the external steel was put togther from pre-fab sections/units...forming an interlocking grid/wall/structure. the exterior was not made from continuous steel I-beams or any such thing. it was put and welded/rivetted/bolted together in sections. these seams being possible weak points/the weak points. the interior of the towers were very much open except for the center elevator shaft/main weight supporting/stabilizing part of the building. the concrete and steel truss floors would provide internal resistance to the planes hitting the building's exterior...thus, creating flex/stress and/or breaking points where the external tubular steel sections joined (especially, between the floors). and so, they failed...proportionate to the force applied to them. if, some exterior WTC material appears to be pointing outward in these openings...i would say it was from the fireballs' blowback/explosions...the outward force of the fireballs/explosions wrought on the building and not the plane's inward largely linear force/momentum/trajectory.
supposedly, the towers occupied roughly a full square acre of space. the area they were built on and enclosed/covered. their ground level "foot print". that's big.
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