aviation fuel burns much hotter than most other fuels, around 980 C. With most steels completely melting around 1300 C, it's very possible that the heat could compromise the structural integrity of the steel lattice. not to mention the gaping holes put in the framework by the initial impact. though less likely due to the high initial temps needed, it's also possible that the aluminum from the aircraft and the iron oxide on the exterior of the WTC reacted together to form thermite.