One problem for controlled demolition theories at the WTC is the shortage of structural engineers offering support for them. This is sometimes explained as people not wanting to speak out, disturb the status quo, maybe they or the company they work for would face serious consequences... But presumably this doesn’t apply to engineers outside the US, or at least not all of them. So what is the view from abroad?
Asif Usmani, a structural engineer from Edinburgh University proposed a theory, suggesting that collapse could be initiated through thermal expansion with temperatures of 600-800 degrees C (perhaps even less). Here’s a report of a talk he gave on the subject.
Dr Asif Usmani, a structural engineer at the University of Edinburgh's School of Engineering and Electronics, told an engineering conference the twin towers that had dominated the Manhattan skyline seem to have been "unusually vulnerable" to a major fire.
"There was a vulnerability in the design of the structural system. It is not materials. It is not about fire protection. It is about the design of the structure," he told Reuters.
In a computer-based analysis of the buildings, Usmani studied the structure of the towers that collapsed after they were hit by two hijacked planes in attacks blamed on Osama bin Laden's al Qaeda network.
"We are not analysing the event. We are saying let's look at the structure itself and see if it has anything unusual that makes it more vulnerable to fire than other structures," he explained.
Usmani said the towers' exterior and internal columns, which make up the core of the building, were joined by a very long and thin expanse of floor. Supporting the floors were lightweight trusses which, when exposed to fire, expand.
But because of the way the towers were built, the trusses did not have anywhere to expand without buckling.
"This was good for constructing a building very quickly and they were strong enough to carry all the loading that was put on them. But when there is a fire underneath the trusses and the steel gets hot...it expands quite a lot and those expansion forces can be quite high if the restrains to expansion are high," he added.
Even at low temperatures of about 200- 300 C (392-572 F) the steel can begin to expand and because the building itself is keeping them in place pressure builds up to a critical point.
"This caused a buckling of the floors which was providing not only the load carrying capacity for the furniture and the people but was also providing lateral support for the columns," Usmani explained.
"That lateral support vanished as soon they buckled and that is why the building fell in on itself."
Here’s an Abstract of the relevant paper from the Fire Safety Journal.
This paper uses a finite-element model to investigate the stability of the Twin-Towers of the World Trade Center, New York for a number of different fire scenarios. This investigation does not take into account the structural damage caused by the terrorist attack. However, the fire scenarios included are based upon the likely fires that could have occurred as a result of the attack. A number of different explanations of how and why the Towers collapsed have appeared since the event. None of these however have adequately focused on the most important issue, namely ‘what structural mechanisms led to the state which triggered the collapse’. Also, quite predictably, there are significant and fundamental differences in the explanations of the WTC collapses on offer so far. A complete consensus on any detailed explanation of the definitive causes and mechanisms of the collapse of these structures is well nigh impossible given the enormous uncertainties in key data (nature of the fires, damage to fire protection, heat transfer to structural members and nature and extent of structural damage for instance). There is, however, a consensus of sorts that the fires that burned in the structures after the attack had a big part to play in this collapse. The question is how big? Taking this to the extreme, this paper poses the hypothetical question, "had there been no structural damage would the structure have survived fires of a similar magnitude"?
A robust but simple computational and theoretical analysis has been carried out to answer this question. Robust because no gross assumptions have been made and varying important parameters over a wide range shows consistent behaviour supporting the overall conclusions. Simple because all results presented can be checked by any structural engineer either theoretically or using widely available structural analysis software tools. The results are illuminating and show that the structural system adopted for the Twin-Towers may have been unusually vulnerable to a major fire. The analysis results show a simple but unmistakable collapse mechanism that owes as much (or more) to the geometric thermal expansion effects as it does to the material effects of loss of strength and stiffness. The collapse mechanism discovered is a simple stability failure directly related to the effect of heating (fire). Additionally, the mechanism is not dependent upon failure of structural connections.
