Dr Asif Usmani, a structural engineer at the University of Edinburgh, proposed a new theory in 2003 suggesting that thermal expansion could have explained the collapse of the World Trade Centre towers ( http://technology.nzoom.com/technology_detail/0,1608,196263-113-380,00.html ). We’re not qualified to say whether he’s correct or not, but it’s an interesting idea, and as some time has passed since his original paper we decided to send him an email question or two:
Have the last two years left you more or less sure that your hypothesis was correct? And what has helped to confirm your opinion, or change your mind?
The following day, Dr Usmani replied. This is the full text of his response.
Date: Mon, 05 Dec 2005 11:33:13 +0000
From: Asif S Usmani
To: Mike Williams
Subject: Re: WTC Query
I and my students and co-workers have written a few more papers since the original FSJ paper and the conclusions derived from the FSJ paper have only been strengthened. In all our work our aim has simply been to understand how the WTC1&2 towers structural system would have responded to a large fire (ignoring any aircraft impact damage). Our computational analyses have repeatedly shown that it was possible for the towers to have collapsed simply because of a large fire of moderate temperatures (600-800C). Here large means in extent, i.e simultaneously over whole floors and over 3 or more stories (as was roughly the case on 9/11). Most analyses of the fire also show that the temperatures were not very high.
So the results of the analyses and the fact that the structure had sustained significant damage lead to a very strong case that the impact damage coupled with the subsequent fires were the only causes of collapse that rational minded people should need consider. The vertically downward collapse (which looks like a controlled demolition) is simply because once a large enough mass starts moving (accelerating because of gravity), it does not change direction unless met by a very large resisting force. The forces generated by a large moving mass are orders of magnitude larger than its weight (called dynamic amplification in engineering). Professor Bazant of Northwestern University (Illionois) explained this well in his paper soon after 9/11.
Behaviour of structures in general and structures in fire in particular is a complex and very specialist engineering mechanics field that requires years study and research to understand well. It is unlikely that any of the conspiracy theories emanate from people who understand these issues.
Our results were initially surprising to us as all our previous research had shown that in general steel frame structures respond very well to fire. 9/11 is the first ever and the only occasion when large steel structures collapsed mainly because of fire (and in case of the 46 storey WTC7 building, only because of fire). However, these structures were quite different from ones we had studied earlier and this was demonstrated by the analyses quite clearly.
Unfortunately structural design of structures to resist fire has been predicated entirely on "protecting" the structure from it, i.e. insulation. No attempt is made to quantify the response of the structure if subjected to fire. This is entirely different from how structures are designed to resist other loads, such as furniture/people, high winds, earthquakes etc. Protecting structures from fire in general leads to very conservative (over-safe) designs, but as no quatification of response is undertaken, no one really knows if a tall building is really safe from fire or not. This in my opinion
is a very dangerous practice and it is only good fortune that more large building failures have not occured because of fire.
I wish you success in your endeavours
No change of opinion, then -- according to Dr Usmani, the fire and structural damage were enough to do the job..