Your jumping to a conclusion without covering the middle ground - are you really saying its literally impossible for the structure to be weakened enough to collapse with a controlled mechanism but still strong enough to survive the impact? personally I don't think we have conclusive evidence that this can't be engineered even if it seems unlikely on the face of it.
i would have to agree with acid, due to the capablities of vibration and resance (cant spell that word to day) Most building have to be otherly strong to conteract the chaninges in load that happen from the smallest of things air pressure humidity.
the towers we hit by a plane, the moment of the force on the aircraft was so great that it caused the concrete to split and twisted the metal structural supports. then the explosion of aircraft fuel striped the concrete from the supports results in a flexible material that cannot stand compression holding up the rest of the tower, gravity kick in and exploits weakness, towers fall, everyone without basic physics knowledge blames the government and gets tin hats.