http://arxiv.org/abs/physics/0609105

The paper linked to above talks about the collapse of tower two in terms of an avalanche and concludes that the total collapse observed at near free-fall speed would not have been possible absent a "wave of mass destruction" that the author does not speculate on. Of course, one can infer that the wave of mass destruction was the result of timed explosions and/or pre-planted incendiaries like thermite.  I'm just throwing this out as a curiosity--the site it is on is an online "journal" that has been the subject of a few disagreements in the editorial/opinion pages of Harvard's student newspaper.  In fact, one junior physics prof at Harvard links to it on his blogger.com site (http://motls.blogspot.com/2006/09/preprint-on-falling-twin-towers.html) in an attempt to "debunk" it.  He claims at the outset of his debunking that the "high temperatures melted the metallic structure".  Hee hee, the best and brightest indeed.  Anyway back to this paper--I think it's funny that it doesn't come out and say EXPLOSIVES could cause a "wave of massive destruction".  I also think it's funny he uses the abbreviation WMD.  Is this a good thing or a bad thing--I don't know but I thought I'd throw it out for comments from the crew here!

So here is his conclusion:

V. CONCLUSION

We conclude that in a tall building like WTC 2 a single avalanche may form under the conditions such as those following the airplane impact. The avalanche can propagate a few floors, at best, before it comes to a stop. The avalanche cannot reach ground zero, however, if the damage is localized only to the impacted floors. For the avalanche to develop and consume whole building in the time of ∼ 1.2 · T extremely high levels of damage to the floors that were not directly affected by the airplane collision are required. If such a damage were delivered slowly to the whole building, this would have initiated one or possibly few slower avalanches sooner. This did not happen, so judging by the magnitude of the damage that had to be delivered rapidly to the building moments before the collapse, we conclude that the airplane impact could not have been a sole culprit for the catastrophe. Rather, we surmise that there was a wave of massive destruction (WMD) propagating throughout the building in the wake of which the avalanche formed. The WMD most likely started somewhere between the impacted floors and the middle of the building in a big explosion, which destroyed a number of floors to create a core of an avalanche. The WMD then propagated downwards destroying 70-90% of load-bearing capacity of the floors that were shortly thereafter consumed by the avalanche.

In other words, the scenario suggested by the 9/11 commission, where the building collapses in a single avalanche in the near-free-fall times, is physically impossible unless other sources of damage to the building are identified.

Finally, it is worth noting that the one-dimensional model presented here overestimates the energies involved in the motion of avalanche. That is, allowing the core of the avalanche to move in other two directions, as well, would open the paths for its kinetic and potential energy to be diverted into the tipping or the rotation of the top part. This would in turn decrease the energy available for destruction of the floors below. Additionally, in a multidimensional collapse/avalanche model the straight-down motion is unstable. Considering that the strongest part of WTC 2 was its center, a random change in the way the floors at the collapse front failed would have scattered the avalanche and disperse the collapse front. This stability question does not arise, however, if the building is pulled rather than pushed to the ground, which goes back to the hypothetical WMD.