The Man who built the Twin Towers
Why am I seeing this page?
This feature can also be viewed as flash video. This page exists for people who have problems seeing broadband flash videos, and for visitors from outside the UK.
Visit BBC Webwise for instructions on how to install the free flash plugin.
The World Trade Center towers were designed to be more modern, more cutting edge and taller than any skyscraper ever built. The chief structural engineer on the project was Leslie Robertson, who was then just 34 years old. We interviewed Leslie Robertson just two months after 9/11. Here are some excerpts from his original interview.
Leslie Robertson talks about his experience of 9/11 and the aftermath.
When the plane hit, I was in fact in Hong Kong. I was having dinner with some friends and some business acquaintances. As is the habit of Hong Kong, everyone has a cell phone. The cell phones went off and a message saying the WTC had been hit by an airplane. Second airplane came in.... and then it became clear it was not some small problem... then this huge ball of fire… it was very very very unsettling to everyone. I left the dinner party, I hope someone paid for it... and I went up to my room and got ready to come back to New York.
I can tell you I was just thinking about the people in the building. I mean I just hoped that the buildings would stand I had no way of knowing that they would or would not. And, the people above, obviously they were suffering terribly, the people who elected to take their own destiny in their hands by jumping... I mean it must have been an incredibly awful place above the impact. It's the only thing I could think about then, it's the only thing I can think about today. I mean I could turn it off and start being rational about it, but it's an effort, it's a real effort.
Ground Zero is a very disturbing place for me. I mean, I probably have more emotional attachment to it than maybe any other person now alive. I've been working with the project, at least peripherally, for 40 years. And I cannot escape the people who died there, even if I'm looking down into a pile of rubble, it's still to me somehow up there in the air burning and I cannot make that go away.
Impact of a plane
Leslie Robertson explains how the building was designed to resist the impact of a plane.
We had designed the project for the impact of the largest airplane of its time, the Boeing 707. The 767 that actually hit the WTC was quite another matter again. First of all it was a bit heavier than the 707, not very much heavier, but a bit heavier. But mostly it was flying a lot faster. And the energy that it put into the building is proportional to its square of the velocity, as you double the velocity, four times the energy. Triple the velocity, eight times the energy and so forth.
And then of course with the 707 to the best of my knowledge the fuel load was not considered in the design, and indeed I don't know how it could have been considered. But, and with the 767 the fuel load was enormous compared to that of the 707, it was a fully fuelled airplane compared to the 707 which was a landing aircraft. Just absolutely no comparison between the two.
Attack in 1993 1:06
Leslie Robertson talks about the World Trade Center bombing in 1993.
We had this bombing in 1993, and at that time, the bomb went off and we thought at first it was maybe a transform works explosion or something like that. I had not the slightest concern about the towers. Never crossed my mind it would be an issue and in fact the amount of damage done by the bomb was insignificant.
The impact was so small on the building that the measuring instrumentation at the top of the building, which is used to record the motion of the building in the wind, it has a trigger on it, it doesn't start recording until you get a little motion, the trigger was not set off so we got no record of that. So, the bombing I think created a lot of confidence in everyone's mind that the Trade Center was pretty sturdy.