World Trade Center Collapse

  • By Susan K. Lewis
  • Posted 09.05.06
  • NOVA

The truth about exactly how the World Trade Center towers collapsed matters greatly, both to the families of those who perished and to the future safety of buildings in America and elsewhere. In the spring of 2002, Congress called upon the National Institute of Standards and Technology (NIST) to conduct a thorough investigation into the disaster. Civil engineer S. Shyam Sunder served as the lead investigator. In this audio slide show, Sunder describes the series of structural and fire-related events within the towers as they progressed toward catastrophic collapse.

Launch Interactive

Watch an expert-narrated slide show of the Twin Towers' final minutes.

Editor's note: As a result of the study, NIST made recommendations for changes to building codes and standards. Shyam Sunder discusses these recommendations and other sobering findings of the study in a separate interview with NOVA. The edited transcript of this interview may be found under Transcript below, following the transcript of the audio slide show.

This feature originally appeared on the site for the NOVA program Building on Ground Zero.



SHAYM SUNDER: I'm Shyam Sunder. I am the lead investigator for the World Trade Center investigation that was conducted by the National Institute of Standards in Technology.

In many typical investigations, you have a partially collapsed building standing there that can be looked at by engineers.

In the case of the World Trade Center towers, the buildings had collapsed. And also there was an immediate need for search and rescue of people who might have been buried under the rubble. So there was also a need to clear the site as quickly as possible. It's very unusual that the evidence is so completely scattered, and that made our investigation a lot more complicated.

We realized that we had to model the complete aircraft impact. We had to model the evolution of the fires over the next hour, or hour and half. We had to model how the steel weakened as a result of the high temperatures. And lastly, we had to model the progression of failures-- local failures in the structure, which then led to the overall initiation of collapse. Each of these stages had very sophisticated models that were validated using the photographic evidence we have, validated using laboratory experiments that we conducted. And validated, ultimately, based on how the buildings actually collapsed using videos.


Through a fairly sophisticated analysis of the video information, we were able to determine the airplane speed, direction of the airplane, and the orientation of the airplane as each of the airplanes impacted each of the buildings.

The north tower was hit at 8:46 by a 767-200 extended range airplane, between the 93rd and the 99th floor. The airplane was flying at about 550 miles an hour, with a total weight of somewhere on the order of 283,000 pounds.

The second building to be hit, which was hit 16 minutes later, was hit on the south face of the south tower. Again by a 767-200 extended range airplane, but this time between floors 77 and 85.

And one of the questions that comes out from looking at this information is how is it here we have two seemingly similar buildings—they were called Twin Towers—and they were hit by nearly identical airplane, but how come one building lasted for 102 minutes, the other building lasted for 56 minutes, which is nearly a factor of two.


On the outside of the buildings, on the perimeter, there were about 59 to 60 columns on each face. The first thing that happened as the airplanes went through the dense grid of column is that they got disintegrated by the columns, which acted like knife edges. Pieces of the airplane actually came out from the other end. For example, we had landing gear come out from the south side of building one, and we had an engine component come out from the northeast corner of building two.

The debris that was generated by the airplanes covered nearly between four and six floors of the buildings. Of course, in addition to the debris, we also had jet fuel that was dispersed in that region.

On the interior, in the core of the building, these buildings had 47 columns-- fairly massive columns inside. In the case of building one, six of the columns were severed and three of them were heavily damaged. In the case of building two, 10 of them were severed, and one was heavily damaged.

What is very important to note is that in the case of building two, that damage to the core was heavily in the southeast corner. So, it's like looking at a three or four legged stool, and essentially taking out one leg. Inherently, the second building was a lot more unstable than the first building, and one of the key reasons why the second building collapsed much faster.


A number of factors contributed to having a very large, multi-floor fire that was uncontrollable. First and foremost, the sprinkler system was not functional, because the water supply to the sprinkler system was damaged by the airplane impacts.

We had dispersion of very large amounts of jet fuel through multiple floors, igniting fires through a large segment of all of those floors, which is very, very unusual.

Most of the jet fuel was burned in a matter of a few minutes-- maybe five or six minutes, but certainly less than ten minutes. Furthermore, about a third of the jet fuel actually burned outside the building, as a result of the fire balls.

So, what really was burning for the next hour or hour and a half was the everyday contents of the buildings. The workstations, the computers, the paper, the carpets and so forth.

In building one, the fires moved from location to location, every 20 minutes, consuming the combustibles in any given location. And as long as the windows kept breaking to provide the air that was needed for combustion, the fire kept propagating. Toward the end of the period when the buildings were about to collapse, the fires were actually on the south side of the building. And it's the south side of the building that played a key role in the collapse.

