Why the World Trade Center collapsed: an interview with Charles Clifton, professor of Civil Engineering at the University of Auckland

by Hammer. An Italian translation is available here.

The collapse of the Twin Towers and of Building 7 at the World Trade Center is one of the most controversial aspects of the events of 9/11. Proponents of conspiracy theories often back their arguments with claims that are apparently precise and scientific but are actually based on superficial and often incorrect knowledge.

To clear the field of these misconceptions, Undicisettembre has interviewed Charles Clifton, associate professor of Civil Engineering at the University of Auckland (New Zealand), who published an independent study on the collapse of the Twin Towers.

As you can read below, Clifton's professional opinion dispels all of the doubts raised by conspiracy theorists regarding the three collapses and makes it very clear that for the experts there is no mystery at all.

We wish to thank Charles Clifton for his kindness and for making himself available for our questions.

Undicisettembre: Hi Charles, thanks for your time and availability. Would you first like to introduce yourself?

Charles Clifton: My background is in performance of steel and composite steel/concrete buildings in severe events, that has been my university background right the way through my career from my Canterbury University undergraduate years, with emphasis on earthquakes. From 1983 to 2007 I was with the New Zealand Heavy Engineering Research Association and since 2008 The University of Auckland.

My areas of expertise are performance of steel buildings in severe earthquakes and severe fires and corrosion of steel.

The combination of the knowledge gained from performance of buildings and earthquakes was very useful for my later research into the performance of buildings in fire. By the time the WTC attacks occurred I'd been involved for ten years very intensively in fire engineering research including developing procedures for allowing composite floors to be put in place safely with supporting beams not insulated against fire, using a failure mechanism I will describe in detail later in regard to the WTC7 failure. That was very helpful in understanding and in analyzing the sequence of events involved in the World Trade Center collapses. We published information very quickly after those attacks to explain to the engineering profession what was likely to have happened to cause the buildings to fail. New Zealand has a performance based building control system which encourages use of Fire Engineering Design and so it was important to understand what had happened and why, in order that we could learn as much as possible from these collapses.

There was an insurance dispute over the payments for the WTC collapses. The insurer initially tried to claim that the collapse of the South Tower caused the collapse of the North Tower and the property developer, who also is a major developer in New Zealand but is originally an Australian company, commissioned a detailed independent investigation into the collapses of WTC1 and WTC 2 to refute the insurer’s contention that the WTC2 collapse precipitated the WTC1 collapse. The consulting engineering company that undertook this investigation published a very short summary of the sequences behind the collapse of the two Towers. This almost exactly matched what I had previously written so I wrote to the Lead Engineer on that investigation, explained who I was and sent copies of what I had written, to show I was contacting them with a genuine interest and for legitimate reasons. He sent back to me a full copy of their report. They gave a lot of information and that allowed me to pack together a fairly detailed package of information and to confirm things that I got right and to make modifications to some of the things that I got wrong.


Undicisettembre: Can you please explain us what caused WTC1 to collapse?

Charles Clifton: Before giving this explanation I've got to give some details of the nature of the construction of the buildings, because that's pivotal for the collapse sequence. WTC1 had a perimeter frame around the four sides of the building that took all the lateral loading; in conjunction with a network of columns in the middle of the building, called the core, that took one half of the vertical loading from the floors. So there was a core in the center and a very stiff and strong box around the outside, and then the floors spanned between the core and the perimeter frame. The core was gravity only, so it was designed to take half of the vertical load of the floors and to provide support for all the services. The perimeter frame was a network of very close steel columns and deep beams to carry the lateral loading and so was extremely stiff and strong in-plane. Both the perimeter frame and the core are supported off each other at intervals up the height of the building by the floors; when you have vertical elements carrying the weight of a building you have to provide lateral support to these at regular intervals to stop them from buckling. Typically this support comes from the floors which are rigid elements that tie the building together, at each floor level.

