilles heel on each of its 4 corners.

Joel: LeMessurier could now see what no one else had.

Roman: No one except for that pesky student from New Jersey.

Joel: And as LeMessurier started looking into the student’s claims about these quartering winds, he realized the situation wasn’t just bad.

Roman: It was a disaster waiting to happen.

William: The return period to failure was 16 years, think about that.

Joel: Here’s what that means. LeMessurier calculated the quartering wind velocities required to topple Citicorp Center. He then matched these velocities to weather patterns to see how frequently on average, winds strong enough to blow the building over, occurred.

Roman: LeMessurier found that a storm strong enough to knock over the building hits New York City on average once every 55 years. But that’s only if the tuned mass damper is working.

Joel: The tuned mass damper needs power to run. And if you get a huge storm, LeMessurier realized, it’s not unlikely that the city could suffer a blackout.

Roman: LeMessurier ran the numbers again, this time imagining the damper without power.

William: It was even worse.

Roman: Without the damper, a storm strong enough to topple Citicorp Center hits New York on average once every 16 years.

Joel: To put this another way, for each year the building was standing there was about a 1 in 16 chance of a storm potent enough to take out the Citicorp Center.

Roman: This building is 915 feet high. If it failed it would topple sideways in the wind, crashing into other Manhattan skyscrapers. A whole slew of buildings from Midtown to Central Park would fall like dominoes.

William: This thing is in real trouble.

Roman: This brand new skyscraper was on the verge of catastrophic failure.

Joel: But the imminent disaster wasn’t the result of one glaring oversight. Rather, it was a sequence of minor issues. Let’s remember how we got here. This building was on stilts because they had to build over the church.

Roman: And stilts placed at the corners would have been just fine except the church was at the corner of the lot, so the stilts had to be in the middle of each face.

Joel: And that could have been A-OK except the offset stilts led LeMessurier to use an ultralight chevron bracing structure.

Roman: Which would have been perfectly adequate except in the end those chevron V’s were bolted, not welded together.

Joel: And even that could have been fine, except no one thought about how vulnerable this crazy design was to wind blowing at its corners.

Roman: No one except an anonymous college student.

Joel: LeMessurier went to Citicorp Chairman, Walter Wriston and told him they would need to open up the building and weld it back together.

William: He said, “How much it’s going to cost?” I don’t think it’s going to cost an awful lot, a million or 2? That’s nothing. Well, a building that cost 175 million and if it falls down–

Joel: From here, LeMessurier and his team worked with the Citicorp bigwigs to coordinate emergency repairs on the building. With the help of the NYPD, they worked out an evacuation plan spanning a 10 block radius. They had 2500 Red Cross volunteers on standby and 3 different weather services employed 24/7 to keep an eye on potential windstorms. They welded through the night and quit at daybreak just as the building occupants returned to work.

Roman: But all this happened in secret over 3 months without telling anyone who worked there.

Joel: Which is the part of the story I kind of have a problem with. I mean isn’t this situation serious enough to warrant informing the building’s tenants? My wife works in a skyscraper in Midtown, Manhattan and, it’s about informed consent. I’d want her to be able to make the judgment call about whether or not to go work in a faulty building, not to have it made for her.

Roman: Their thinking was a contingency plan was in place and ready to be invoked as soon as a storm strong enough to blow the building down was in striking distance of New York except–

Joel: There was a giant storm racing up the East Coast, Hurricane Ella. LeMessurier says the powers that be were just hours away from calling in the evacuation. But the hurricane veered into the Atlantic before it hit New York and in the end, the public was never notified.

Roman: I know what you’re thinking. If only there was some kind of independent organization that serves the public’s right to know that might wonder why Citicorp Center was aglow with blow torches all night every night. Something like a newspaper, except–

Joel: The New York Times was on strike.

William: Not only did the New York Times go on strike but all the newspapers in New York went on strike until October, so we had a press blackout. And that was the greatest thing that ever happened.

Roman: So, word never got out and construction was finished.

Joel: And Citicorp Center has remained standing ever since, long enough to be renamed Citigroup Center and later 601 Lexington. The whole thing remained a secret for almost 20 years.

Roman: In the early 90s, writer Joe Morgenstern overheard the story being told at a party. He interviewed LeMessurier and broke the story in the New Yorker in 1995.

Joel: LeMessurier went on to tell the story publicly like he did at MIT which is how we have him talking about this. And, after the story got out, it was written up as a textbook case of good ethics in structural engineering. New York was spared a tremendous loss of life and the annihilation of its skyline all because Bill LeMessurier was humble enough to give time to the inquiry of an undergraduate student.

