Dr Shyam Sunder led the investigation by the National Institute of Standards and Technology into the collapse of the three World Trade Center skyscrapers in New York on 9 September 2001. This interview was conducted in February 2006.

Dr Sunder began by explaining the aims of the investigation.

1. Our investigation had three objectives. One was to understand why and how the World Trade Center’s towers collapsed, second was to understand the evacuation and emergency response procedures that were used and how it affected life safety. And third was to evaluate and document the procedures and practices that were used throughout the life of the buildings so we could compare them with accepted procedures and decide if those procedures were appropriate or not.

2. With regard to the first objective we carried out a fairly sophisticated analysis of the airplane impact into the buildings, into each building, the subsequent fires and then eventually the conditions that led up to the collapse.

3.The impact of the airplane did multiple things; it damaged structural components on the façade of the building but it also damaged structural components in the core of the building. In addition to damage to the core and to the structure it also damaged the fireproofing in the path of the debris of the airplane. The fireproofing on the structural components was dislodged. The third thing it did is it damaged the partitions that prevented fire spread so it made it easier for the spread of fire that happened subsequently. The fourth thing that the airplane did is that it distributed jet fuel throughout the building in multiple floors of the building particularly on the five or six floors where the impact was most severely felt. And the combination of the jet fuel there led to the ignition of fires on multiple floors. As a result of the fires which then spread and expanded the conditions were created for the eventual collapse of the building.

Q: Right. So you are suggesting that the impact of the airplanes knocked out the fireproofing and dumped enough fuel to keep fires going. I take it this then led to the heating of the steel structure which lost its strength. Is this right?

Dr Sunder

4. That is correct. There was significant structural damage to the building as a result of the airplane impact but it was not sufficient to collapse the building. But it was substantial damage and that damage was particularly felt in the exterior wall and the core of the building as well as in some of the floor system. The fireproofing, yes, was knocked out. But the dislodging of the fireproofing, what it did was it contributed to the faster heating of the steel components due to the fires that had been set by the jet fuel that was sprayed into the building.

5. What is important to note is that the jet fuel burned in a matter of minutes, no more than five or six, maximum ten minutes whatever jet fuel that burned initially.  What was burning throughout the rest of the time, the rest of the hour or hour and a half before the building collapsed was really the contents of the building, the everyday contents of the building.

Q: According to your report the steel you tested only reached a temperature of about 250 degrees C. Can you confirm how you came to the conclusion that there was sufficient heat to impair the metal?

6. Most of the pieces of steel we had were from various parts of the building. The fire itself was restricted or limited to only about five or six floors up on the top of the building. The number of pieces of steel we had from the exterior face of the North Tower of World Trade Centre 1 was considerable. We did not have a substantial amount of steel from the core columns. In fact we had very few core columns for which we had actual steel in the fire zone. And we certainly did not have any significant evidence of the floors, the steel that was in the floors of the building. Because in most of these floor systems – which were made up of concrete and steel trusses – the concrete was mostly pulverized and the steel was mostly mangled beyond recognition. So we could not identify any location of any such steel. So what I am trying to say here is that there was very limited steel that we have in the fire impacted regions.  The temperature in the steel that our models predicted was very consistent with the evidence we had. In fact there was very little of the physical evidence in the regions in which we were predicting high temperatures.

Q: Are you saying that in the regions where you predicted high temperatures you didn’t find any high temperatures?

7. What I am saying is that there is no inconsistency between the physical evidence we have and the modelling results.

Q: I want to approach the theory that the buildings were blown up.  Why was the concrete and steel pulverized and shredded to such an extent?

8. The floor itself as you know was made of about four inches of concrete – light weight concrete and normal strength concrete in those days was in the order of about 3000 pounds per square metre of concrete. The floor trusses were made of rods and angles which were typically  in the order of 1 inch to two inches in cross-section. So these were very lightweight materials. They were sufficiently strong in terms of load sharing capacity to be able to handle the design loads in the building. They were efficient in carrying their loads so you didn’t have large sections. You had lots of the steel but you had very thin sections or very narrow sections And it is not unusual for concrete that is compressed. You know due to the force of gravity, you know you have large pieces of the building coming down. Once you started initiating the failure the force of gravity was sufficient to compress the concrete and pulverize it. That’s not inconsistent. The potential energy that was unleashed into kinetic energy due to the top section of the building collapsing was very, very large. And its not inconsistent with the fact that the steel in the trusses which were very, very thin sections was in fact mangled.

Q:  What about the larger steel columns.  Why did they break into tiny pieces as was reported?

