http://infowars.com/articles/sept11/squibs_new_wtc_collapse_video_highlights_unmistable_implosions.htmConservative estimates put the collapse of the south tower at around 14 seconds, a rate of collapse that almost rivals freefall speed. Simple physics dictates that for a building to fall at near freefall speed, the resistance of the building below needs to be removed before the collapse point reaches it.