The whole building was prefabricated in sections. They prefabricated the frames in large elements and bolted and welded them together on site. The floors were basically a two way truss system, very lightweight truss with a lightweight steel deck on top and then lightweight concrete on that. So the floors were basically a two way steel truss system with a concrete topping; they were connected to the core and to the frame with a relatively light angle connection which was site welded; there was a corbel bracket welded onto the columns of the core and the perimeter frame and at each end of the main trusses there was a welded connection between the end of the truss and the column corbel and then there was minimum reinforcement in the floor slab that connected into the frame. The connections were relatively light in the sense that they were designed to carry the vertical loading from the floor into the core and the perimeter frame and to tie the building together under normal conditions. They weren't designed to hold together in the event of severe distortion or overloading; there is no reason why they should have been.

So there was this very light floor construction, very stiff external frame and quite a dense network of columns in the middle: half of the load from the floor goes inward and into the core and the other half goes out into the perimeter frame.

What happened with the World Trade Center 1 is that the plane hit between the 93rd and the 99th floors, flying pretty much level and cut straight though the perimeter frame. The floors offered effectively no resistance to the plane at all and so the core took much of the impact, so the plane destroyed a large chunk of the core immediately at impact and severely weakened the rest of it. There are some cellphone recordings of people who subsequently died, they were trapped at the top of the North Tower, who said that at the moment of the impact the center of the building sank slightly and made getting into the lifts and the stairs impossible immediately. The loads from the upper floors that under normal conditions would have been transmitted to the core had to find another path so they tried to go out to the perimeter frame, this started to increasingly overload the connections between the floors and the perimeter frame; so the floors started to try and hang off to the perimeter frame. The connections started to be overloaded in all of the floors above the impact zone until finally something gave way. Because all of the floors above the impact zone were close to the point of failure it would have taken just one small event to trigger a near simultaneous collapse of each floor. The floors fell down inside the perimeter frames, portions of which remained standing for a few seconds before they collapsed in on top of the void where the floors had been.


Undicisettembre: What do you think about conspiracy theories according to which the upper block could not have enough momentum to make the whole building collapse?

Charles Clifton: From a momentum point of view, if one floor collapses on another in a building, the force that that floor invokes on the floor below is slightly greater than what the floor below is designed to withstand. So if one floor fell into another floor the chances of the first floor causing the second floor to collapse are possible but not particularly likely. But if the second floor is almost on the point of failure when the first falls on it they will both collapse. If you have two floors falling on a third one the ability of the third floor to hold the top two is very low. By the time you have three floors falling on a fourth is just not physically possible for the fourth floor to hold three. So by the time you have twenty floors falling on the intact floors below they will just go straight through.

There were three firemen in the stairs of the North Tower low down that survived the collapse. They said that the collapses sounded first like a loud sort of explosion and then like an out of control train, so they could hear the crunch and this crashing noise getting increasingly faster. The first loud bang could have been the top giving way and then the crunch would have been the mass falling on each floor and destroying them. I cannot guarantee but I'm pretty sure that was the sequence of what the noises were.


Undicisettembre: Can you please explain us what caused WTC2 to collapse?

Charles Clifton: While in the case of WTC1 the fire itself had very little influence on the collapse, it was the weakening of the core leading to the floors hanging in tension and leading to the failure of the building, in the case of the South Tower the fire did play a significant part in the collapse.

What happened there is that the plane hit between the 77th and the 85th floor at the south east corner of the building and it only caused severe damage to the core in the vicinity of the south east corner. Survivors from above the impact region used the stairwell which was the farthest away from the point of impact to escape. However the impact destroyed a large portion of the floors over six or seven levels. The debris from the plane and the office contents were pushed up towards the north east corner and started a severe fire. There were reports of molten steel but in fact it would have been either molten aluminum which is 660 degrees centigrade or potentially lead from storage batteries but not steel, which melts at over 1600 degrees. The fire itself would have gotten to over 1000 degrees, a typical fire in an office does get to over 1000 degrees; fires in offices are usually much hotter than most people think. Failure of individual steel elements in a fire starts at around 550 degrees but whole buildings in steel can get much hotter, without failure, as testing has shown.

In 1995 in Cardington in the UK researchers built, inside a large test facility, an eight story steel frame building and set a series of increasingly severe fires in the building to determine what would happen. In the last of those fires they tried to generate the most severe fire possible with the design fire load for an office and managed to get the air temperature to 1200 degrees centigrade. The floor above it was not protected and sunk significantly but held together and fulfilled its role as a stable floor, and the steel members in that floor reached over 1100 degrees centigrade. So things get much hotter in a fire than most people realize.

