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.

22 commenti:

Chris Sarns ha detto...

Charles Clifton: What happened with the World Trade Center 1 is … the plane destroyed a large chunk of the core immediately at impact and severely weakened the rest of it.

This is incorrect. And since the rest of his theory is built on this fallacy, it is invalid.

Cumulative structural damage to floors and columns of WTC 1 - NCSTAR 1-2 p. 189
3 core columns severed
4 core columns severely damaged
5 core columns moderate damage
1 core column light damage
34 core columns undamaged

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?

Get that man a dictionary.
A conspiracy is an illegal or subversive act planned by two or more people. That has nothing to do with the laws of physics which is what the question is about. "bin Laden 19 hijackers" is a conspiracy theory but "The Trade Towers and building 7 were controlled demolitions" is a collapse theory, not a conspiracy theory.

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.

No one has shown that scenario to be valid. Bazant's analysis requires 12 feet of free fall but that requires explosives to remove all the supporting structure simultaneously. Forty plus bending steel columns preclude free fall so Bazant's theory is invalid.

Charles Clifton: The first loud bang could have been the top giving way.

A collapse due to column failure would be a creaking and groaning noise, not a bang. An explosion is a better explanation for the first loud bang.

Charles Clifton: 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.

Incorrect: Molten aluminum and lead are silver/gray in daylight. NIST admitted that aluminum glows silvery in daylight but falsely claimed that aluminum mixed with organic material, causing it to glow orange.
A 21. http://www.nist.gov/el/disasterstudies/wtc/faqs_wtctowers.cfm

There is no known case of aluminum mixing with organic material and no scientific evidence to prove that could happen - but there is a simple experiment that showed it does not. http://www.stj911.org/jones/experiments_NIST_orange_glow_hypothesis.html
NIST and Mr. Clifton cannot just make baseless proclamations. They must show with experiment that their hypothesis is valid.

Charles Clifton: The fire itself would have gotten to over 1000 degrees (centigrade)

There is no evidence to support that claim.

"The microstructures of the steels known to have been exposed to fire, based on the pre-collapse photographic evidence, were characterized. These microstructures show no evidence of exposure to temperatures above 600oC for any significant time." NCSTAR 1-3C p. 281

"From the limited number of recovered structural elements, no conclusive evidence was found to indicate that pre-collapse fires were severe enough to have a significant effect on the microstructure that would have resulted in weakening of the steel structure."
NCSTAR 1-3C p. 235

Charles Clifton: All I am certain of there is that it wasn’t steel.

Is that a scientific evaluation? What is your evidence? "It can't be because" is not scientific.

Charles Clifton: The initial impact had destroyed in the south east corner and along the east side some six levels of floors. … Finally, the perimeter frame along the Eastern side failed in an elastic buckling mode over some six stories.

This is incorrect. And since the rest of his theory of the South Tower collapse is built on this fallacy, it is invalid.
Floors 80 and 81 were damaged by the fuselage between the outside wall and the core. The east floor area between the core and exterior wall was undamaged. NCSTAR 1-2 p. 230

Leonardo Salvaggio ha detto...

Chris Sarns,

This is incorrect. And since the rest of his theory is built on this fallacy, it is invalid.

That's the "base case". You probably ignore what it is. Check what the same document says at page 204.

"The core had extensive damage in the region close to the impact point. The columns in line with the aircraft fuselage failed on the impact side, and several of the core beams were also severely damaged or failed in the impact zone. In some cases, failure of the column splices located on floors 92, 95, and 98 contributed significantly to the failure of the core columns."

If this is your level of attention I could argue the rest of your comment is invalid.

Get that man a dictionary.

Well, I guess you are the one who needs a dictonary. It would help you understand what the NIST said about the "base case".

No one has shown that scenario to be valid. Bazant's analysis requires 12 feet of free fall but that requires explosives to remove all the supporting structure simultaneously. Forty plus bending steel columns preclude free fall so Bazant's theory is invalid.

Answer by Charles Clifton: "The collapse started with top of the building unzipping from the perimeter frame and free falling onto the floor below which was also close to the point of failure. All the floors above the impact zone in WTC1 were close to failure so when the first one went and fell onto the others there was negligible resistance available to prevent an unzipping collapse"

A collapse due to column failure would be a creaking and groaning noise, not a bang. An explosion is a better explanation for the first loud bang.

Answer by Charles Clifton: "That’s probably correct but the collapase was not initiated by a column failure."

Once again, you don't know what you are talking about.

They must show with experiment that their hypothesis is valid.

