Shockwave

One of the more interesting theories advanced to explain the explosive collapses of the Twin Towers is that a "shockwave" tore down through each tower faster than the rubble fell (and perhaps faster than the rate of free-fall) pulverizing the building ahead of the falling rubble.

The following excerpt is from an article originally published on the University of Sydney's Department of Civil Engineering website. The latter article was later changed to remove the mention of the shockwave theory.

e x c e r p t

title: World Trade Center - Some Engineering Aspects

authors: University of Sydney

This photograph shows the south tower just as it is collapsing. It is evident that the building is falling over to the left. The North Tower collapsed directly downwards, on top of itself. The same mechanism of failure, the combination of impact and subsequent fire damage, is the likely cause of failure of both towers. However, it is possible that a storey on only one side of the South Tower initially collapsed, resulting in the "skewed" failure of the entire tower. The gigantic impact forces caused by the huge mass of the falling structure landing on the floors below travelled down the columns like a shockwave faster than the entire structure fell. The clouds of debris coming from the tower, several storeys below the huge falling mass, probably result from the sudden and almost explosive failure of each floor, caused by the "shockwave". [emphasis added]

site: www.nyc-architecture.com page: www.nyc-architecture.com/GON/GON001A.htm