In the impact region on World Trade Center 1 the maximum temperature reached by the fire wouldn't have been higher than 450 degrees centigrade, at that temperature steel is just starting to weaken but the core was so heavily damaged by the impact that any weakening at all would have been very detrimental.

In World Trade Center 2 the fires would have reached the full fire severity you get in an office and it would have been over 1000 degrees very easily and that's hot enough to melt quite a few metals that go into computers and to melt aluminum of the window frames. Steel itself still maintains some degree of strength at 1200 degrees and doesn't melt until it gets to 1600 degrees. So there's no question that any steel melted in the buildings. Something that looked like molten metal was seen at the World Trade Center 2 from the north east corner and that may have been the window frames melting.


Undicisettembre: In an article we wrote several years ago we made the hypothesis that since at that floor there were big Uninterruptible Power Supply batteries maybe the glowing fountain was also made of molten lead from the batteries.

Charles Clifton: Yes, you could be right, that is more likely than the window frames. Lead melts at a lower temperature than aluminum and it would explain the localized source of this relatively large quantity of molten metal. All I am certain of there is that it wasn’t steel.

The initial impact had destroyed in the south east corner and along the east side some six levels of floors. Since vertical elements of the building, walls and columns, rely on the floor for lateral support now the area on the eastern side of the building lost lateral support for six consecutive floors and at the same time it was affected by severe fire. Also the core in that corner was weakened, transferring the load from the floors above to the perimeter frame. Finally, the perimeter frame along the Eastern side failed in an elastic buckling mode over some six stories.

This elastic buckling failure caused the top of the Tower to tilt. As soon as the top tilted, the connections between the floors and the walls were torn apart, there's no way those connection could withstand any degree of tilt. So as soon as the top starts to tilt the floors above detached and came up loose and started to fall down and then you have the same sequence as in the North Tower, except that some of the top fell outside the line of the perimeter frames on the East side.

So the initiating mode for World Trade Center 2 was an elastic buckling failure along the whole eastern side of the building. There was another characteristic of the building that made it vulnerable, if you look at the building you see that that dense network of columns didn't extend around the corners, the corners actually had quite large windows. If you take a shoebox and put a lid on it and twist it it, the sides that come into the lid try to move past each other. If you take a square hollow section and cut a slot in it, so it's still square and hollow but now it has a cut on on one side, and then you twist it the two sides move past each other. It's called shear lag effect. If you tie buildings with perimeter frames together at the corner you get extra stiffness, but generate very high internal actions at the corners. You either have to tie the two sides together very robustly or you keep them separate and allow them to move relatively to each other. In this case, the perimeter frame didn't need the extra stiffness of tying the frames together but keeping the frame separate and having windows on the corners gave the most valuable office space to rent, because these are corner offices with the best view.

This meant that the east frame was not tied back to the north or south frames at the corners, making it vulnerable to collapse when the floors were destroyed by the impact at the south end of this frame and the floor to frame system degraded by fire at the north end. The loss of lateral support from the floors initiated the buckling collapse.


Undicisettembre: What do you think of conspiracy theories which claim that once the upper block started tipping over it should have kept rotating and this proves that the towers were intentionally demolished with explosives?

Charles Clifton: That's physically impossible because of the very limited strength of the floor to frame and floor to core connections. To rotate as a rigid body floors had to remain rigidly attached to the frame and to the core and there's no way that that could happen. Those connections were never designed to handle anything like that, as soon as the top started to rotate the floors were torn straight out.


Undicisettembre: Can you please explain us what caused WTC7 to collapse?

Charles Clifton: The collapse of World Trade Center 7 is the most complicated by far, it needs a bit more of explanation of the construction of the building. In World Trade Center 1 and 2, the floors were a very lightweight two way truss system. This was very innovative for its time and even now. World Trade Center 7 was a much more conventional composite floor with a concrete slab on a steel deck and supported on a network of steel beams and girders. These beams and girders were structurally tied into the concrete slab with shear connectors, which are headed steel rods that are welded into the top of the steel beam and embedded in the concrete slab and prevent slip between the slab and the steel beam.

