Leslie Robertson, Engineer

Engineer of record for the World Trade Center, Leslie Robertson's work portfolio includes hundreds of buildings around the globe. A pioneer in structural design innovations, his firm has designed three of the six tallest buildings in the world. Producer Ric Burns interviewed him during the making of New York: The Center of the World.


The Trade Center's Sculptural Quality

It's often in the world where I live, in the world of architecture, that an idea jells really fast. And the concept of the World Trade Center came together with enormous rapidity. Sure, after we got started on it, it got taller and there were changes, of course. Constantly evolving. But the fundamental of it was born almost immediately. And it was born, in my view, more as a sculptural form than as an architectural one. And indeed later on, when the architectural world kind of turned away from the project -- although it later returned, but it turned away for many years -- it still remained in the heart and minds of sculptors as a beautiful place in New York City.

Close Columns and Narrow Windows

We had done a variety of projects with quite narrow windows. And indeed if you look at [Yamasaki's] work, it's pretty much like that. He's not a -- never was a great window-wall kind of person. I think he felt that people would be more comfortable at high, with more control over the kind of physical enclosure. And glass is sort of but not quite a physical enclosure.

For example, during the construction, when there was no glass in the windows, just the steel work was there, you could get almost anyone to walk up to the outside wall of the building, put their hands on the column, and look out. But if there was a little bit of breeze from behind, just a little bit, maybe five feet from the edge is as close as a lot of people would get, just -- I think people are quite sensitive to that.

And Yama was sensitive to that. I've heard it said on the television that Yamasaki was afraid of heights. I think he was insecure with heights. Fearful, I don't think he was fearful of heights. He -- because he walked up to the outside wall. I don't recall that he walked up with a wind from behind, but probably he did at some time. So I think it was just his feeling that it would create a more -- a warmer and more satisfying workplace, to have the windows close together -- have the columns close together.

The World Trade Center and Empire State Building

The Empire State Building was designed and constructed in an incredibly short length of time. And I think it's a real tribute both to the architecture and the engineering and the people who financed it, and the city of New York, for that matter. It's a magnificent building. It's entirely different -- in terms of my work, the structural work -- from the World Trade Center.

For example, it has in it a steel frame. But that steel frame was designed to carry all of the loads that the designers thought would be on the building. They were wrong by a significant margin, in terms of the magnitude of the loads. But they created their design based on experience in the past. And so even though the steel frame was in fact not adequate to carry the loads that would in fact go on the building, the system -- which consisted of the steel frame and all of the masonry in the building -- was quite strong enough to do the job.

The two buildings (Empire State and World Trade Center) were in one way the same, in that they were symbolic of the city of New York. But inside, inside of the guts of it, if you will, the structure, entirely different. Entirely different buildings.

And all of the work that had been done from the 30s -- stopped with the Second World War and picked up again -- basically was built on that technology. Remember, the engineers did not even have computers. They used slide rules. I knew how to use a slide rule. Bet I did. But I'm sure that there was no engineer in New York City that had a computer, except us. We were the only ones. We were the only ones who knew how to use it, even. We were, in a sense, many steps ahead of the rest of the profession.

The Skin Structure of the Trade Center

There are photographs that one can see of the World Trade Center where from the outside, you look at it and it's a totally opaque object, or objects. And that's particularly the case when the sun was on the facade. With the sun behind, you could see right through the building. It was literally almost transparent, almost transparent. You could look right down through the elevator corridor to the other side of the building. Nothing like that had ever been created before. Before, there were columns that -- relatively close spacing -- 30 feet on center, something like that. And those systems worked pretty well. But the Trade Center had a different kind of structure. It was built more like the wing of an airplane. And the wing of the airplane, the strength is all in the surface of the wing, or the fuselage, on both cases. You can't have columns down through the center of your passenger airplane.

And so the Trade Center being that kind of structure, being a kind of skin structure, it was important that it have as well, a lot of load on it. It had to have weight on it, because if it were very light and you push on it, it would just tip right over. So it was important to have a lot of weight on it. And all of the interior columns that had been used in the past were a detriment. They were harmful to the design, because we didn't want those interior columns. We wanted that weight out on the outside, where it would do some good for this -- for the stalwartness of the building in resisting these giant loads from the wind.

Wind Load and the Towers

The issues of wind loading were enormous because, it being an entirely new kind of building with just a structure and not all that masonry, we had to rethink the entire process. How much can a building move in the wind? Well, when it moves downwind (leans with the wind, if you will), the way older buildings did (and still do), not so important. I mean, you have to design for that motion, but that's just something to do. But how much would they oscillate? No one had ever found out. No one had ever tried to find out, even, or even thought there was an issue to find out about. Not only how much does it move; how much can it move? And so we did something that was, I'd say, incredibly brilliant. We brought into the firm a young man, Alan Davenport, who was at the University of Western Ontario in Canada, came from South Africa through the U.K. And he was the rising star in wind engineering. We were able to convince him to leave the university and come with us. And I'd say, between us, he and me, we redirected the whole thinking process about tall buildings. We organized and had built an experiment that was produced to move people, and find out how much they could accept in terms of building motion.

Before that, I went to all of the places that have motion simulators -- the automobile industry, the ship industry, aircraft industry, space industry -- and rode in their simulators and tried them out and I learned right away that the advice that I had gotten, which was that people would accommodate to building motion, was not good advice. That was not good advice. That people would not accommodate to the motion. Accept it, yes. Accommodate, no. They would find another job as fast as possible and get out of the blooming building.

And so trying to find out how much the buildings moved, how much people would accept, and then develop techniques for reducing the building motion down to the acceptance limits, was a task that was unique in the engineering world. We developed, eventually, a damping system, sort of like the closer on a screen door in, if you have a house and one of those old screen door closers. Shhh. Closes the door softly. That kind of a device -- but of course very much stronger, each one capable of carrying two and half tons, and -- and using material that had been developed by 3M Company. We were able to get quite reasonable devices, and increase (we call it) the damping, the ability of the building to dissipate the energy of oscillation -- the wind putting energy in to the building, making it wanting to oscillate, and the damper taking energy out, decreasing that oscillation or holding it to a given limit. You know, that's -- things like that had never been done before.

Differences Between the Two Towers

People looked at the buildings and called them twin towers. I never thought of them as twin towers. To me, they were -- each had its own personality. Mental twins. But they were not the same. So if you took an element (say, one of the columns) in identical location -- on each facade there would be -- two, you would think, would be the same. They were not. And then you go around the corner. They were not, nor were these, nor were these. And the two buildings were different, because the wind loads on the two buildings were different. Because one, in a sense, shielded the other from winds in one direction, and the other shielded it from winds in the other direction. So to me, the two towers were quite unique. And I thought the thought of them that way. They were not -- We called them A and B or North and South, and I always hoped that they would eventually get names like real things. Never did, I guess. 


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