text: The fire combined with major structural damage caused the collapse.  The fire was obviously initiated by the jet fuel, but was further sustained by combustible objects in the building.\n\nWhile it is not frequently discussed, the Twin Towers were rare in that their outer walls were structural (load-bearing).  Typically, buildings have widely spaced columns.  This type of design was heavily dependent of on the steel floor truss systems to distribute the load.  As the fire weakened (important note - not melted) the floor trusses, the vertical members gave way to excessive loading caused by a failure to distribute the loads.\n\nThe excessive weight above the points of weakening caused a rapid, catastrophic failure - observed as pancaking of the floors below the initial point of failure.  \n\nIt has been argued that because the building fell at free-fall speeds, which is largely accurate, it could not have possibly been failure of members due to vertical loading.  However, consider the sheer weight and momentum applied to each floor as the one above it fails.  These are extreme values that no building could withstand.