Reply #207: Not the entire floor.

[VTMechEngr]

The planes hit perpendicular to the area involved. If anything, the floors helped channel the debris right into the core columns. Sure, some of the debris punched through, but 4 inches of concrete is not a whole lot. The floors were made that thick to properly encase the re-bar, and provide a solid surface for heavy office equipment. The floors were not made to withstand an impact, just support the needs of offices above and hanging pipes, ducts, and cabling below.

The columns did the work of supporting the building, the floors just support a floor of stuff.