So you put the network of steel beams up, then you lie the decking down and then you weld shear studs through the decking to connect the decking to the beams. And then you pour concrete over the whole thing and when the concrete hardens around the shear studs, the concrete and the decking and the beams are combined together into what we call a composite floor system.

The decking is very light weight and designed to carry the wet load of the concrete during construction at a relatively short distance between the supporting beams, so you have a network of beams at typically three meters (10 foot) centers and they support the decking. And those beams frame on to bigger beams, typically at right angles, so they carry the load on to the columns and down to the ground. We call the closely spaced beams “secondary beams” or “beams” and the beams they span on to “primary beams” or “girders”. “Beams” and “girders” is the North American terminology and I will use it for the rest of this discussion. So you have a network of beams and then they span on to girders that carry the load down into the columns. The load path used in design is slab to beam to girder to column.

The WTC7 building, as you know, was not rectangular, it extended out on the north side, so the core was not in the middle of the building. On the North side between the core and the North edge of the building there were three very big interior girders, that spanned on to very large columns. This line of three girders carried approximately ¼ of the total floor area on each floor. And those girders had a network of beams coming in from each side. Normally beams span onto girders at close to right angles on each side. So, when you have a girder in the middle of the building, you have a series of beams coming from the right and a series of beams coming from the left; so the girder is effectively held in position in plan by the beams. In the case of World Trade Center 7, the very large girder that fell off its connection and initiated the final failure sequence had beams coming in from the left side at nearly 90 degrees, but on the right side the beams changed orientation and came into the girder almost parallel to it. So in effect these very large interior girders had beams framing in from one side only. However, all the other interior girders had beams coming in from close to right angles on both sides. Because of the unusual orientation of this girder relative to the beams, the girder was non-composite to the floor slab and did not have any connecting shear studs into the concrete slab.

Normally we use bolted connections for the end connections to these beams and girders. These are called “simple” or “shear” connections in that they are designed to carry vertical loads only between the beams/girders and the columns and they are designed to be quite rotationally flexible. When you put load on a beam, it sags between supports and the ends rotate. With simple connections you want to make sure that that rotation doesn't introduce moments into the columns, as this increases the design demands on the columns. What they did for those girders, because the vertical load to be carried from the girder into the column at the girder ends was so high, instead of using a normal bolted connection they used a heavy corbel coming out from the column and they set the beam onto the corbel. And that connection was used for the three very large girders that each supported this very large area of floor.

The other thing I have to talk about is tensile membrane action, as in my opinion it was an important mechanism in explaining the final collapse sequence of WTC7. If you take a piece of thin A4 cardboard, such as the backing on a writing pad, and support it on two pencils (in engineering terms these are roller supports), one at each end, and then put a weight in the middle of it (such as a rubber) it will sag down onto the desk under the rubber; but if you take two more pencils and you lay them along the other two sides that you haven't initially supported, so that all 4 sides are now supported by pencils (roller supports), and you put the same weight on it you'll find out it won't sag to the desk; in fact it will hardly deflect at all. To make it sag to the desk when all four sides are supported you will have to increase the weight by some 10 to 15 times. Floors usually are designed for one way action, but when you build it you actually have a two way system, so floors usually can carry much more weight than they are designed for under large deflection. In a fire or when you have loss of internal supporting columns but the edges of the floor are effectively supported against downwards movement you generate this extra reserve of strength, which increases with increasing deflection until something finally breaks and leads to collapse. It is called tensile membrane action and it greatly increases the load carrying capacity of the floors in a severe fire.

Another way of visualizing this action: For instance, you have a lot of domed buildings in Italy where you have a compression dome and then a ring beam around the outside the dome to stop the dome from collapsing. If you turn the dome upside down and you replace stone with something that can carry tension and you replace the ring beam with something than can carry compression, then you have tensile membrane action. Structurally it's the inverse of the dome.

A composite floor system behaves like that on fire, if you leave the beams unprotected but you support the four sides you can get very large increases in fire resistance from what you'd get from a standard one way spanning system.

That's vital in my opinion to explaining why World Trade Center 7 collapsed uniformly over its whole length.

