Getting back to initial failure and the PBS doc that bill linked to;
That doc did not refer to a massive largely remaing core bill. At least not the part you linked to.
However I do note that the roofline is seen to move downward and the core is not seen spearing up through it so we do know that the core did fail.
I am refering to the fact that when one sees the top section begin to drop one does not see the core punching up through the roof. One does not see the hat truss being held up by the core either.
THUS it is obvious that at the initial collapse the core columns on the fire floors failed. They did not fail lower down during that initial failure. This is obvious for two reasons;
1) the PBS doc shows that the core did not collapse at the same rate as the perimeter. Had the core failed low down then this could not have been the case
2) had the core failed, say in the basement, the perimeter columns on EVERY floor would have been pulled inward
So we know for certain then that there were no explosives or thermite used on lower floors and certainly not in the basement in order to cause that initial collapse.
Done and done, then, with any such suggestion.
Were there explosives in the core in order to ensure that not only the perimeter and the floors came down thus killing anyone inside, but also the core? This would require that said explosives be mounted such that despite the destruction occuring to the floors (debris would be raining down through the core floors as well,between the beams) they were not stripped off the columns. There might be one way to do this by mounting them inside the box columns, any wireless timing mecahnism would also have to be mounted outside the columns(and a wired system is even more vulnerable) not to mention that a wireless system would have to work through the dense falling mass of debris and dust.. However now this is not the simple task of smuggling a cartload of explosives past security which included bomb-sniffing dogs, it now also requires extensive and noisy work to be done to the core columns. Given that this now becomes a task that would have a miniscule probability of not being discovered ( even bill agrees that some conjectures are just way too complicated to be credible) I believe that we can be assured that it did not occur.
We are left then only with explosives at the fire/impact floors.
Once again we run into the problem of how to mount them such that they, and their timing mechanism, are unaffected by either the chaos of an airliner impact, or the subsequent, immediate, large area, multi-floors fires. We are alos confronted with the problem of having explosives mounted on the floors that are impacted. One might mount them on many floors in a target zone but that would require absolute certainty that one could call upon the ones at the actual impact zone and only those ones.
Once again this gets to very complicated it is an exercise in ridiculousness to seriously conjecture that it did occur.
On the other hand we do have the impact and despite the incredulity of some that an aluminum aircraft could cause significant damage to a steel structure it indeed can and did.
The fuel was dispersed over the impact floors and ignited thus accellerating the office fires in a way never seen in 'normal office fires'. It meant that the fires were immediatly at a stage that noramlly takes hours to reach, in a structure significantly damaged by the impacts. The impacts had also compromised the active and passive fire suppression built into the structure.
