1. About 60% of the 60 columns of the impacted face of framed-tube (and about 13% of the total of 287 columns) were severed, and many more were significantly deflected. This caused stress redistribution, which significantly increased the load of some columns, near the load capacity for some of them.
2. Fire insulation was stripped during aircraft impact by flying debris (without that, the towers would likely have survived). In consequence, many structural steel members heated up to 600±C (NIST 2005) (the structural steel used loses about 20% of its yield strength already at 300±C, NIST 2005, and exhibits significant visco-plasticity, or creep, above 450±, especially at high stresses that developed; see e.g. Cottrell 1964, p. 299; the press reports right after 9/11, indicating temperature in excess of 800±C, turned out to be groundless, but Bazant and Zhou's analysis did not depend on that).
3. Differential thermal expansion, combined with heat-induced viscoplastic deformation, caused the floor trusses to sag. The sagging trusses pulled the perimeter columns inward (by about 1 m, NIST 2005). The bowing of columns served as a huge imperfection inducing multi-story buckling. The lateral deflections of some columns due to aircraft impact and differential thermal expansion also decreased buckling strength.
4. The combination of six effects
a) overload of some columns due to initial stress redistribution,
b ) lowering of yield limit and creep,
c) lateral deflections of many columns due to sagging floor trusses,
d) weakened lateral support due to reduced in-plane stiffess of sagging floors,
e) multi-story buckling of some columns (for which the critical load is an order of magnitude less than it is for one-story buckling), and
f) local plastic buckling of heated column webs finally led to buckling of columns (Fig. 1b). As a result, the upper part of tower fell, with little resistance, through at least one floor height, impacting the lower part of tower. This triggered progressive collapse because the kinetic energy of the falling upper part far exceeded the energy that could be absorbed by limited plastic deformations and fracturing in the lower part of tower. (Bazant, Verdure, 2006)