9. Well the large columns did not break into tiny pieces. In fact the exterior columns which were typically about 14 inches square, in fact many of them. most of them remained intact. I mean there was damage where there was a direct hit by the airplanes, which is as you would expect, but we have evidence of, we have in our hands many large exterior columns from the direct impact region that survived pretty much intact. The condition of the pieces in our hands, after they went through the complete fall from the heights they were at, and being shipped  and transported to the salvage yards and from there to NIST  is remarkably in similar condition as we see in photographs of those same pieces of steel on the building. So they did not suffer the same extent of damage as the thin sections did. And with regard to the core columns which were even more massive then the exterior columns, those columns survived pretty much intact with not much damage. Clearly some pieces that were hit by the air plane did suffer damage but not most of the other ones.

Q: So why are there reports these columns ended up in 30 foot lengths?

10. That’s not unusual because the external panels of the building were 3 feet high by 3 columns wide. They were prefabricated elements – that were 3 feet high by 3 columns wide – and these panels which helped fast construction were actually connected together by bolts. So when you have a significant impact – and when you snap these pieces- when these pieces` are snapped they tend to snap along the lines of their connection, the bolted connection, so that’s not inconsistent. With regard to the core columns, these core columns were welded together for the most part at their connections, and so what gives way is the weld. And so that also is not inconsistent, that you would see them in pretty large lengths.

Q:  Some people believe controlled demolition is a credible alternative explanation. You interviewed many witnesses. Were there any reports of explosions?

11. We have no evidence at all either from witnesses we spoke to or from photographs or from video which would suggest there was a controlled demolition or controlled explosion of any kind. Neither was there any evidence that we found of something that might look like a missile striking the building from outside or from any other source.

Q: Is it unusual for three skyscrapers  to come down neatly into their own footprints without leaving columns sticking up or large pieces of debris?

12. Well let me first speak for the towers before I speak about – you talked about a third building, I assume you’re implying World Trade Centre 7 – but let me first talk about the towers. Most of the prior experiences with building collapses deal with buildings that were subjected to fires only. In this particular case you must remember that these buildings were hit by very large commercial jetliners that were fuel filled. Boeing 767 airplanes are among the largest airplanes in the air barring I suppose the 747s and in the future Airbus 380s. But at the time that 9/11 happened these were among the largest airplanes flying and they contained a lot of fuel. So buildings are not designed to withstand airplane, jet airline impacts. None of the building codes in existence in the United States required buildings to be designed to withstand airplane impacts impacts. So it is very important to recognise a major distinction between prior building collapses and the collapses that we are talking about. Second, besides the damage, the significant damage that was done to the structure by the airplane impacts, there was a significant dislodgement of the fireproofing in the building. And those two factors are quite different from those prior fires – tall building fires that did not lead to the collapse of those buildings. So there was very substantial differences between what happened on 9/11 and the historical data which people reference.

13. With regard to World Trade Centre 7, our investigation of that collapse is still ongoing. We have not released our findings and recommendations from that investigation. We expect to have that completed by the end of this calendar year. We will be looking at all potential mechanisms or factors to have caused that building to collapse. And I’m hoping by the time we release our report we will have considered all of those factors and hopefully we will have a definite answer. In investigations like this there always remains the possibility that we will not reach a specific conclusion. That’s simply because the buildings don’t exist anymore and we don’t have access to all of the evidence. The building doesn’t exist anymore so we can’t go and check it. We have to only look at a study based on what we already know – and there are some things that are unknowable, even if the building’s debris was there.

Q: Most of the steel was removed before you were able to visit the site. Is there anything sinister about this?

14. Not really. At the time 9/11 happened the primary agency that decided to study the failure was the Federal Emergency Management Agency, Fema, in partnership with the American Society of Civil Engineers who pulled together a private sector team of experts – world class experts. NIST was, one of our staff members was a member of the team that was pulled together jointly by the ASCE and Fema.  And that team did visit the site 3 weeks after the failure, 2 or 3 weeks after 9/11. But you have to keep in mind a few things. One the primary focus of the days and hours immediately after 9/11 was on search and rescue. That was the highest priority accorded by the local government and all of the agencies working together at the time. It was felt that during that time access for other purposes would be very difficult and that was why it was not possible.

15. Secondly to accomplish the search and rescue in the most efficient fashion possible, and in the most timely fashion possible, the decision was made I think by the City of New York in conjunction with the Port Authority (who were the building owners) and Silverstein Properties that were leasing the buildings to remove the debris as fast as possible so they could in fact do their search and rescue. And for removing the debris that included structural steel they assigned 3 or 4 salvage companies to take the steel and transport it to the salvage yards. NIST was given the legal authority to conduct a full investigation by Congress after it held its hearings in the spring of 2002. And we in fact initiated an investigation 11 months later after we received funding from Congress and the authority from Congress to do so. And during the interim we actually helped transport -went to the salvage yards – searched for pieces of steel that would be of interest to us, worked in partnership with ASCE and Fema. And in fact transported a lot of the steel that we have – somewhere in the order of 236 pieces – to our site here outside of Washington DC over a period I would say of 10 to 12 months.

