"Although the structural damage inflicted by aircraft was severe, itwas only local. Without stripping of a significant portion of thesteel insulation during impact, the subsequent fire would likelynot have led to overall collapse Bažant and Zhou 2002a; NIST2005. As generally accepted by the community of specialists instructural mechanics and structural engineering though not by afew outsiders claiming a conspiracy with planted explosives, thefailure scenario was as follows:
1. About 60% of the 60 columns of the impacted face of framedtube and about 13% of the total of 287 columns were severed,and many more were significantly deflected. Thiscaused stress redistribution, which significantly increased theload of some columns, attaining or nearing the load capacityfor some of them.
2. Because a significant amount of steel insulation was stripped,many structural steel members heated up to 600°C, as con-firmed by annealing studies of steel debris NIST 2005 thestructural steel used loses about 20% of its yield strengthalready at 300°C, and about 85% at 600°C NIST 2005;and exhibits significant viscoplasticity, or creep, above450°C e.g., Cottrell 1964, p. 299, especially in the columnsoverstressed due to load redistribution; the press reports rightafter September 11, 2001 indicating temperature in excess of800°C, turned out to be groundless, but Bažant and Zhou’sanalysis did not depend on that.
3. Differential thermal expansion, combined with heat-inducedviscoplastic deformation, caused the floor trusses to sag. Thecatenary action of the sagging trusses pulled many perimetercolumns inward by about 1 m, NIST 2005. The bowing ofthese columns served as a huge imperfection inducing multistoryout-of-plane buckling of framed tube wall. The lateraldeflections of some columns due to aircraft impact, the differentialthermal expansion, and overstress due to load redistributionalso diminished buckling strength.
4. The combination of seven effects—1 Overstress of somecolumns due to initial load redistribution; 2 overheatingdue to loss of steel insulation; 3 drastic lowering of yieldlimit and creep threshold by heat; 4 lateral deflections ofmany columns due to thermal strains and sagging floortrusses; 5 weakened lateral support due to reduced in-planestiffness of sagging floors; 6 multistory bowing of somecolumns for which the critical load is an order of magnitudeless than it is for one-story buckling; and 7 local plasticbuckling of heated column webs—finally led to buckling ofcolumns Fig. 1b.
As a result, the upper part of the towerfell, with little resistance, through at least one floor height,impacting the lower part of the tower. This triggered progressivecollapse because the kinetic energy of the falling upperpart exceeded by an order of magnitude the energy thatcould be absorbed by limited plastic deformations and fracturingin the lower part of the tower."

In sostanza qui si afferma che già a 600°C l'acciaio perde l'85% della tensione di snervamento, il punto in cui inizia a deformarsi plasticamente, e già a 450°C presenta una vistosa plasticità, segno che non serve che l'acciaio fonda per far crollare una struttura. Vengono altresì citate le strutture che nel momento dell'impatto furono distrutte (punto 1), aumentando il peso sulle restanti strutture, aumentandone così la debolezza e abbassandone il punto di rottura. Questo insieme agli altri 6 effetti presentati ha portato al collasso della parte superiore all'impatto, portando la struttura intera al collasso perché si legge che "the kinetic energy of the falling upper part exceeded by an order of magnitude the energy that could be absorbed by limited plastic deformations and fracturing in the lower part of the tower"
l'energia cinetica della parte superiore cadente superava di un'ordine di grandezza l'energia che poteva essere assorbita da deformazioni plastiche limitate e dalla fratturazione nella parte basse della torre.