Answer by Charles Clifton: "In my opinion it is not possible from the video footage I have seen of the molten metal to say from the colour and flow pattern what type of metal it was. I should have said “could have been” rather than “would have been” molten aluminium or lead. I have seen melted aluminium from fires in buildings (window frames melting) and that is in a single storey house fire and also molten aluminium from smelting but the video footage is not clear enough to say what the molten material was. From my extensive knowledge of fully developed fires in buildings I can say the fire temperature at that location would have been around 1000 Deg C and that structural steel melts at over 1600 Deg C. "

Is that a scientific evaluation? What is your evidence? "It can't be because" is not scientific.

Answer by Charles Clifton: "It is based on the melting point of steel being much higher than the peak temperatures that would have been reached in the fire. The 1996 Cardington Demonstration Furnature Test set out to generate the highest fire temperatures possible in an office type building; peak temperature at any place of recording was just over 1200 Deg C and peak average temperature just over 1000 Deg C."

Floors 80 and 81 were damaged by the fuselage between the outside wall and the core. The east floor area between the core and exterior wall was undamaged. NCSTAR 1-2 p. 230

Answer by Charles Clifton: "This buckling mode of failure has been recorded in photographs and recreated in numerical modelling. I call it an elastic buckling mode as it is generated by Eular buckling of the columns due to cumulative loss of lateral support along the east side. In practice it would have been a mixed elastic and inelastic mode of failure but generated through buckling instability of the frame along that side."

Leonardo Salvaggio ha detto...


one more thing:

The east floor area between the core and exterior wall was undamaged. NCSTAR 1-2 p. 230

That's the base case, again.

I found this description you give of yourself "I saw the video of WTC7 for the first time about a year ago. I recognised immediately that it was a controlled demolition. Since then I have dedicated my life to getting out the truth about 9-11."

Amazing! You were able to discover such an intrigue just by watching a video!

Ok, no one can take you seriously.

Leonardo Salvaggio ha detto...


1) You ignore what a "base case" is and you also ignore my remark

2) You don't even understand that the phrase "conspiracy theory" was used by me, not by Clifton (it was in the question, is it too difficult to notice?)

3) You think you are able to distinguish a natural collapse from a controlled demolition only by watching a video on Youtube

4) You keep quoting the NIST Report (which explains why the Tower collapsed because of impact and fire) as source of information to support your claims that it was a controlled demolition (this is simply delirious)

So I hope you don't mind if I don't consider you a serious collocutur, so please stop sending messages because they won't be taken into account.

Go back to investigating on Youtube.


Chris Sarns ha detto...

I now know what base case is and I quoted the more serious case. But it did not alter the points I had made - that Mr. Clifton had made false statements.

I corrected YOU for incorrectly labeling a collapse theory as a conspiracy theory and your not knowing that OBL and 19 hijackers is a conspiracy theory.

Anyone with a knowledge of structure who has seen a few controlled demolitions can see that WTC 7 is a controlled demolition.

The NIST report does NOT explain how the towers collapsed. They stopped at collapse initiation.

You won't post my comments because I showed that Mr. Clifton was wrong many times.

Vaya con Dios

Paolo Attivissimo ha detto...

Mr Sarns,

Anyone with a knowledge of structure who has seen a few controlled demolitions can see that WTC 7 is a controlled demolition.

A classic Dunning-Kruger effect. In your opinion, experts are blind but the less competent have 20/20 vision.

Does it not seem strange to you that all the real experts in the field disagree with the "controlled demolition" theories?

Consider the options:

a) all the world's structural engineers are incompetent, wrong or corrupt and you an a bunch of non-experts are right

b) you are wrong and the world's structural engineers are right

Ask yourself which of these scenarios is more plausible. Have a nice day.

Chris Sarns ha detto...

Mt. Sarns: Anyone with a knowledge of structure who has seen a few controlled demolitions can see that WTC 7 is a controlled demolition.

Mr. Attivissimo: A classic Dunning-Kruger effect. In your opinion, experts are blind but the less competent have 20/20 vision.

You cannot assume that "that all the real experts in the field disagree". There is NO evidence to support that outlandish assumption.

A majority of experts that have gone on record agree with me. There are over 2,000 architects and engineers who do not accept the official explanation have signed a petition calling for an independent investigation into the possibility that explosives were used.

What experts are you talking about? How many experts have gone on record supporting the NIST "WTC 7 collapsed from normal office fires" explanation other than the 13 who signed off on the final report? Those who just contributed cannot be counted because they had no say in the final decision and they have not publicly endorsed it. They only vouch for their contribution, not the conclusion.

Dave ha detto...

Chris Sarns wrote "Anyone with a knowledge of structure who has seen a few controlled demolitions can see that WTC 7 is a controlled demolition."

Thanks for this superb example of the "No True Scotsman" logical Fallacy!

papageno ha detto...

