Fire induced progressive collapse of tall buildings

Abstract eng:
The tragic events of September 11, 2001 that culminated in the total collapse of three very tall buildings in the space of a few hours, left many unanswered questions for structural engineers in their wake. The recently concluded investigations by NIST[1] have partially addressed some these questions but have also raised many new ones. A much smaller scale parallel investigation has also been carried out at the University of Edinburgh [2, 3] primarily to answer specific questions about the role that fire may have played in the collapse of these buildings. In the course of this primarily computational work progressive collapse mechanisms have been discovered whereby tall buildings may suffer progressive or disproportionate collapse because of the effect of fire in single or multiple floors. This paper will explain these mechanisms which have identical antecedents but result in different final outcomes, described formally here as a weak-floor collapse mechanism or a strong-floor collapse mechanism. A structural frame with a weak floor system is shown to fail progressively in a buckling mode, whereas frames with stronger floor systems fail by forming hinges in the perimeter column. This produces interesting analogies with earthquake engineering design in terms of the weak-beam strong-column concept, which will be explored in the paper. The conditions under which these collapse mechanisms may occur in general frame structures will be discussed and simplified analysis methods will be presented in order to identify the risk of such a collapse in existing or future tall buildings.

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