In the case of building two, a significant amount of the combustible debris was piled up on the east face of the building. And therefore, there was a persisting fire in the same location for a very, very long time. And the second reason for the early collapse of the second building was really this persisting fire on the east face of the building.


The World Trade Center towers would likely not have collapsed under the combined effects of aircraft impact damage, and the extensive multi-floor fires if the fireproofing had not been widely dislodged by the aircraft impact.

What was applied to these buildings was spray-applied fireproofing. And the use of this kind of fireproofing for the floor system, the floor trusses, was very unusual.

When you heat steel up, and particularly when there's no fireproofing, the steel tends to soften, and loses strength and stiffness. It becomes very much like play dough.

In addition to the columns weakening, the floors that were in the vicinity of those faces also weakened. And as a result of that, the floors sagged. And the sagging of the floors actually helped pull the external columns inward. And caused an inward bowing of the external columns. That is captured in photographs.

What then happened after the inward bowing is there was a stage at which a critical amount of inward bowing took place, and the columns snapped. And essentially the columns, once they snapped, the inwardly-bowed columns suddenly sprung back and out. And once that happened, the top mass, that rigid mass of somewhere on the order of 10 to 20 floors just started moving downward.

And the structure below, because of the fracturing of the columns just before it, that had snapped, was unable to withstand the energy that was released.

The primary reason for the collapse was the fact that you had massive airplane damage. A massive amount of fireproofing that was dislodged, and a really huge fire, that all three combined initiated the collapse. There was no one particular event that played a more significant role. It was a combination of events that led to a general system collapse of the building.


Our investigations are geared toward fact finding, and they're geared toward enhancing safety of buildings in the future. They're not geared toward fault finding. However, what I am able to say is that we did not find a deficiency in the building, in terms of the building being inconsistent with the New York City building codes, in a way that influenced the outcome of 9/11.

We made 30 recommendations as a result of the investigation. Both in the area of structural design, and in the area of evacuation and emergency response, we can make considerable improvements with minimal investment. And the return is going to be exceedingly high for those minimal investments. Particularly since, when we are dealing with tall buildings, we're dealing with the lives of a very large number of people.

We should be looking at rare events when we design tall buildings, much as we already do. We look at rare hurricanes. We look at rare earthquakes, events that are 500 years, or 1,000 years, or 2,500 years return period. We should be designing the structure and the evacuation system for such kinds of rare events that a building might see over its life.

But I should hasten to add, nowhere are we suggesting that buildings be designed to withstand airplane impact. We feel it's much more important to keep terrorists away from airplanes, and airplanes away from buildings.

We certainly hope that the legacy of our study is such that the recommendations are acted upon, and that future buildings will be safer. And those that actually rise on Ground Zero will be safer than what were in place on September 11th.

EDITOR'S NOTE: Below is an edited transcript of an interview with Shyam Sunder in which he discusses recommendations for new building codes and other matters related to the WTC collapse.

Goals of the Investigation

Q: What was the mandate that your investigation had from Congress?

S. Shyam Sunder: Our investigation had four objectives. First, to determine why and how the World Trade Center towers collapsed. Second, to analyze the loss of life and injuries, particularly through studying the evacuation and emergency response. Third, to determine the procedures and practices that were used throughout the life of these buildings, so that we would have a basis on which to compare how the buildings performed on 9/11. And lastly, to make recommendations for improvements to building codes and standards.

Q: What resources did you rely on most in reconstructing the events?

Sunder: First and foremost, we had a very large collection of documents on the design, construction, operation and maintenance of the towers that were provided to us by the Port Authority of New York and New Jersey as well as their consultants and contractors. And we also received documents from the city of New York, primarily from the Fire Department of New York.

We then had a very large collection of photographs and videos—somewhere on the order of 7,000 photographs and 150 hours of videotape. This was a unique collection of evidence that was very, very helpful to us in terms of understanding the damage caused by the airplanes, the progression of the fires through the buildings, as well as of the building collapse.

We also had two other very, very important sources of input. We had about 236 pieces of steel that represented the 14 different grades of steel that were used in the building. That evidence enabled us to determine the proper grades of steel for use in our models. And the final source of input to our study was interviews. We carried out over 1,000 interviews with surviving occupants and about 116 interviews with emergency responders. So this was a fairly extensive set of eyewitness evidence, complemented by published media accounts.

Q: Were there clear design and engineering flaws that accounted for the collapse?