This proves nothing, of course -- even Usmani only says that his conclusions are provisional. And we have no idea whether his theory is correct or not (in theory you’d expect the NIST computer models to be more accurate than anything Usmani could produce). However, as a University professor and structural engineer who lists his interests as "Computational structural mechanics using FEM, Analytical and Computational models for structures in fire, Computational heat/mass transfer and CFD using FEM" ( http://www.civ.ed.ac.uk/research/fire/asif.html ) he’s certainly as qualified to comment as most of the proponents of the WTC demolition theories.
December 2005 - we decided to check and see whether Usmani still believes his explanation is correct, and it turns out he’s more certain than ever. Read his email response here.
The Japanese Kajima Corporation produced a paper called "Structural Responses of World Trade Center under Aircraft Attacks" in January 2005. All that's available online, unless you pay for the document, is this abstract:
At the September 11, 2001, terrorist attacks on the New York World Trade Center (WTC) Towers, extensive structural damage, including localized collapse, occurred at several floor levels directly impacted by the aircraft. Despite this massive localized damage, each structure remained standing for approximately 1 h or 1 h 30 min. Although the damage to the beams and columns in the perimeter tube of each tower were clarified in the published ASCE/FEMA report, the damage to the floor system and inner core columns were not estimated. The purpose of this study is to determine why the towers remained standing after impact through several analytical studies, including impact analyses using a simplified model to estimate the overall damage, a rigorous finite element model to estimate the local damage, and stress analyses after some structural members are lost. The results of the stress analyses show why both buildings did not collapse immediately after impact, and WTC2 collapsed sooner than WTC1.
But it seems essentially the researchers modelled the towers, found redistributed loads after impact resulted collapsing floors, columns exceeding their yield stress, and progressive collapse (go here for a commentary). However, it’s important to note that the previous link says "many assumptions were made [in the building model, presumably], as the authors did not have access to much of the specific design data of the towers".
G Charles Clifton, a Hera structural engineer from New Zealand produced a paper at http://www.hera.org.nz/PDF%20Files/World%20Trade%20Centre.pdf . Despite clear warnings that "the explanation given is only my considered opinion,
based on the information available six to eight days after the event" (although some minor tweaks were made later), parts of his explanation are still a good match for the much later NIST accounts. In particular, he pointed out that the fires were relatively minor, but with some fireproofing stripped away a collapse could still have occurred.
The effect of the initial impact was so severe that the fire did not have to do much additional damage to collapse the buildings. It is likely that the fire was a more significant contributing factor for the South Tower, however for the North Tower this is less obvious and it was probably progressive shear failure of the system connecting the floors to the perimeter frames that initiated the final collapse.
In fact it is likely we will never be able to establish with absolute certainty the exact sequence between impact and collapse in either case. The above are simply my best guesses based on the information available and my experience of steel building behaviour in extreme events.
In 2002, Pryian Mendis (an associate professor with the Department of Civil and Environmental Engineering in the University of Melbourne) constructed a computer model of "a standard 52-storey Australian-designed building" and tested out what would happen under "extreme loading from bomb blasts or collision from a light aircraft". Their conclusions?
"Under the extreme loading experienced during such events most Australian tall buildings would probably suffer progressive collapse, which occurs when a section of the building fails to support the load above and triggers a cascade of failures leading to the collapse of most or all of the building. The Murrah building in Oklahoma, which collapsed in 1995 after a bomb went off, and the World Trade Centre suffered this fate," says Mendis.
Mendis is talking about "Australian-designed" 52-storey buildings, so this says nothing about the WTC, and doesn't prove that he's correct about Australian buildings. It does show that concern over progressive collapse isn't some kind of US-only mirage, though.
Meanwhile Dr Frank Greening, a Brit living in Canada, has produced a detailed study revealing his own views on what it took to bring down the WTC. Click here to take a look (PDF reader required).
And while none of these papers (or any others) will in themselves prove what happened on 9/11 (and we’re not qualified to assess the accuracy of any of them) , they do at least illustrate one point: it’s not only US institutions who think that the combination of fire and structural damage could bring down the towers.