TECHNICAL NOTES
Stability of the World Trade Center Twin Towers Structural
Frame in Multiple Floor Fires
A. S. Usmani1
Abstract: This technical note presents the results from a nonlinear finite element analysis of a very simple two dimensional model of the
World Trade Center Twin-Towers structural frame subjected to fire. The analysis was carried out for a large range of fire scenarios and was
reported in detail in a recently published paper. The paper further investigates the results of this analysis to obtain the details of the
collapse mechanism found. An interesting series of events leading progressively to overall collapse are discovered and described in detail.
The main reason for the failure is found to be the low membrane capacity in compression of the composite steel truss and concrete deck
slab floor system.
DOI: 10.1061/͑ASCE͒0733-9399͑2005͒131:6͑654͒
CE Database subject headings: Buildings, high-rise; Fire resistance; Structural response; Progressive failure; New York; New York
City; Collapse.
Introduction
The collapse of the three largest buildings in the World Trade
Center ͑WTC͒ complex has posed new questions over the stability of large steel frame composite structures in fire. It is therefore
important to understand the possible causes that may have led to
these collapses in order to ensure the safety of existing and future
structures in fire. A recent paper ͑Usmani et al. 2003͒ highlighted
a possible collaspse mechanism of the external frame of the WTC
twin-tower structure as as result of a fire on multiple floors. This
technical note extends those findings to shed further light on the
collapse mechanism discovered. It should be noted that the aim of
this ͑and the previous͒ note is only to develop a better understanding of the mechanical response of the frame of the twin towers
and possible collapse mechanisms. There is no specific implication that the collapse mechanism described here may actually
have occurred or had been...