Sunder: We did not observe any flaws in terms of the structural design with regard to the collapse. We checked out the entire design, including that of the floor system, the connections of the floor systems to the internal columns, to the exterior columns. All of the elements seemed to have been properly designed. We also found no areas where the buildings were not consistent with the New York City building codes that had an impact on 9/11. We did find some areas where there were differences from the New York City building code. But those differences did not really play a role in 9/11.

Q: So were they sound buildings?

Sunder: The structural design of the buildings was consistent with the codes, and they were sound.

The Evacuation

Q: Of the roughly 17,400 occupants present in the towers the morning of the disaster, how many were able to evacuate?

Sunder: Well, approximately 87 percent of the towers' occupants were able to evacuate the towers very successfully. And that included some 99 percent of those below the floors of impact. Only about 118 people below the floors of impact lost their lives, approximately 107 in Building One and 11 in Building Two.

Q: For those people who were on the floors above the impact, were there any passable stairwells, and were all the elevators knocked out?

Sunder: In the case of Building One [North Tower], none of the stairwells were passable after the airplane impact, in that impacted region. And in that same building, of the 99 elevators, only one elevator was functioning, and I think it went up to the 16th floor.

In the case of the second building [South Tower], there was one stairwell out of three that was marginally passable, for at least some period of time. And approximately 18 people from above the floors of impact were able to get through that stairwell and make their way out of the building. Once again, of the 99 elevators in that building, only one elevator was functioning, and that elevator took people up to the 40th floor.

Q: If the buildings had been at full capacity, would there have been adequate time to evacuate all of the occupants?

Sunder: The full capacity of these buildings on a typical day was on the order of 20,000 occupants per building. On 9/11, there were only between 8,500 and 9,000 occupants in each building, and the primary reason for that was it was 8:45 or 8:46 in the morning. Many offices hadn't opened, it was primary election day, first day back to school for children. And tourists don't show up until about mid-day; they certainly don't show up at 8:46.

So we wanted to analyze what would have happened if, in fact, the buildings had been full to capacity of 20,000 people each. Our analysis showed that it would have taken somewhere on the order of three hours to evacuate the buildings in that case. And, of course, we only had 56 or 102 minutes available on 9/11. If the buildings had been at full capacity, our analysis showed that almost 14,000 people would have died on 9/11.

Interestingly, about half the people would have died below the floors of impact, and half the people would have died above the floors of impact. The capacity of the stairwells would not have been sufficient had the buildings been full.

Emergency Response

Q: Roughly how quickly did the firefighters make their way up the stairwells?

Sunder: In the case of occupants, we found that people were walking down about one minute per floor. In the case of firefighters, we observed that it takes much longer, somewhere on the order of one and a half minutes per floor if you're not carrying gear. And it could be as much as two minutes per floor, if you're carrying 100, 150 pounds of gear, which many of them do, particularly hoses and air bottles.

Q: Were the firefighters hindered by evacuees moving down the stairwells?

Sunder: Counter-flow was an issue. You had a very large number of occupants going down. Now these evacuees only encountered a firefighter once in a while. It didn't affect them as much. But in the case of the firefighters, they saw this very long and steady stream of occupants coming down. It did make it harder for them to get up.

Q: I was struck by something I read in your report about the time it probably takes a firefighter to reach the 60th floor of a burning building. Can you describe this?

Sunder: If a fire is raging on the 60th floor of a high-rise building, and all of the elevators are not functioning, it will probably take close to two hours for the firefighters to show up on your floor. That's presuming that they can keep to the two minutes per floor rate, which is going to be very difficult. Most firefighters we have talked to would suggest that they can climb up 15 stories fairly well. They would then need to take a break, maybe then climb up three or four or five more stories. But over about 20 stories is very, very difficult for firefighters to continue at that same rate. They will need to take a very, very long break.

And more importantly, even if the firefighters do end up reaching the 60th floor, they will probably not be in a physical condition to be effective in the service that they're trying to provide—whether it's putting out the fire or helping somebody with a medical condition.

Q: Are there lessons to be learned from the World Trade Center evacuation?

Sunder: Oh, there certainly are broad lessons to be learned from looking at the evacuation. It's taught lessons about the need for adequate stairwell capacity to evacuate an entire building and lessons about providing adequate access for firefighters. We also need to think more about the self-evacuation of mobility-impaired occupants. There were nearly 1,000 mobility-impaired occupants in the buildings, which is a very, very large number. It's about 6 percent of the occupants. And these are not necessarily people who use wheelchairs. These are people who have a medical condition, have had recent surgery, they may be on blood pressure medications, they may be pregnant, they may have asthma, or they may be obese. There are multiple reasons beyond the more conventional definition of mobility impairment.

Beyond that, there's also the need to make stairwell enclosures more robust and keep them sufficiently remote from each other, so that if one is damaged, there's a good chance that another is not.


