ELIZABETH FARNSWORTH: Those twin towers, which were completed in 1973 were 110 stories tall, 1,360 feet. They were the tallest buildings in New York City, roughly 50,000 people work in them.
They were designed to be a hub for international trade and were part of a seven-building complex, which was completed in 1988. In addition to the twin towers, one other of the seven buildings in the World Trade Center complex also collapsed late this afternoon.
For more on the buildings we turn now to two structural engineers, Ron Hamburger, chief structural engineer at UQE, an engineering firm, and Hassan Astaneh, professor of engineering at UC Berkeley who is helping develop guidelines for the American Institute for Steel Construction, guidelines that would help structures withstand terrorist attack.
Ron Hamburger, you've seen the video and the plane hitting. You've seen the fires and the collapse. What do you think happened?
RON HAMBURGER: Well, incredible as it may seem, the buildings survived the aircraft attack. Both of them were able to stand for the better part of two hours after the crash. What they just were not able to survive was the incredibly intense fires that ensued from all of that burning jet fuel. Structural steel, these buildings were steel billings.
Structural steel when it gets hot loses strength. The steel elements that held up the building where the crash occurred got hot from the fires at about the 90th floor. They were supporting 20 floors of building above it. And when they lost the ability to support that, all of that mass, like another building, came down on top of the rest of the structure like a pile hammer and just essentially drove the rest of the building into the ground.
ELIZABETH FARNSWORTH: Now, we don't know for sure, do we, that, Mr. Astaneh... We don't know for sure that there wasn't some kind of a bomb. But you didn't think there had to be a bomb for this happen?
HASSAN ASTANEH: That's exactly case. We are not sure, of course, what was in those planes but the amount of fuel that came and was delivered to this building was enough, in my opinion, as I agree with Ron, that the cause of this collapse and tragedy was really what we call progress of collapse. What happened here was the initial impact did not cause much damage; it just ignited the fire. The fuel was supplied.
The fire on almost four hours - and at that time the temperature of the columns - they have reached the critical level which is 1000 degrees Fahrenheit, and when steel reaches that level of temperature, it loses its strength, and of course the upper floors, the weight of those upper floors completely collapsed on the lower part and hammered it down and collapsed it.
ELIZABETH FARNSWORTH: And Mr. Astaneh, was it important where the aircraft hit the towers?
HASSAN ASTANEH: Exactly. What was really amazing to me this morning watching the footage was that actually - I don't know by design or by accident - but they really hit the worst part of these towers. If you hit these towers at the top, very top, you might lose several floors, but that will not collapse the whole, entire building.
If you hit them at the base, the columns at the base are so strong, as we saw during the past bomb attack on this building; that really those columns will not collapse. You hit in the middle, these columns are not very strong as the base but at the same time they have very heavy weight of upper floors on them. So this was the worst combination of strength reduction and increased weight on them.
ELIZABETH FARNSWORTH: Mr. Hamburger, anything to add to that?
RON HAMBURGER: Well, not really. I think Dr. Astaneh said it very well. Really the terrorists picked the perfect place to strike these buildings.
ELIZABETH FARNSWORTH: Tell us about the buildings. Were they more or less vulnerable than other tall buildings like them? I read that the structural engineer that designed them said they were designed to take a hit from a 707.
RON HAMBURGER: That's correct. These buildings actually were very strong. The steel columns that support the building were spaced at about three feet apart all around the perimeter of the building. Typically on a building like this, you'll see column spacings on the order of three to four times that. So this was an exceptionally strong building. As I said, it did actually survive the impact of the aircraft both towers did.
ELIZABETH FARNSWORTH: They even survived the impact going right through it. You saw that picture of the aircraft going through it.
RON HAMBURGER: Yes, that's right.
ELIZABETH FARNSWORTH: Anything to add about the buildings themselves?
HASSAN ASTANEH: All I can tell you is if there's any positive thing here today is that actually the fact that these buildings were steel structures. When we had the Oklahoma City tragedy, that structure was concrete. When it happened the concrete could not tolerate the impact and the columns were pretty much collapsed and the whole building collapsed and there was no time for people to get out of the building.
In this case, because the structures were steel structures, the columns were able to tolerate easily the impact. Even they could tolerate the fire if we were able to reach the fire and extinguish the fire. But since it wasn't possible, the fire was too intense, and then the steel lost its strength and collapsed after one hour.
But that one hour apparently was enough for many people, as I heard, in fact, from Ron when we were sitting outside, that his firm had people in that building and they were able to evacuate from the 91st floor after the fire started. So they were out before the collapse.
So one positive thing I see is that at least we were lucky in a sense that the collapse actually happened in a progressive way, not in a very sudden, immediate after attack. So I see a very, very positive point in the design of these buildings that they were really strong, as Ron mentioned, and they were really designed well. But unfortunately they could not tolerate that intense fire due to the jet fuel perhaps.
ELIZABETH FARNSWORTH: Could any building designed in the future in a different way-- I'm really asking what needs to be done in the future-- have withstood the heat of that fire.
RON HAMBURGER: Well, really, I don't think you would have to do much to the design of the structure -- what you would have to work with would be the protective coatings that are put on the steel to guard them against fire.
ELIZABETH FARNSWORTH: There won't those protective coatings on this steel.
RON HAMBURGER: They were there but they're designed for the type of fire you would have in an office building: Burning paper, carpet or furniture not burning jet fuel. They're designed to resist that for a period of two or three hours. It would be possible to put additional coatings on the steel that would allow them to survive such a fire. But you'd have to weigh the cost of that against the likelihood of the repeat of such an occurrence.
ELIZABETH FARNSWORTH: You're designing guidelines, which are supposed to help prevent such a building from collapsing if there's a terrorist attack. Structural engineers worry about these things, right?
HASSAN ASTANEH: Yes. Basically, the American Institute of Steel Construction, that organization develops guidelines and design recommendations for profession to design structures for everything.
Now since Oklahoma City collapse, the Professional and American Institute of Steel Construction has started a committee and I'm a member of that committee and our work is to develop guidelines and provisions that structural engineers can use in order to prevent what we call progressive collapse -- which means if you... for either due to car attack, car bombs or rocket attack or other means, if you remove a column or part of a building, can you prevent the full collapse of a building?
These guidelines are in the process and hopefully they will help in the future to prevent progressive and catastrophic collapse.
ELIZABETH FARNSWORTH: Well, Hassan Astaneh and Ron Hamburger, thanks very much for being with us.