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CNN Live Today

Remembering 09/11: Look at Collapse of Two Towers

Aired September 09, 2002 - 10:29   ET

THIS IS A RUSH TRANSCRIPT. THIS COPY MAY NOT BE IN ITS FINAL FORM AND MAY BE UPDATED.

ANDERSON COOPER, CNN ANCHOR: Not long after the collapse of the World Trade Center, researchers began studying the reasons why. The answers could well create a new monument, in the form of tougher building codes.
CNN's Garrick Utley explains.

(BEGIN VIDEOTAPE)

GARRICK UTLEY, CNN CORRESPONDENT (voice-over): As the towers burned, and people watched in horror, who imagined that one millions tons of buildings, more than 1,000 feet high, could come crashing down?

GENE CORLEY, STRUCTURAL ENGINEER: We'll go in a frame at a time. The aircraft completely buries itself in the building.

UTLEY: Gene Corley, a structural engineer, saw the fire on television.

CORLEY: I said to one of my colleagues, "Well, they didn't collapse under the impact of the aircraft, but if they don't get the fire out, they're going to collapse.

UTLEY: When it was designed in the 1960s, the World Trade Center was the state of the engineering art. The traditional steel-grid system used in skyscrapers since they began going up was abandoned. Instead, each tower had a central core of columns to support half of the buildings weight, while columns on the outside walls supported the other half.

Under each floor, support trusses ran from the core to the walls, which provided lateral strength to the steel skin of the building. The design provided vast open office space on each floor.

(on camera): The architects and engineers anticipated every blow they thought the towers would have to absorb. A hurricane with winds up to 140 miles an hour? No problem. A 707 jet ramming into one of the towers? No problem. What no one imagined was the kind of intense fire that the aviation fuel would ignite, and what that would lead to.

(voice-over): When the first plane struck the north tower, it flew right into the core columns, cutting off elevators and stairwells. The second plane, which struck the south tower, hit closer to the edge of the building, destroying nearly two thirds of the outside columns on one side. In each building, 90,000 gallons of jet aviation fuel became a giant flame thrower, igniting the fire which spread upward. The fire, estimated to have burned at up to 1,500 degrees, weakened the steel columns and the supports connecting them. The fire-resistant material, which had been applied to the steel, was knocked loose by the impact of the planes.

CORLEY: Once that fireproofing is gone, the steel heats up very rapidly and loses its strength in a few minutes, rather than in a few hours.

UTLEY: The immediate threat was not in the vertical columns, but in the steel trusses under each concrete floor. As those cross beams began to weaken, they no longer supported the columns on the exterior wall. The south tower, which was struck lower and therefore had more weight bearing down on the failing steel, was the first to fall.

Gene Corley was a member of the team that investigated the collapse.

CORLEY: The collapse starts there and then spreads across. The building tilts to the east and then disappears into the cloud.

UTLEY: And then, 15 minutes later, the north tower.

CORLEY: We believe in the north tower that the core, which was supporting the antenna, lost its strength first and started to collapse. That transferred the load to the exterior columns of the building, the tube surrounding the building, and then the whole building started down on the north tower.

UTLEY: Engineers say there was never a real danger that the towers would tip over. They were so heavy they could only come straight down, although no one could ever imagine that they would.

Garrick Utley, CNN, New York.

(END VIDEOTAPE)

COOPER: Now, someone who worked on the Twin Towers from the drawing board to the postmortem study of their collapse, Dr. John Fisher is a civil engineering expert, and served on a panel of national experts that investigated the collapse, and he joins us from Lehigh University.

Thanks very much for being with us, Dr. Fisher.

JOHN FISHER, ENGINEERING EXPERT: Thank you.

COOPER: I think Garrick Utley did a very good job of explaining why the towers came down. Let's talk a little bit about the future, in terms of we can learn from what happened, in terms of building new buildings or strengthening existing buildings. First, is it possible for any building to be made that is indestructible?

FISHER: Well, I don't think that it can be maid into indestructible, we can harden buildings, and perhaps resist some loads and perhaps even resist some fires, but the massive amount of impact we are talking about from a large aircraft from the sides of those two towers is probably impractical, try turning towers into tanks, and we know that even military vehicles, such as tanks, can be breached.

COOPER: Is it even safe to build buildings which are that tall? Is there something fundamentally unsound about a structure that is that large some.

FISHER: Well, no, I don't think that is the issue. I think that the issue is we should not be allowing aircraft to fly into tall building structures. Presumably, that's the objective to the steps that are being taken now at airports to prevent that from happening again. That seems the most logical solution.

COOPER: But with the structure that large, can a building like that be fire-proofed, for instance?

FISHER: Well, it can be fire proof for normal fires, but I don't -- this is fire that was developed in both of the towers, the World Trade Center, was unique. Never before in the history of fires have call buildings have we had simultaneous ignition of fires on four or more floors of the first tower that was struck and six or more floors in the second tower. And so when you have jet fuel being spread across that many fore plates, you have a massive fire that suppression is probably impossible, and the impact of the -- massive impact of the plane on the towers, as has been pointed out, probably knocked loose all of the fire proofing.

So you have condition that's are probably impossible to design against, and probably not desirable. We can't turn our towers into fortress.

COOPER: What should be done, though, in terms of building new buildings? What should be learned from what happened?

FISHER: Well, I think that we do -- there are obviously are many things that we've learned that are -- steps are being taken even as we speak to improve. The coordination between the structural engineer and those that have been involved in fire engineering has not really been very great in the past.

And so I think that the dialogue between those individuals can be significantly improved, and structural engineers need to be more aware of the consequences of fires and what they may do to the structure itself.

So that's certainly an area that needs to be done. There can be improvements in fire proofing. And obviously, the most critical in all is how we connect things together. Can we make the structural system more redundant, having more alternate load paths to be able to carry the loads when there is severe damage in localized areas.

COOPER: John Fisher, thank you very much for joining us this afternoon.

FISHER: Thank you.

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