The final point needed to explain the WTC7 collapse is the use of transfer members in that building. Most of the times in a building, the columns are in the same position in plan from the top to the bottom and so carry the accumulated compression loads down the columns to the ground. You don't want to have to change the position of a column in plan because that's structurally very expensive, as you need very stiff and strong beams or trusses to carry the high column loads across to the new supporting columns. These are called transfer members. What happened when they built World Trade Center 7 is that there was an existing electricity substation that supplies power to lower Manhattan and that substation had to be kept operational throughout the construction and subsequently. Elements of that substation were in the position where columns in the core should have gone, so they had to build these transfer members to take the weight from those columns and transfer it across the substation and down into the ground on each side. Those transfer members were very big beams or trusses, they were massive systems.

In 1988 there was a fire in a building in Los Angeles, it was the First Interstate Bank, some 60 stories high, and the first thing the fire did there was to cut the electricity into the building, so the emergency pumps had no diesel that kept them working so the Fire Service personnel had to manually supply the diesel to the building pumps. So after that, they commissioned a review in America to the emergency systems and put multiple levels of redundancy to the fuel supply to make sure this didn't happen again.

So what happened with the substation in WTC7 is that after the attacks it was destroyed but the fuel supply system continued to operate and fed fuel into the fires at substation level. These fires burned for some seven or eight hours, and would have progressively weakened at least some of the transfer members. There is supporting eyewitness accounts that say that there was damage to the core in the interval between the Twin Towers' collapses and World Trade Center 7 collapse.

So on the basis that the core was progressively damaged following the WTC 1 collapse, the final collapse sequence was initiated by the girder spanning between columns 79 and 44 that fell off its corbel seat either at level 12 or level 13. It was pushed off to to the west as the fire came around the North East corner of the building and heated all the secondary beams on the north side where they spanned into the girder at a right angle. Now, normally a beam or a girder cannot be pushed around in a fire like this because it would be connected by shear studs into the decking; and for an internal girder if it was pushed on one side the beams on the other side would stop it from moving sideways; but in this case you had girders with beams coming at right angles on one side but with no corresponding beams on the other side. Also, as mentioned above, there was no shear studs connecting the girder to the floor slab, so it could be pushed sideways relative to the slab.

So it was pushed around on its corbel. Probably, while it was being heated it would also have been pushed into the column as it expanded on heating, while at the same time deflecting downwards, but as the fire started to burn out and cool down in that region the now deflected girder would start to cool down and reduce in length. This would have led to its falling off the corbel and initiating the collapse. There are eyewitness accounts of noises that corroborate the collapse beginning at the point. What happened then is these girders fell off their corbels, collapsing the floors around these three big columns that supported in total about a third of the floor area, and the collapse starts moving across towards the core. As these columns fail, the floors at the top of the building progressively loose internal support, while the perimeter frame remains intact and the connections of the floors to the frame are much more robust than they were in the Twin Towers, because the WTC7 connections were standard composite beam to column connections. So those floors start to resist the loading from the progressive loss of internal support by tensile membrane action, with increasing load being carried by the outer perimeter frame.

There was a penthouse at the top of WTC7 that was observed to disappear from sight some thirty seconds before the final collapse. This was caused by downwards deflection of the top floor away from the perimeter frame. This would have generated increasing tensile membrane action occurring over the full extent of the building with the perimeter frames acting as the vertical supports. It would have involved vertical deformation of two to three meters in the interior regions of the floors and that would have been enough for the Penthouse to disappear from view from below, but with the whole system remaining temporarily stable in the increasingly deformed state. Meanwhile, the collapse progressed across the building internally taking out the core and progressively dropping support to the perimeter frame with the top floors hanging increasingly in tensile membrane tension across the whole building. Finally the perimeter frame failed in the lower levels (probably around the levels 12 or 13 where the initial interior girder collapse occurred) due to loss of lateral support. So then the building drops down uniformly as observed. The difference between WTC7 and the top of WTC2 is the much greater robustness of the floor system and floor to perimeter frame connections, which allowed tensile membrane action to develop in WTC7 when it couldn’t develop in WTC1 or WTC2 due to the weakness of the floor to perimeter frame connections.