Q:  Are you satisfied that you had enough steel to test and draw conclusions.

16. I think we need to step back a little bit. This is probably the most comprehensive and most complex and sophisticated building failure investigation ever conducted. In most prior building failures or construction failures the problem is very isolated. And in most cases people I don’t think have done the amount of due diligence that we have done to collect pieces of steel because the goal is never to reconstruct a failure physically. That has never been done in building construction where you obtain all pieces of steel and try and reconstruct from looking at the reconstructed building. That would have been an endeavour that might have taken ten times longer, ten times larger in terms of budget costs, more costly, and that’s not done usually. What is done is a field investigation looking at the evidence, and then beyond that some amount of analysis and modelling. What we did is far more comprehensive. In fact we transported these pieces of steel.

17. By the way these pieces of steel were not selected ad hoc. They were selected with great care in terms of their relevance to the study. And what was relevant to our study was to make sure that we had pieces of all different kinds of steel from the building. That was in the order of 14 different types of steel that were used in the building and we had them all. And that helped us to carry out tests in our laboratories to determine the properties of those steels; to determine if those properties were consistent with what was required in the design and to use those properties in our analysis. For example we needed properties at room temperature to see if the buildings had been designed properly for wind. We needed properties for fast rates of loading at very high speeds to model the airplane impact. And we needed the properties at high temperatures to model thermal properties as the fires spread in the buildings.

18, So I think we have done a level of study that has not been done before. The only example I could see is in the case of airplane crash. They did try to reconstruct TWA 800 – I think that went into the water. And that study took long, it took over $50 million just to get the pieces of the airplane to try and reconstruct the bomb. We were not looking at a bomb. We knew what the source of the problem was, which was the airplane impact. There was substantial evidence for that. We didn’t really need to understand what caused the building to collapse. We knew what started the process.

Q:  It has been suggested that the steel columns were cut by explosives. Was there any test you could have conducted for explosives?

19. Well let me tell you. The fact that we had 236 pieces of steel does not mean that’s the only pieces of steel we saw. We went to the salvage yards and we also went to a special hangar at JFK Airport where there were pieces of steel. We have probably looked at several orders of magnitude more pieces of steel from the buildings in the salvage yards and at the JFK hangar. So what we have is only a small fraction of what we have reviewed and seen. And this steel was seen not only by NIST experts , it was seen by experts from the private sector as well. There were leading engineers and designers in the US who design tall buildings, who investigate failures of buildings, who investigate fires and so forth. So these people went and saw all of the evidence, most of the evidence that was at the salvage yards, over multiple visits and did not find any evidence that there was blast related evidence in any of that steel. I mean you don’t need to do, its not essential to do a chemical analysis to look for residue. If blast did play a significant role in the collapse of the buildings then you should be able to see that evidence in the pieces of steel that you can see, in the manner in which the steel is fragmented. You should be able to pick that up. And I’m going to guess there were `a large number of people in the order of at least 20 to 25 people at different times who visited the salvage yards and none of them, absolutely none of them have found and reported that they saw evidence of a blast.

Q:  Are you saying that if there were any tell-tale signs of a blast your people would have investigated further.

20. Yes we first absolutely look at tell-tale signs. If there were any tell-tale signs that sent red flags up then obviously as you suggest we’d have gone further and looked at additional testing to see in fact if they was residue and that sort of stuff. There was no sign. A lot of the steel that was there in fact had the primer paint on it. And that’s red colour – the chemicals there. In fact we used changes in the primer paint to determine what temperatures they reached. But clearly the impact of a blast would have been substantial in terms of the fragmentation of the steel pieces and we really did not see any such evidence.

Q: The buildings were designed to withstand multiple airplane impacts. What do you say about that.

21. The buildings were designed according to code, the New York city building code and the worst case design condition is a hurricane hitting New York city. And we reviewed the hurricane wind loads that were used in the original design of the buildings. And we found that the hurricane wind loads that were used in the original design of the buildings were substantially larger than what was required by the New York City building code. They had  actually done wind tunnel tests and used more accurate estimates of what the wind load should be. We did an independent review of what the wind load should be based on some wind tunnel testing that was done after 9/11 on those same buildings by other private sector laboratories. And our estimates of wind loads – design wind loads for those buildings – in fact were very consistent with what was used in the original design, were different by about 5 or 10 percent, which in turn were much larger than what was required by the building code.

22. Now with regard to specifically your question that these buildings had been designed to withstand airplane impact, yes we did see a couple of documents. And I would say there were about two documents each about three pages long which suggested such an analysis had been done, and that the buildings would have survived the impact of a Boeing 707 airplane at the time maybe flying at 600 miles an hour. But it appears that what they did not account for in that analysis was the impact of the fires. They said there would be local damage structurally, not an overall collapse. But it appears that the issue of fire that resulted from the jet fuel ignition was not considered in that analysis. I should hasten to say this is not required in any building code but they seem to have done this study. I also need to hasten to add we found lots of evidence of the calculations they have done for wind and a lot of other data that we have. We found very little substantiating calculations or analysis with regard to the design of these buildings for airplanes – other than the two pieces of paper I have just said each about three pages long.

