[Azure]

I can't say it much better...

Simply put: 1) Each structure sustained a high speed impact from a large airliner. This caused: A) Severe structural damage including the severing of exterior and core columns. B) Stripping of fireproofing material from floor trusses. C) Large fires starting simultaneously across multiple floors. 2) Fed by the jet fuel fires, and supported by the wind feeding into the gaping hole in each tower, the fires igniting the building contents, resulting in an inferno spreading across multiple floors. 3) The badly damaged, and unprotected floor trusses began to soften in the heat, sagging as they did so. 4) The sagging trusses pulled the exterior columns of the towers inwards across an entire face. 5) With additional loading on the exterior columns due to other severed columns, and increasing lateral loading due to bowing, the exterior columns failed across an entire face of each building. 6) The upper structure twisted and fell through the destroyed impact floors, hitting the first fully intact floor with 10 GJ (WTC1) or 30 GJ (WTC2) of energy. It failed virtually instantly, adding its own weight to the mass falling on the next floor. 7) The floor mass collapsed down inside the tube created by the exterior columns. 8) The force of the collapse forced exterior columns below the impact point outwards, peeling them away from the structure in multi-story sections. 9) The debris fell to the ground, leaving the badly damage core standing to at least its height. 10) The core section, unable to stand on its own, collapses.

Posted by a very knowledgeable person on... the JREF forum.