Without definitive knowledge, my strong inclination is buildings 1, 2 and 7 of the WTC complex were brought down by means of controlled demolition involving pre planted explosives.
1) The Purdue simulations postulate the likely result of the North Tower impact was the loss of between 9 and 11 interior columns (on the 3 floors affected) out of a possible 47:
http://news.uns.purdue.edu/html4ever/2006/060911.Sozen.WTC.htmlIn and of itself, this does not seem enough to cause collapse in a highly redundant structure such as one of the Twin towers. This seems even less likely upon considering the 15 or so floors above the impact zone in the North Tower are the lightest in the building.
2) It is highly debatable whether jet fuel ignition and resultant hydrocarbon fires would have the intensity (both in terms of temperature and duration) to weaken the remaining structure to the point of collapse.
3) Given the Empire State Building was struck by a B52 years before, the Twin Towers were designed with the possibility of airliner impact in mind. In January of 2001, WTC Construction and Project Manager Frank De Martini (who died on 9/11) said in an interview, "The building was designed to have a fully loaded 707 crash into it. That was the largest plane at the time. I believe that the building probably could sustain multiple impacts of jetliners because this structure is like the mosquito netting on your screen door. This intense grid and the jet-plane is just a pencil puncturing that screen netting. It really does nothing to the screen netting."
http://www.911eyewitness.com/samples/demartini.php4) Witnesses such as last survivor William Rodriguez testified before The 911 Commission regarding explosions in the North Tower basement before the first impact:
http://video.google.ca/videoplay?docid=8536008651248732897This testimony was ignored in the final report.
5) Here is a clip outlining the molten metal discovered at ground zero by first responders, contractors and volunteers as well as John Gross of NIST denying any knowledge of its existence. This is of particular importance because the fires in the Towers were nowhere near hot enough to melt steel, a sentiment John himself reverberates:
6)To pre-empt those who would put forth this molten metal is not steel and possibly aluminium which burns at a lower temperature within the range of “dirty” hydrocarbon fires such as those at the WTC, please address the NASA thermo imagery findings which put the temperatures at ground zero beyond that range:
http://911research.wtc7.net/wtc/evidence/rubblefires.htmlTo be sure, having a computer model reveal the mechanics of collapse without explosives does not negate the possibility that collapse was the result of explosives; keep an open mind. If anyone is interested in a scientific dissection of the NIST report on the collapse of the twin towers, watch Kevin Ryan (formerly of Underwriters Laboratories, the company charged with certifying the steel):
http://video.google.ca/videoplay?docid=718236659434732032
