### Aircraft Impact Analyses of the World Trade Center Towers

Abstract

Although
some official statements have been released by the
Federal Emergency Management Agency (FEMA) and the National Institute of Standards and Technology (NIST), the
9/11 tragedy of the New York World Trade Center (WTC) towers still remains as
an unresolved mystery since the progressive collapse sequence of the towers was
unnaturally rapid. We sought the true cause of the collapse by performing
full-model, dynamic collapse analyses of the WTC towers due to aircraft
collision. The numerical code used in the analyses is based on the ASI-Gauss
technique, a modified version of the formerly developed adaptively shifted integration
(ASI) technique, which computes highly accurate elastoplastic solutions even
with the minimum number of elements per member. The ASI-Gauss technique gains
still a higher accuracy particularly in the elastic range, by placing the
numerical integration points of the two consecutive elements forming an
elastically deformed member in such a way that stresses and strains are
evaluated at the Gaussian integration points of the two-element member.
Moreover, the technique can be used to express member fracture, by shifting the
numerical integration point to an appropriate position and by releasing the
resultant forces in the element simultaneously. The numerical results show
instant disconnections between the core columns and the main double trusses
during the impact, and a spring-back phenomenon of the core columns due to a
very rapid unloading is observed. The spring-back phenomenon is fatal enough to
produce a gigantic tensile force on the columns, which might have triggered the
total fracture of some connections, eventually leading to the total collapse of
the towers.

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