Why the World Trade Center collapsed: an interview with Asif Usmani, professor of Structural Engineering at the University of Edinburgh

by D.B. Cooper. An Italian translation is available here.

Today we publish an interview with Asif Usmani, Professor of Structural Engineering at the University of Edinburgh and author of some of the most influential papers on the causes and dynamics of the collapse of the World Trade Center.

We think it is a very interesting interview: not only it clearly explains the collapse and why it cannot have been caused by a controlled demolition of any kind, but at the same time it shows that while there is no doubt among specialists as to the general nature of the collapse, there is still a lively debate about its exact details (e.g., what was the exact role of the structural damage caused by the planes, and which structural elements of WTC7 were the first to fail), proving that there is no such thing as an "official version" passively accepted by the scientific community.

Undicisettembre: Thank you for your time and availability. Would you first like to introduce yourself?

Asif Usmani: I am a Professor of Structural Engineering at the University of Edinburgh, School of Engineering.


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

Asif Usmani: There is no consensus on the exact cause of the collapse amongst experts in the field. In general the causes espoused by the various experts have a bias towards their own expertise. Therefore, while not absolving myself of the same bias, my contention is that the collapse was caused by the fires that took hold in the buildings after the aircraft impacts.
The mechanism of how the collapse may have initiated is explained in a number of my research papers.


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?

Asif Usmani: This view is contrary to elementary mechanics (inertial forces exerted by moving masses can be orders of magnitude greater than their static weight, which is primarily what building structures are designed to resist).


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

Asif Usmani: Based on my theory the cause of collapse is identical for both towers, namely fire, but the exact mechanisms of initiation could be different. Given that WTC2 was impacted lower down, it had roughly double the load above it so it makes sense that it collapsed earlier and it also had apparently more extensive impact damage (with corner columns severed).

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

Asif Usmani: The upper block tipped over because WTC2 was impacted by the plane in the corner region severing more columns than in the case of WTC1, so when WTC2 began to collapse, the part of the structure with intact connections was pulled down by the collapsing structure and the portion above the severed columns was initially tipped to the side, but then fell back because of its own weight to fall vertically down with the rest.


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

Asif Usmani: Again there is no consensus on the actual mechanism. I am not fully convinced by the explanation provided in the NIST report. I suspect that the failure of transfer structures (bridging the building over the Con Edison power station) is a more convincing cause, than simply the failure of one column (column 79 at the 13th or 14th floor). The failure of transfer structures could have occurred because of potential oil fires but there is not sufficient evidence for that from the pictures and videos (perhaps because significant portions of the building facade were obscured by surrounding buildings).


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?

Asif Usmani: Again this contradicts basic physics. Whether the collapse is caused by controlled demolition or because of fire there will be little difference in the time it takes as gravity rules once the collapse starts and as mentioned earlier buildings are not designed to resist inertial forces exerted by moving masses.


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?

Asif Usmani: It should be possible to distinguish as the collapse mechanisms could be
very different. In a controlled demolition one could theoretically make a building collapse in an infinite number of ways, some of which may look like a natural collapse.


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

Asif Usmani: Depends upon the building, a building can be designed to resist fire without protection while a protected building using traditional rules that border on the unscientific can be unsafe in a fire, as I believe the WTC structures were and who knows how many more such structures are out there. Statistically however unsafe structures would be a very small minority as traditional approaches to fire protection are in general extremely wasteful of resources and produce overly conservative designs. So in most cases you could have a much safer structure at much lower cost if rational and scientific design approaches are used, however this requires regulatory changes and many more better educated and trained engineers to do it.

Undicisettembre: What do you think about conspiracy theories that claim that the Twin Towers and WTC7 were demolished with explosives or thermite? Are they even vaguely plausible?

Asif Usmani: As mentioned earlier, with the proviso that I have no knowledge of explosives, it may technically be possible to carry out demolitions that look natural. However it would never work the first time on a specific building exactly as planned, because of the enormous number of uncertainties in the highly chaotic and unpredictable process of a building collapse. The only way it could work is if they made replicas and tested the demolition scheme extensively first. Clearly the logistics of all this makes this theory entirely implausible.


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?

Asif Usmani: The collapse of building is a complex phenomenon, you cannot write a simple formula for it. However I have personally published quite a few papers with relatively simple explanations of tall building collapse because of fire which should help answer this kind of question. The following is a list of some of them.
1. Stability of the World Trade Center Twin Towers structural frame in multiple floor fires, Journal of Engineering Mechanics, ASCE, Vol 131, pages 654-657, 2005.
2. A very simple method for assessing tall building safety in major fires, International Journal of Steel Structures, Vol 9 pages 17-28, 2009.
3. Tall building collapse mechanisms initiated by fire: Mechanisms and design methodology, Engineering Structures, Vol 36, pages 90-103, 2012.
4. The World Trade Center 9/11 Disaster and Progressive Collapse of Tall Buildings, Fire Technology, Vol 49, pages 741-765, 2013.


Undicisettembre: Do you think that, apart from what a layperson would consider technical details, we have a wide consensus within the scientific community about the causes of the collapses?

Asif Usmani: As mentioned earlier there is no consensus in the scientific community, being humans even the scientific community has its biases. The best one can say is that most people believe that fires caused the collapse. Some people think that the damage caused by the aircraft in combination with the fire caused the collapse. I am inclined towards the view that if there was no fire there would have not been a collapse; and the corollary to that is, had there been no aircraft impact and the WTC1 or WTC2 structures was subjected to a large accidental fire, they could have collapsed. This is the really scary part as it begs the question, how many other such buildings may be out there.

Undicisettembre: How did the tragedy changed our understanding of fire safety measures for tall buildings, and the relative regulations?

Asif Usmani: So far nothing much has changed. I have proposed very simple engineering checks (reported in some of the papers above) but this is yet to be accepted. More research needs to be done and more people need to be saying the same thing before regulations change to enforce these sort of checks.