Gravity driven collapse

25 Story Building Collapses in Brazil

http://news.yahoo.com/video/world-15749633/25-storey-block-collapses-in-brazil-24012047#video=24025778

This is interesting because the video clearly shows very large chunks of concrete being lifted from the rubble pile, which is to be expected in a gravity driven high-rise collapse. Of course there were no such large concrete pieces in the rubble pile of Towers 1 and 2, and Building 7; it was all reduced to powder.