Exactly as the post below, they were designed to take an impact of an airplane when they were built. A tower within a tower so to speak. The core columns were immense! If you think the steel was weakened 80 odd floors below then think again. Aircraft are not strong structures on impact, sure the main wing spars are immensely strong to carry the weight of the wings and fuel, but also flexible enough to deal with movement.  A good comparison is the airliner that hit a block of flats in Amsterdam, google it and you will see large elements of the airframe recognisable, and the block of flats still standing to a point. But for me it's the impossible speeds, unattainable at the heights and pressure altitude quoted. It's the known immoveable facts of Vmax (no aircraft can travel at over 150kts and higher) without specific aerodynamic forces act on the airframe, like flutter and catastrophic failure, control surfaces wouldn't work. To do what we are told happened at the Pentagon with the official story is pretty much impossible.  So many unanswered (truthfully) questions, it is nonsense to believe it happened the way we were told.