Abstract
The 9/11 terrorist attack on the New York World Trade Center (WTC) towers
caused an unprecedented tragedy in the history of architecture. The twin
towers WTC 1 and 2 stood in flames caused by jet fuel until finally, both
collapsed totally to the ground with thousands of people trapped in the
buildings. Both towers collapsed at an unnaturally high speed, which was
observed to be nearly equal to that of free fall. Official statements have
already been released by the Federal Emergency Management Agency (FEMA)
in 2002, and also by the National Institute of Standards and Technology
(NIST) in 2005, regarding the incident. FEMA concluded that the heat of
burning jet fuel induced additional stresses into the damaged structural
frames while simultaneously softening and weakening these frames, and this
additional loading and the resulting damage were sufficient to induce the
collapse of both structures. Many detailed numerical analyses were carried
out in the NIST report, and the report was concluded by stating gthe WTC
towers likely would not have collapsed under the combined effects of aircraft
impact damage and the extensive, multifloor fires, if the thermal insulation
had not been widely dislodged or had been only minimally dislodged by aircraft
impact.h But how wide should the fire be spread in the towers to cause
such perfect destruction? How high should the heat be to reduce the structural
strengths of core beams and columns? What really caused the free-fall total
collapse? Many questions still remain unresolved.
Isobe and his collaborators are seeking for the true cause of the total
collapse by conducting several numerical simulations, including the full
model aircraft impact analysis of the WTC tower. Isobe suggested in the
paper that the spring-back phenomenon due to rapid unloading caused by
aircraft impact possibly have destructed member joints in the core structure,
which might have caused the towers to become brittle and unstable. However,
it does not give answers to the total collapse of WTC tower 7, which collapsed,
reportedly, only by fire.
In this study, the investigation is made from a different point of view
and the influence of fire and heat is taken into account. The main objective
of this study is to conduct some fire-induced collapse analyses using the
ASI-Gauss technique, to investigate how the structural strength reduction
of member joints gives influence on the collapse phenomena. It is to be
noted that the region of New York City rarely has earthquakes and high-rise
towers only need structural design to withstand wind loads. To investigate
the influence of how the structures are designed, the base-shear coefficient
which is the general index to indicate the earthquakeproof strength of
buildings is taken into consideration in the numerical analyses. Member-joint
strengths of the models are also expressed by changing the coefficients
in the yield functions. Fracture points of axial tensile strain, shear
strain, and rotational angle estimated from some experimental data are
used in the criterion of member fracture. Reduction rate curves of elastic
modulus and yield stress related to temperature shown by NIST are adopted.
The results of the fire-induced collapse analyses of high-rise towers show
a clear difference between the models with strong and weak member-joint
strengths. The strongly designed model withstands high temperature without
any sign of collapse, whereas the weak model initiates its collapse when
the temperature reaches nearly to the highest, and ends, eventually in
a total collapse.