Q: Broadly, what recommendations did you make as a result of the NIST study?

Sunder: We made 30 recommendations for improvements to building codes and standards and practices as a result of the investigation. They run the gamut of improving the design, operation, and maintenance of buildings such as the World Trade Center tower, but also more normal types of buildings.

Q: What do you think are the most important recommendations?

Sunder: There are probably four or five areas of recommendations that really stand out. The recommendations on fireproofing are very important. We are calling for better fireproofing in terms of fireproofing having greater bond strength, and also for inspections of fireproofing in a building after it is applied, so that we are sure that the fireproofing that is supposed to be applied is actually there.

We've asked for the development of new technologies, new kinds of coating materials for fireproofing. There are more paint-like materials—as opposed to spray-applied, fluffy fireproofing—which might have certain advantages. These coatings are already used in the United Kingdom.

The second area of recommendations that I would stress deals with active fire-protection systems, such things as sprinklers, smoke-management systems, and fire alarms. In general, we're calling for a greater redundancy. In the case of the World Trade Center towers, as a counter example, the sprinkler systems on a particular floor were connected to the water supply at one point only. So if there were a failure at that single point, then the entire floor would be devoid of water for the sprinkler system.

The third area where we would suggest improvements is in better communication systems for emergency responders. A lot has been said publicly about the need for interoperability, which would allow firefighters to talk to police. But as important is intra-operability—the ability of firefighters to talk to other firefighters. This is really difficult in the case of tall buildings and the kinds of events we saw, for two reasons. One, as many of us know, cell phones have difficulty getting signals in tall buildings, because they have steel. For the very same reason, radios have communications difficulties. So we need better technology and better use of existing technology.

Secondly, most times when emergency responders are using radio communications at a site, they have five, 10, 100 people, 200 people maximum, so a limited number of frequencies—half a dozen, three or four—are adequate. But when you have nearly 1,000 people on site, as we did on 9/11, the limited number of frequencies is a problem. It becomes very difficult to understand what the protocol should be for communication. So there are some real challenges there that need to be addressed.

The fourth area of recommendations that is very important involves the ability of tall buildings to be evacuated fully in an emergency. Our recommendations include the use of adequate stairwell capacity for this purpose. We're not specific as to how one accomplishes this, whether it's through more stairwells or wider stairwells or scissor stairs, where the same shaft has two stairs, one maybe reserved for firefighters. There are many ways to accomplish it.

We also recommend that there be specially designed elevators that are protected against fire and impacts. These should be considered for firefighter access as well as for the mobility impaired. But they probably could even be considered for normal building occupants during evacuations.

And finally, our recommendations call attention to the need to have redundant structural systems to prevent progressive collapse. We have, over the decades, been building taller and taller buildings, and one of the ways we accomplish that is by making the structures very efficient. And efficiency translates to less use of materials, so that we can minimize the weight of the buildings. By increasing efficiency, in short, we probably have reduced redundancy from buildings that were built decades ago.

Q: Some people may feel that the NIST recommendations are too costly to implement and make part of regular building codes. How would you respond to that?

Sunder: Well, the NIST recommendations are appropriate, they're reasonable, and they are realistic. They were designed with prudence in mind. The safety improvements that they will ensure will be of lasting value to our society. Absolutely, whenever you invest in safety, you certainly are going to increase cost. Our understanding is that the cost implications of these recommendations are not prohibitive. They are on the order of a few percent of the total construction costs. Obviously, if you have a building that has unique risks beyond the average, the cost will be higher.

We are already seeing changes happening with the building of the new World Trade Center Seven, at Ground Zero, and the proposed Freedom Tower. And the recommendation with regard to progressive collapse is something that the United Kingdom has been doing for decades. The numerous tall buildings that represent the skyline of Hong Kong are all designed with progressive collapse requirements. So we know the technology exists. We know the requirements can be implemented.

Q: Do you believe that the NIST study of the collapse of the Twin Towers will ensure that what rises in their place will be a safer structure?

Sunder: We certainly hope that the legacy of our study is such that the recommendations are acted upon, and that future buildings will be safer—and those that actually rise on Ground Zero will be safer than what were in place on September 11th. And we are heartened by the fact that the first wave of major code changes are now making their way through the private sector building code development process, as we speak, in the country.



(S. Shyam Sunder, all top and side view illustrations of plane impact, temperature model)
Courtesy NIST
(Ground Zero aerial, firefighters)
© Reuters/CORBIS
(clearing Ground Zero)
(analyzing structural failures, viewing video of collapse)
© WGBH Educational Foundation

Related Links


You need the Flash Player plug-in to view this content.