The heat from the jet fuel fire did not "melt" the steel structure. It was hot enough to weaken the steel and cause it to flex, especially under the weight of the 50 or so floors above the crash. Combine the weakening steel with the fact that many supports were removed by the crash itself and you have a recipe for disaster. Once the supports started to weaken there was a chain reaction. The weaken steel structure started to flex and the floors above the crash fell about 30 feet or 3 floors (the floors taken out by the crash) once that occurred it was all over. The remaining structure below that was not able to handle the energy of 50 floors falling 30 feet and it brought the rest of the structure down.

For those of you that don't think a 767 could take out the steel structure here is a physics lesson for you.

Force = Mass * Acceleration

Mass of 767 - 255,000 LBS
Estimated speed @ impact - 470 MPH
Estimated pounds of force at impact - 5.5 MILLION pounds!

[link to www.ajdesigner.com]
[link to www.simviation.com]

On a side note, thermite is created by small shavings of aluminum and steel, that being said a plane is made out of aluminum and the building out of steel so it makes sense that there was "thermite" residue simply due to the fact that the metals required to make thermite were at the crash sight. When the residue was analyzed it wasn't thermite that was found, it was the residue of steel and aluminum that was found.

Let the shilling begin...