Q:  The buildings came down fast and smoothly into their own footprints.  Does this raise any suspicion of a controlled demolition for you?

23. Let me speak again to the towers. The two buildings were hit in the top third of the building – both WTC 1 which was hit around the 90th floor and WTC 2  which was in I believe the upper 70s. The impact of the fires were in the top third – around 4 or 5 or 6 key floors. Its a 110 story building and the top third had probably 30 to 35 stories on the top and the fire was mainly raging in about five or six floors. The collapse initiated in that top third of the building in the fire impacted region. We have irrefutable evidence, video evidence and photographic evidence – that we have gone public with – which shows where that initiation took place. Now when that initiation takes place at that height what you have is about 15 to 30, 25 floors above that region. So you have a section of the building that’s about 20 stories tall – huge mass with all the furniture there is – coming down. And if you just do a very simple calculation you will see the amount of potential energy that is released by that mass and that potential energy is converted into kinetic energy. You can do the calculation, its very simple.

24. That kinetic energy has to be resisted by the elastic and plastic strain energy of the structure below. If you calculate the strain energy capacity of the structure below you will find its very small to absorb this huge kinetic energy that’s been unleashed. And so its not at all surprising that the big piece of structure, the building that’s coming down, would just crush the building down below, and floor by floor and pulverize everything it sees on its way. And that’s exactly what it did. So it appears like a controlled demolition because you have air or puffs of white dust coming out every time you pulverize a floor, and again you suddenly jump to the next floor and you fall to the next floor and so forth. What happens is the concrete is getting pulverized, the fireproofing is getting pulverized, the furniture is getting pulverized and everything else in between. So that is being sent out in jets, much like whirlpool jets, and you see whirlpool jets coming out on the sides. And that is exactly what we believe is the primary source for people’s perception that there was some kind of a controlled demolition. We don’t have any evidence there was anything other than a normal failure of the building as a result of the airplane impact and the fires.

Q: Did you actually study the collapse itself?

25. Well the primary processes that took place are the initiation of the collapse. The primary things that happened to the building are the things that happened from 8.46 in the morning in the first building till 10.29, and for the second building from 9.02 till 9.58. That is what set the stage for the collapse. The physics of the problem has to do with understanding the physics between that time frame. Once you get to the point of initiation, once you get to the point that this large block of building at the top has started moving downward, the calculations are obvious. You don’t need to do a calculation to figure out that the kinetic energy far exceeds the strain energy of the capacity of the structure below. This is the equivalent of adding two plus two equals four in elementary kindergarten school.

Q: So there was no need to scrutinize the collapse because the top would simply crush the rest of the building below?

26. Absolutely, absolutely.

Q: Can you give an overview of how the buildings collapsed?

27. The main reason for the collapse of the buildings was because of the airplane impact followed by the damage to the structure and the fireproofing which was then followed by a horrendous fire that weakened the structural columns and the floors and caused the building at the top to  initiate collapse. Subsequent to that gravity, gravitational forces, brought the building down vertically.

Q: Did the 9/11 Commission come to the same conclusions before you?

28. The 9/11 Commission did not do an analysis of the building collapses. Most of their work was focused on a whole range of other issues. There was an area where we did overlap and that has to do with the emergency response on 9/11. In fact Chapter 9 of the 9/11 Commission Report is based on a bunch of interviews that we did with over a hundred first emergency responders from the fire department of New York and the police department of New York and the Port Authority police. Those interviews were scheduled by NIST in cooperation with the City of New York and with the Port Authority. 9/11 Commission staff were present when we carried out those interviews which typically lasted two, two and a half hours. Once we had finished our interviews they would follow up with about half an hour of interviews, questioning. That was the typical model of all those interviews. So the basis of Chapter 9 in the 9/11 Commission Report is very consistent with -is based on the same interviews we report in our project 8 – which has to do with fire service technologies and guidelines. But the 9/11 Commission did not study the collapse of the building itself.

Q: And yet they came to the same conclusion about the collapse of the towers?

29. They had access to the Fema, earlier Fema report, the ASCE/ Fema report that was written by a group of about 20 private sector experts as I’ve mentioned before.And that was also the conclusion reached in terms of the failure mechanisms based on their six month study and they did not do a rigorous analysis. They did a review and some simple calculations. That was their conclusion as well. So in many regards our study actually has gone on to provide the specifics of how things happened, and in fact in many regards, reinforce those findings  from the private sector ASCE/ Fema study with a lot more detail.