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Why world trade center collapsed

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Tod Rittenhouse: Why the World Trade Center collapsed

Tod Rittenhouse is an expert in blast engineering from the international consulting engineering firm Weidlinger Associates and has been the blast engineer for a number of embassies and government buildings. He has been called to discuss such problems as the Oklahoma City bombing and the previous World Trade Center calamity.

CNN: Welcome to our discussion, Mr. Rittenhouse. We're pleased to have you with us today.

RITTENHOUSE: I'm glad to be here and hopefully can answer some of your questions.

CNN: When you learned about the airplane hits and saw the pictures, what did you think about the structural soundness of the World Trade Center buildings?

RITTENHOUSE: When the event first occurred, naturally we all wondered how sound the building would be given the structure. We were concerned about the damage and in getting the people out in time before some type of collapse occurred. Like most people, I did not want to believe that a complete collapse could occur. But these were large bombs, strategically placed -- the bomb being the airplane and the placement being in a vulnerable spot in the building. The port authority has worked to secure the perimeter around the base of the building so the only way to attack the building is at a higher elevation -- such as an air attack.

CHAT PARTICIPANT: Can you explain why the buildings collapsed?

RITTENHOUSE: The exterior structure is comprised of columns. The vertical load bearing members and the horizontal elements called "beams." When the plane impacted the building, it severely damaged those exterior columns. The following fire further damaged the support columns. So it was a two step event; initial damage by plane and further damage or subsequent loss of structural stability that caused the building to fail.

CHAT PARTICIPANT: Was it due to the structural engineering that the building collapsed relatively straight down?

RITTENHOUSE: There are two reasons why it fell straight down. One is the structural engineering --how it was designed. And how it fell is really a phenomenon. The other reason is because the impact zone was so high up in the building that the weight of the uppermost floors fell onto the impact zone. Had the impact zone been lower in the building, the structure may have fallen in a tree-like effect, rather than crushing down on itself.

CHAT PARTICIPANT: I am amazed the buildings didn't collapse immediately when the planes crashed into them. Is this more or less unique for these two buildings?

RITTENHOUSE: No. They are very big buildings. They were carrying a lot of weight. And so the structure was acting as it was designed. In most buildings, you might be able to lose a column and have the building remain standing for a period of time. But given the structure of these buildings, and that is called a "tube structure," the remaining structural elements were able to carry the load. A tube structure building is like a garbage can, very rigid around the outside but once the damage starts, it is very easy to crush it. And this time that time to crush, that is, the time to achieve structural instability, was about an hour.

CHAT PARTICIPANT: Do you think the towers could have withstood the plane crashes if the fire hadn't burned so hot and so long?

RITTENHOUSE: Very difficult question. I think that if the fireball was not so great, that they could have contained the fire. Fires are meant to be fought in localized areas. In other words, if a fire breaks out in a 15th floor, the sprinklers will go off on the 15th and 16th floors and so on, up the building as required to fight local fire. However in this case, there were fires located on 15 to 20 different floors. So there was never enough water to arrest the fire to prevent structural instability.

CHAT PARTICIPANT: What can you tell us about the escape routes for people in a high rise building such at WTC? How safe is it really to be in the upper levels of such a tall building during an emergency?

RITTENHOUSE: The emergency egress requirements are well thought out. And every building has an emergency egress plan. The time to egress the WTC, for example, is approximately two hours. And that is why we have requirements to fireproof buildings for one or two hours to allow orderly egress. Had the building collapse occurred two to four hours after the initial event, they would have been able to evacuate everybody.

CHAT PARTICIPANT: Are the water systems enough or do we need another fire suppression system in such buildings as well?

RITTENHOUSE: For a conventional fire, these water systems should be enough. This was not a conventional fire. Other systems have been investigated but have been recalled because of other health risks. So the current water deployment system may be the best we have. Perhaps we need more water.

CHAT PARTICIPANT: How many other buildings may collapse? And do we know yet how many buildings will have to be demolished when all is said and done?

RITTENHOUSE: It is impossible to know how many buildings will need to be razed rather than rehabilitated. There are engineers from the Structural Engineering Association of New York (SEAONY) as well as other local engineering firms, such as my own, that have volunteered to inspect buildings and determine if they are safe for rescue personnel and subsequent tenants.

CHAT PARTICIPANT: What, specifically, would cause surrounding buildings to collapse?

RITTENHOUSE: Damage from the various events. Damage from aircraft parts, the fireball explosion, the building falling itself, causing damage to other buildings, the ground shaking, and potentially high winds could now cause other buildings to fall.

CHAT PARTICIPANT: Why can't we determine if more folks are alive within the structure? How long before we get to the folks?

RITTENHOUSE: It could take a very long time. You have to be concerned about the stability of the neighboring buildings. We don't want them to fall on rescue people. You have to be concerned that removing rubble doesn't collapse on air pockets below where victims might be. You have to be careful that vibrations from machinery do not cause further failures. So many of the rescue efforts are being done by hand and small tools to quickly get to victims.

In Mexico City, there was once an earthquake where up to eight days later, they found survivors, many of them infants, located in the hospital. So there is a good chance that people who are located may still found, though it is a dim chance. That area of Manhattan has many underground tunnels where people could be. We just need to get to them.

CHAT PARTICIPANT: There must be multiple basements under the towers. Is it possible that people have fallen into the basements with debris on top of them?

RITTENHOUSE: Yes, there are many utility tunnels, subway tunnels, below grade and several, maybe as many as seven basement levels. So there is a strong possibility that if they can get to them, they will be people there. Unfortunately there is a lot of debris and now water circulating as they fight the fires. So it is possible that these void places where people are located could be filling with water.

CHAT PARTICIPANT: Tod, I keep looking at ground zero and my mind cannot fathom how two 110-story buildings are reduced to nearly ground level. How is that possible, that such massive buildings are now nearly gone?

RITTENHOUSE: It's partly because of the type of structure. I'm sure a lot of it has filled the hole that was the basement. It is as surprising to me as well, but had they fallen over, it would have caused greater damage and far many more deaths.

CNN: Do you have any final thoughts for us today?

RITTENHOUSE: This has been a terrible tragedy for many many people. I have been pleased at how people have united to help in their own way either rescue workers, or fellow engineers, or individuals who are lining the streets with thank you cards to show appreciation to the rescue workers. I hope that we can rebuild.

CNN: Thank you for joining us today, Tod Rittenhouse.

RITTENHOUSE: Great... If I can be of any help, please contact me. Our Web site is:

Tod Rittenhouse joined via telephone. The above is an edited transcript of the interview on Thursday, September 13, 2001.

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