Chris Sarns(08/2/14 20:42)
"A conspiracy is an illegal or subversive act planned by two or more people. That has nothing to do with the laws of physics which is what the question is about. "bin Laden 19 hijackers" is a conspiracy theory but "The Trade Towers and building 7 were controlled demolitions" is a collapse theory, not a conspiracy theory."

This is the kind of word games creationists played, when they tried to pass off 'intelligent design' as a scientific theory. It did not fool anyone, because anyone could see that it was just creationism in a clown suit.

And you are not fooling anyone here: the only people trying to sell the "controlled demolition theory" are conspiracy theorists.

Chris Sarns(08/2/14 20:42)
"The NIST report does NOT explain how the towers collapsed. They stopped at collapse initiation."

More pathetic word games.
They showed how the collapse started, which was the engineering problem they wanted to understand, because the whole point of building safety is to prevent or at least delay collapse initiation.

Instead of providing valid alternatives for the collapse initiation, like real engineers would do, conspiracy theorists cling to any possible excuse to complain about the bad, bad "official story". Basically, they keep throwing their poo at the wall, hoping that some will stick.

Leonardo Salvaggio ha detto...


as I have already said no one can take you seriously because you are not debating the issue with the seriousness it deserves. So please stop sending long and useless comments because they won't be published.

Leonardo Salvaggio ha detto...

By mistake I rejected a comment by Uknown, this is the full text of his comment:

Did Richard Gage come to New Zealand and speak to hundreds of people in Christchurch, Wellington and Auckland (as Charles Clifton admits) or did he not?

If Richard Gage actually came to New Zealand in 2009, who paid the thousands of dollars it would have taken to get him here, print 10,000 flyers, organise articles in local papers and the interviews on TVNZ and Radio NZ? Such was the interest in Mr Gage’s presentation that in November 2009 over 600 people attended the exhibition at the National Museum in Wellington with another 100 turned away due to lack of room. The Auckland presentation organised by Auckland activists at short notice after the Wellington presentation attracted 200. Mr Gage’s New Zealand tour was subsequently written about in articles in the Listener, Architecture Magazine, discussed on Radio New Zealand’s afternoon panel and talked about on science and political blogs.

Who runs and financially supports the NZ911Truth website and has distributed many thousands of DVDs and flyers regularly every month on the streets of Wellington since 2008? Who organizes the annual 911 exhibitions in Wellington and Rotorua which attract growing numbers every year?

Who paid for the recent Rethink911 posters displayed on the streets near the Radio New Zealand and Dominion Post newspaper offices in central Wellington? Who invited Mr Clifton to the Auckland presentation and has corresponded with him since? In addition to a good number of activists throughout New Zealand there are also hundreds of New Zealanders who have signed ae911truth’s petition calling for a new, genuine 911 investigation.

We have given up long ago on trying to disabuse Charles Clifton of his Magical Thinking on 911. However, we must refute his false assertion - a statement he knows to be false - where he denies that New Zealanders who support the work of ae911truth actually exist. He has had enough contact with us and has sufficient knowledge of our activities to know that he is making a false statement.

and this is Charles Clifton reply to this comment:

Yes the reader is correct that Richard Gage’s visit attracted large numbers. It was the first visit from anyone with an international profile in regard to the 9/11 attacks and as such had considerable attractive power.  However the central tenet of his assertions was not taken seriously by any of the official engineering organizations or by more than a handful of individual engineers and the interest died very quickly after his visit. That is what I meant by my comments.

We hold directly opposing views and they gave no arguments that undermined or called into question my understanding of the sequence of events leading to each collapse. Equally I could not convince them to revisit their deliberate sabotage theory. That was the frustrating bit and it was not worth carrying on the discussion.

They put a lot of effort into getting people along to Richard Gage’s visit and were successful in getting good numbers. However there was little agreement with him and the interest died down very quickly after his visit.

Paolo Attivissimo ha detto...


Scusate sto interessandomi da poco tempo di questo argomento. Avete mai analizzato le questioni sollevate da questo video?
Sto cercando di farmi una opinione informata su questo argomento e mi sembra che qui siamo in presenza di considerazioni fondate basate su una semplice questione fisica.

Ho rimosso il link al video dal tuo commento per non regalare pubblicità all'ennesimo video di rimontaggi complottisti senza basi tecniche. La "questione fisica" è talmente semplice che se fosse vero quello che sostiene il video, l'intera popolazione mondiale di ingegneri dovrebbe essersi rimbecillita per non accorgersene. Dato che questo è altamente improbabile, si presume che ci sia un errore nelle asserzioni dei complottisti. Ma al di là di questo semplice ragionamento non mi spingo e credo che non voglia spingersi nessuno per manifesta perdita di tempo.

