The 208-foot wide facade is, in effect, a prefabricated steel lattice, with columns on 39-inch centers acting as wind bracing to resist all overturning forces; the central core takes only the gravity loads of the building. A very light, economical structure results by keeping the wind bracing in the most efficient place, the outside surface of the building, thus not transferring the forces through the floor membrane to the core, as in most curtain-wall structures. Office spaces will have no interior columns. In the upper floors there is as much as 40,000 square feet of office space per floor. The floor construction is of prefabricated trussed steel, only 33 inches in depth, that spans the full 60 feet to the core, and also acts as a diaphragm to stiffen the outside wall against lateral buckling forces from wind-load pressures.
See:
http://www.greatbuildings.com/buildings/World_Trade_Center.htmlWhen considering the above, then add a fully fuelled plane hitting the side of the building, the elevator shafts alone would have probably acted as a blow torch to heat up the steel framing of the building causing the structure to buckle and collapse.
Bombs in the building as well, I doubt it.
Firefighters hearing explosions from gas pipes rupturing might be a little more plausible.
