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American Morning
America's New War: Recovery Effort Going Slow for More Reasons Than One
Aired September 24, 2001 - 11:51 ET
THIS IS A RUSH TRANSCRIPT. THIS COPY MAY NOT BE IN ITS FINAL FORM AND MAY BE UPDATED.
THIS IS A RUSH TRANSCRIPT. THIS COPY MAY NOT BE IN ITS FINAL FORM AND MAY BE UPDATED.
PAULA ZAHN, CNN ANCHOR: The recovery effort at the World Trade Center is -- site is going slowly for more reasons than one. Engineers want to make sure that the rubble being removed does not further damage the towers' collapsed basement, which basically holds back the Hudson River.
CNN's David George gives us a look now at the structural stability of the trade center.
(BEGIN VIDEOTAPE)
DAVID GEORGE, CNN CORRESPONDENT (voice-over): The World Trade Center's North Tower shook and swayed, but thanks to a design concept engineers call "redundancy," did not fall. Not right away.
UNIDENTIFIED MALE: If the load carrying capability is disturbed or destroyed in one area, within reason, it should be able to -- the structure should be able to transfer the load from that point and carry it down to the foundation using a different path.
GEORGE: That's what happened at the World Trade Center. After the impact, the North Tower stood for more than an hour, long enough for hundreds, perhaps thousands to escape. The collapse, went it came, was caused by fire.
RICK ZOTTOLA, STRUCTURAL ENGINEER: The fire was very, very intense and burned for a long time. The fire weakened that portion of the structure, which remained after the impact. It was weakened by the fire to the point where it could no longer sustain the load.
GEORGE: When the World Trade Center towers did fall, they pancaked on to themselves, minimizing damage to adjacent structures, just like the centers' designers had intended should disaster ever strike.
In the '90s, after Oklahoma City, many government buildings were retro fitted to better withstand bomb blasts. In the days after the World Trade Center tragedy, Georgia Tech's Barry Goodno suggested it would be technically possible, but too expensive to design a totally disaster-proof building, unless, he said, the building were built underground.
David George, CNN. (END VIDEOTAPE)
ZAHN: For more now on the structural concerns involving the World Trade Center let's turn to David Gockel. He is the geotech engineer at Langham, which is an engineering firm.
Welcome, good to have you with us this morning.
DAVID GOCKEL, GEOTECH ENGINEER: Good to be here.
ZAHN: Can we start off by talking about the bathtub or this giant sea wall that exists below what used to be the World Trade Center.
GOCKEL: Certainly. There has been a lot in the media regarding the basement or sea wall beneath the building. It has been referred as a bathtub, but essentially, as you can see here, it's a six story basement structure that is essentially a 16 acre, 70 foot deep basement that had to be constructed water tight to keep out the waters and the ground water and the Hudson River.
ZAHN: All right, you have another picture that we'll ultimately see that will show how close the World Trade Center was to the Hudson River. And you can carry on and see -- talk over what you're looking at now.
GOCKEL: Certainly. This is just an aerial, but I think what's important -- here we go. This image here shows the proximity of the Hudson River. This was taken back in the 1970s, and all that land in the foreground was actually filled in. So you can see here the Trade Center was only about 100, 150 feet from water's edge. And in fact, back in the late 1700s, the Hudson River shoreline coincided with the assignment of the trade centers.
ZAHN: So let's talk a little bit about the stability of the sea wall or the bathtub. Is there any indication that that thing could blow, and there could be major flooding in the subways that are attached to it?
GOCKEL: Well, it is certainly a concern, but the indications to date have been very, very positive. The city has gone about this in a very responsible and cautious way.
One of the concerns is that floor slabs in this basement actually retained or braced the walls of this giant basement from falling in from the earth pressure.
Now that many of the floor slabs collapsed from the building collapse, the floor slabs themselves are gone, and it is essentially the debris pile keeping the walls stable. So as the debris is removed, they must do it in a very cautious and slow fashion.
ZAHN: Well, what do you replace that with?
GOCKEL: Well, as it is removed, it is likely they will go ahead and install tiebacks, which are structural tendons that are anchored into rock, they're drilled through the wall and give the wall a stability. In fact, it is very, very similar to the actual construction procedures that were done in the late '60s when the building was built.
ZAHN: All right, now if you were to ever rebuild on this site, because Larry Silverstein, who has the leases on what used to be the World Trade Center, says he does plan to build, and I guess, one of the ideas that he has in mind is four smaller towers, maybe 50 story towers, being built in the same area. Would you change the way this sea wall was built, or do you live with that?
GOCKEL: Well, my guess would be you would try to go ahead and live with it. You have a slurry wall, a foundation wall that provides for this basement, plaza area, and in fact, the structural integrity of it has not been dramatically compromised, and that is what we believe to be the case. It can be reused as a platform or a basement or a new project at the site.
ZAHN: A quick question for you, David: As you stood, and I know you have seen these pictures over and over again of the towers collapsing, what went through your mind?
GOCKEL: Disbelief, total disbelief.
ZAHN: And you understand how these buildings are built.
GOCKEL: Like many in retrospect, it made so much sense. But at the time it was just such a shock to us all. I think we were numb from the initial strike, that it hadn't crossed many people's minds.
ZAHN: David Gockel, good of you to join us. I know we kept you waiting a long time because of all of the various news conferences we've been dipping in and out of today. I appreciate your insights.
GOCKEL: Thank you so much.
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