Roman: Thank goodness for this uppity college senior and his thesis and kudos to Bill LeMessurier who through his humility and heroism saves the day.

William: That’s really the end of the story.

Roman: Except-

William: Wait a minute, there are a couple of more things here.

Joel: Let’s take a step back and remember how this story starts.

William: I got a call from a student. It was a real student from New Jersey. I think he was an architectural student. His teacher had given him this building to study. I do not know the school. I wish he would call me.

Joel: The way LeMessurier tells it and how Joe Morgenstern told it in the New Yorker that a college student, the young hero of that story, was lost to history.

Roman: Okay wait for it. Wait for this moment. It’s a good one. Here it comes.

Diane: It would have been sometime in the early 90s. I remember being upstairs in my bedroom and had one of my sons, he was sort of hanging off me as I was trying to put him down to go to bed. And suddenly my husband who had been downstairs started yelling a little bit saying, “Diane, quick. Turn on the TV. Your thesis building is on TV.”

Joel: This is Diane Hartley.

Roman: As far as we can tell, she was the student in LeMessurier’s story.

Diane: And so, I was holding the baby with one arm, I fumbled for the remote and got to the channel, just in time to hear-

Man 1: This building could have killed tens of thousands of people.

Man 2: The extraordinary chain of events began with a phone call out of the blue from a student in New Jersey whose professor had told him to write a paper on the Citicorp Tower. William LeMessurier.

William: I explained to this student at the telephone call that he could tell his professor.

Diane: I was aghast. I’d nearly dropped my baby.

Joel: This was the first time Diane had ever heard of the emergency retrofits, the round the clock weather monitoring, the evacuation plans-

Roman: And the involvement of a student.

Joel: So, when she heard LeMessurier reference a student, a male student.

Diane: I, of course, assumed “Gosh, there was some guy studying the building as well. And wow, how could I miss this? Wow.” So it was pretty remarkable but I, of course, assumed at the time that there as another fellow who would have been a better researcher than I had been.

Joel: In fact, Diane never even considered that she might be the student in the story. That is until she went to a Princeton event honoring her thesis adviser, David Billington.

Diane: When I showed up at the event, David said, “Well, have you ever heard of the problems with the Citicorp Center?” And I said, “Yes, isn’t that pretty remarkable? I wonder who it was that discovered this problem.” And he said, “You know, Diane, there are very few engineering schools in New Jersey and quite frankly I know all of them. I know the heads of the programs of all of them and I’ve talked to all of them. There was no other student from New Jersey that was studying the building except for you. So it must have been you.”

David Billington: Yes, I don’t know of any other student.

Joel: This is David Billington, the thesis adviser.

David: Yes, my name is David Billington. That’s correct and I’m an engineer, spent 53 years at Princeton as a faculty member. I was Diane Hartley’s thesis adviser. She wrote the most outstanding thesis [chuckles] anybody, was very long, have you ever seen it? Two big volumes, yes anyway.

Joel: To hear Diane Hartley and David Billington tell the story, Diane wasn’t some in your face know it all.

Roman: Here is her version of the events.

Diane: So, I was 21 years old and I was a senior at Princeton University where I was studying architecture and urban planning. My thesis, I had selected the topic of the Citicorp Center in New York, which had only recently been completed and occupied.

Joel: She reached out to LeMessurier’s firm to get the calculations that they had run. And then she tried to replicate the math herself. To do the work again from the start and see if she got the same answers as LeMessurier’s engineers. David Billington, her adviser, asked her to ask LeMessurier’s firm about the quartering winds.

David: The quartering wind, I thought that was a problem and that it should be looked at.

Diane: So, I obtained calculations from the design engineer from LeMessurier’s office and I concluded the governing loading condition was the wind hitting the corner of the building.

Joel: But the math just wasn’t coming outright.

Diane: I was struggling with the calculations. I assumed of course that the engineer had done the right thing and any inconsistency that I might find would be my error. The building was standing.

Joel: After all, she was just a college senior and LeMessurier and his people were established, internationally renowned professional structural engineers.

Roman: Diane got back in touch with LeMessurier’s firm although she never talked with LeMessurier herself.

Diane: I never called LeMessurier. I never said these things that were documented in the various reports that have been published to date.

Joel: Diane did talk to a junior engineer, LeMessurier’s New York office named Joel Weinstein. She showed him her calculations.

Diane: But at the end of the day, no, I didn’t feel that I had put my finger on a major problem. I assumed I was wrong and wasn’t understanding things.

