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Part of my justification for delaying examination of mechanical resistance is that it doesn’t have to be the truss seats that failed. If we imagine a falling pile of rubble between perimeter and core, rather than a regular pancaking/stacking of whole floor slabs, it’s the centre of the long floor spans that are most vulnerable; the rubble would smash through rather than decoupling floor assemblies from the vertical core and perimeter columns at the truss seat connections.
If indeed the floor slabs were getting smashed through, that’s entirely within the realm of “deformation of materials”, which is where kinetic energy dissipates to when inelastic collisions occur. In this scenario, momentum transfer and overcoming mechanical resistance are pretty much the same thing.
Whichever failure demands least energy is the one that will occur quickest; a chain is only as strong as its weakest link.
Remember that we’re considering failure, but the ordered geometry of the building is its main source of strength, especially in structures that are as lightweight as possible like the Twin Towers were. Any event which disrupts that order weakens the structure. The second law of thermodynamics is on the side of collapse – disorder tends to increase.