Se un sostenitore delle tesi alternative, invece di fabbricare video su Youtube, riuscirà a farsi validare e pubblicare un articolo tecnico su una rivista di settore peer-reviews, seguendo insomma l'iter di qualunque discussione scientifica, allora varrà la pena di esaminare le sue tesi insieme agli esperti. Fino a quel momento no.

ALBERTO49 ha detto...
Questo commento è stato eliminato dall'autore.
ALBERTO49 ha detto...
Questo commento è stato eliminato dall'autore.
Leonardo Salvaggio ha detto...


Ah, ho capito, tu sei davvero sicuro che si tratti di rimontaggio? Ma sicuro davvero? E puoi dirmi dove ne avete parlato sul vostro blog, anche della questione, visto che non ho trovato nulla sull' argomento?

"Rimontaggi" nel senso che sono sempre i soliti video proposti dai complottisti, e il sito che hai linkato tu propone video triti e ritriti già postati da altri.

Vedo che siete molto sicuri e tranchant e la cosa, detta da uno che ha 40 anni di esperienza in grandi aziende, e che ha dovuto sempre dimostrare le cause dei fatti, non depone a favore di una immagine professionale.

Tiratela di meno. Sono ingegnere anche io, lavoro per una multinazionale anche io e ho molti partner commerciali. Non credere di avere conoscenze ed esperienze superiori agli altri, perché stai facendo una pessima figura personale.

Fra le altre cose, si parla nel vs blog solo di velocità di caduta per l' edificio 7 e non per gli edifici principali ovvero quelli del video. Solo per chiarezza.

Mai hai letto il blog prima di commentare? Ne abbiamo parlato in moltissimi articoli tra cui le FAQ, Zerobubbole e le interviste a Clifton, Usmani, Coppe e Blanchard.

Paolo Attivissimo ha detto...


ho respinto il tuo commento offensivo e inutilmente polemico per evitarti di fare una pessima figura.

Se vorrai, ne riparleremo quando ti sarai dato una calmata e avrai dati tecnici concreti, al posto di vaghe e strampalate congetture, su cui ragionare.

Paolo Attivissimo ha detto...


di nuovo, ho respinto il tuo ulteriore commento. Il tema di questo blog è l'11 settembre, non i bisticci e gli insulti. Eventuali altri tuoi commenti di questo stesso tenore verranno cestinati senza ulteriori richiami.

Se vuoi che i tuoi commenti siano pubblicati qui, sii civile, non insultare, presenta documentatamente le tue tesi. Non è difficile.

ALBERTO49 ha detto...

Questo link al lavoro di Zdenek P. Bazant, non funziona.

Esattamente come quest' altro, sul suo curriculum.

Per cui aggiornateli.

Il paper di Bazant, non so se originale o semplificato, è qui.

Senza fare tante altre polemiohe col sottoscritto che non ne aveva alcuna voglia, dopo la mia cortese richiesta che ho lasciato, per dimostrare la mia assoluta correttezza, bastava che mi citavate il lavoro di cui sopra, il quale ha il pregio di mostrare senza ombra di dubbio che il collasso strutturale delle torri gemelle è avvenuto per cause assolutamente ovvie e dimostrate ad abundantiam.



Leonardo Salvaggio ha detto...


ho aggiornato i link.

Grazie della segnalazione.


ALBERTO49 ha detto...

Credo sia opportuno inserire, non so onestamente dove nel blog, anche questo lavoro di H.Y.Hung, Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China, che mi sembra non apparire e che apporta notevoli info sugli aspetti legati al collasso a causa dell' indebolimento strutturale delle due torri principali.

brain_use ha detto...


Personalmente lieto tu abbia trovato la documentazione che cercavi e che abbia dimostrato che chi cerca "risposte", non appena trova la documentazione tecnica ormai disponibile da molti anni, abbandona subito le panzane complottiste.

ALBERTO49 ha detto...

Non sempre accade ed i motivi sono legati a mio avviso, soprattutto in Italia, e non solo in questo specifico campo, tutto sommato di nicchia, alle carenze di analisi dei media. Pensate al terremoto di Casamicciola per esempio, bufale di varia natura, incluse quelle del sistema sismologico. Credo che il punto focale della questione WTC, al netto di altre meno o punto importanti, sia quella che mi sono posto dopo aver visto La7, da tecnico e su cui Mentana, secondo me, avrebbe dovuto spendere un tempo maggiore ed al limite organizzare un confronto tra le parti in conflitto.
Io sono venuto sul vs. blog per accelerare la ricerca di documenti tecnici inconfutabili, vista l'analogia molto insidiosa tra una demolizione controllata ed un collasso strutturale; questo lavoro di Bazant, al du là dell'impressionante evidenza dwi carichi dinamici, porta anche le curve relative a variw tipologie di collasso dinamico. Ciò che cercavo.
Buon lavoro!