Basically the perimeter frame finally gave way down at the lower levels of the building (probably around level 12 to 13, where the initial girder off supports occurred), causing the whole building to come straight down.

There is still an unanswered question on WTC7 in my opinion, which was that when the girder in the North East corner fell off its corbel and initiated the final collapse sequence, why did the core not constrain the collapse to the east side of the building? The first theory was that the fires in the substation beneath the core weakened the core and triggered the collapse but the NIST report states that didn’t happen, based on the observed amount and intensity of smoke released from the base of the building during the interval those fires were burning. However, computer simulations with the core intact don’t generate the final observed collapse sequence. I think the NIST report is wrong in this regard and that those substation fires did cause a progressive failure in the core, so that when the gravity system failed on the north east side and put more load suddenly into the core, it gave way at the lower levels. The two way strength of the slab would then have temporarily supported the upper floors off the perimeter frame long enough for a complete failure to occur at the lower levels both internally and externally, causing the building to drop straight down. This two way strength of composite slabs is now being recognized and designed for in fire and is very large compared with the traditional one way load path mechanisms assumed in design.


Undicisettembre: What do you think about conspiracy theories that claim that the collapse of WTC7 was too fast to be caused by fire and damages from the previous collapses?

Charles Clifton: I've heard this theory and it puzzles me why they say that. Once the floors start to fall or the frame collapse occurs, the collapse will occur rapidly. The speed of the collapse is consistent with that collapse sequence, so it's not unreasonably fast.


Undicisettembre: Generally speaking, is it possible to distinguish a natural collapse from a controlled demolition based on the time the building takes to collapse completely?

Charles Clifton: No, it's not. If the collapse sequence follows the same sequence it would follow in case of a demolition that the time has to be same.


Undicisettembre: What happens to unprotected steel buildings in case of fire?

Charles Clifton: That's a very interesting question! Basically it depends on the type of building and how severe the fire is. If you protect the columns and have unprotected beams supporting the floors and the whole building has been designed for robustness (ability to sustain large deformations) in case of earthquakes (we call that ductility), the building would remain standing in a severe fire. In the Towers, the fire protection was removed from many columns in the impact region and they weren't designed for ductility on earthquakes as there was no reason why they should have been. The system used to connect the floors to the frames and the walls was a non ductile connection that cannot sustain being overloaded or plastically deformed. We use a similar floor called Speedfloor here, which is a light truss, but the connection between the trusses and the supporting system is designed for robustness in both earthquake and fire through additional reinforcement in the slab back into the supporting structure.

In the World Trade Center building they would have had no reason to do that. Even if there would have been an earthquake in the area, the buildings were so high and so flexible that they would have remained elastic during an earthquake. So there was no need to design those connections for ductility and it would have added quite an extra cost to the building.

In the new World Trade Center tower they put ductility in it but it's probably doubled the cost of the building. You can argue it's an overkill and I don’t expect that the level of robustness in the Freedom Tower will become standard practice in high rise buildings. However, I expect that additional ductility will become standard practice worldwide for these buildings – we do this in New Zealand for earthquake and fire. This ductility is imparted through design and detailing and is well established in our Standards for seismic design, as it is in the codes and standards of all most seismically active countries. We are implementing this year formal requirements for “fire ductile” detailing in a new Composite Standard under development.


Undicisettembre: Conspiracy theorists keep demanding to "see the math" that proves that buildings can collapse and can do so as rapidly and completely as we saw on 9/11. Is there a mathematical formula (or a set of formulas, or laws of physics) that explains, at least in principle, the collapse of buildings? Or to rephrase, how would you answer such a request?

Charles Clifton: I'm sure the answer is “No there isn't”. It depends totally on the structural system of the building and on the kind of damage and how the building reacts to the damage. The very different collapse mechanisms of WTC1, WTC2 and WTC7 are good examples of this variability.


Undicisettembre: Are conspiracy theories popular in New Zealand?

Charles Clifton: No, they are not. Richard Gage came to New Zealand for a speaking tour. I couldn't go to his presentation due to other commitments but he got very little publicity, little credit and not much following. As far as I am aware there is not a serious group of WTC conspiracy theorists in New Zealand.