Roman: Even David Billington who told Diane to look into the quartering winds in the first place didn’t think there was any imminent threat.

David: I mean I don’t think either of us really were in a position to say at the time, “You got to do something, Bill, right away.” We didn’t have that feeling. I had a feeling that she should talk to the designers and find out whether this quartering wind had been considered and if so, what was the outcome.

Roman: But somehow Diane’s discovery got silently passed up the totem pole within LeMessurier’s firm.

David: Weinstein must have said, “Well, I better check with Bill.” That would have been the logical thing to do and he probably did that. I never heard that. You never heard it I guess.

Joel: We asked Joel Weinstein the engineer about meeting Diane and what happened afterward. But he says he has no memory of this either way, though he does acknowledge that if Diane remembers speaking to him, then it probably happened. That, in such a circumstance, he would have passed her message onto LeMessurier.

Roman: So it could be that’s why LeMessurier remembers speaking to a male student. He was conflating Diane Hartley the student with Joel Weinstein his own staffer.

Joel: Now, of course, it is possible that there was a male student.

Roman: A third man.

Joel: He somehow had the same data as Diane and actually did talk to LeMessurier on the phone.

Roman: But I find it really hard to believe that such a person didn’t come forward and claim the spotlight after the story broke.

Joel: Gender aside, Diane definitely doesn’t fit the image of the precocious student. She identified a problem without even recognizing the significance of what she had done for decades.

Diane: It’s not that I did something. It’s that perhaps my questions helped someone figure something out. Perhaps my questions were meant to be. I think if I had really realized that there was a problem and put my finger on it and raised it up the flagpole, I could pat myself on the back for having done something that was pretty remarkable. But at the end of the day, I feel like I was a bit player in a play that had a happy ending.

David: So yes, she deserves more credit than she’s gotten. [chuckles] You could tell her that, include that I think that that’s right. That’s I think the coda for this.

Roman: 99% Invisible was produced this week by Joel Werner and Sam Greenspan. With Katie Mingle, Avery Trufleman and me, Roman Mars. Special thanks to Michael Vardaro who wrote the Area Trust white paper about Citicorp Center, Shamil Raghavan at the National Academy of Engineering for helping me track down archival audio of William LeMessurier and Allen Bellows of website Damn Interesting for talking to me about this about 2 years ago at this point. We are Project 91.7 local public radio KALW in San Francisco and produced at the offices of Arcsine, a beautiful architecture firm in beautiful downtown Oakland, California.



Joel Werner (@joelwerner) is an Australian science journalist/radio producer living and working in Brooklyn, NY, Joel has produced for the likes of WNYC, BBC Radio 4, BBC World Service, and ABC RN. He is responsible for a podcast he calls “an occasional non-narrated storytelling mixtape;” you can subscribe to Your Own Voice here. He also does sound design for the wonderfully quirky PBS Digital show, Brain Craft.

  1. So happy for you all. Well deserved. I am so glad one of my favorite episodes won! Thanks for posting the other two winners. The Criminal story happened minutes from my house, and I had caught the Love + Radio story on PRX chanell 123 on Sirus, All I can say is WOW what a moving story.

  2. Okay I have a serious question: Why didn’t they just design the structure using the same placement of the stilts but with the upper portion of the building rotated around 45º? That way the corners would line up with the legs, giving the structure the same “4-legged table” design we all know and love. Was it really that important to have the sides of the building parallel to the streets? Or could they just not spare having 50% less floor space?

    1. Will Shea

      Because that building wouod have a far smaller footprint and would be a less efficient usage of space. Try drawing a diagonal square inside of another square. See how much less area it fills.

  3. Lucas

    As a fan from Brazil, I want to congratulate you guys for one of your best episodes. I was unbelievably scared of a tragedy that we know never happened. It was a well deserved award, thank you for telling this story.

  4. Amanda

    I am a civil engineer and I remember hearing this story and a recording of LeMessuir’s speech at MIT in one of my structural design classes (about 5 years ago). The focus for the day was engineering ethics. I was fascinated by the story, but I remember being completely confused about how the student who brought up the corner winds was never searched out or given some kind of recognition! How could this person remain anonymous?? This episode was so satisfying to me for that reason. I’m extremely glad that Diana finally got the recognition she deserved, and I of course am also thankful that this potential catastrophe was averted. Great episode!!

  5. Laura

    This is quite possibly the coolest podcast story i have ever heard. the award was totally well-deserved.

  6. Ben

    Why do the article and podcast refer to the student as a he? Diane Hartley was identified long before this article was published.

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