How did the WTC towers collapse: a new theory

Abstract

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.

The World Trade Center 9/11 Disaster and Progressive Collapse of Tall Buildings

“…This study expands further the concepts that were presented by Usmani et al [10] based on the same assumptions. The parametric treatment of fire, representing…”

“…Finally, this work also showed the capability of OpenSees to perform progressive collapse analysis under fire since the same global collapse mechanisms seen in previous work [10,13,21] have been reproduced without any significant difference. A primary reason for OpenSees being advantageous for this parametric study compared to ABAQUS is that it is more computationally efficient.…”

“…The model presented in this paper is identical to the one used in Usmani et al [10], however a completely different software (OpenSees, [23]) is used. A twodimensional sub-structure representing a 12 storey slice of the tower along the longest span floor area is modelled, assuming the core to be rigid.…”

“…A summary of the structural design of the WTC complex and NIST's findings can also be found in the current special issue [5][6][7][8]. Other smaller scale independent research studies were carried out by Quintiere et al [9], Usmani et al [10], Kodur [11], Usmani [12] and Flint et al [13]. The FEMA report and Quintiere et al studied the large deflections that were developed in the composite floor during the fire but did not present a clearly defined collapse mechanism.…”

Structural safety in case of extreme actions

“…Other cases where this approach should be considered refer to the design of complex structures, where an additional knowledge on the structural behaviour can be required: it is the case, for example, of high rise buildings, which can be particularly susceptible to progressive collapse (Usmani et al, 2003;Izzuddin et al, 2007) or of infrastructural systems such as a long span bridge, where the loss of functionality or even a temporary closure would imply high economic losses and may have social repercussions (Bontempi et al, 2008;Petrini and Bontempi, 2010). This approach, which is referred to in the following as inductive or bottom-up approach, has been used in this study in order to perform some investigations of the sensitivity of some simple structural systems to the entity of the initial damage.…”