The core columns were made up of three stories tall sections which were butt welded together. I believe that some type of explosive charge was used every third floor to break the welds of the outer core columns, after the collapse was underway for a few floors. As they were inside the tower, any blast from the charges would not be visible, and the debris falling outside would mask any escaping ejections and the collapse itself would mask the noise. I don't think much would have to be done to the perimeter columns except to separate the orthogonal walls at the corners every 10 to 20 floors. This could have been done by attacking the spandrel splices at the corners, allowing the perimeter walls to petal outward.
The breaking of the welded joints of the outer core columns every third floor would very effectively bring down those towers. The remaining spires were comprised of only the interior core columns. All of the much more robust and heavier outer core columns collapsed to the ground with the collapse wave.
I do think artificial heat weakening was used to initiate the collapses. This heat weakening could have been used to weaken the joints of the structure. It would not have to mean cutting the columns with thermite. I am inclined to believe that the temperatures from this heat weakening were much higher than what fire could produce and that is why little to no steel evidence was saved from the fire affected areas for analysis.
In essence, I think the demolition was achieved by attacking joints with heat generating mechanisms and explosive charges rather than cutting columns.
wtf is "artificial heat weakening" - heat is heat no matter how you produce it
how can you attack a 1 inch bead of weld (or smaller) without damaging the 2 ends butted together?
you cant
furthermore the welds were in addition to the rivets that they used to actually fasten the columns together
you actually think that the core columns were held together by welds alone??
the wind loads would have busted the welds if thats all there was long before 911 (and would have had no chance once the building started to fall)
remember a weld fills in a seam but its not the same strength as casting a piece
example:
i went to a job in brooklyn at a recycling sort facility
they had a large payloader (articulating body for steering, 2 solid axles)
they had cracked the bell housing for the differential on the front axle
the axle was a single cast piece (including the bell housing)
their plan was for me to remove the axle and they wanted to send it to their weld shop to try to repair it
i told them it wouldnt work cause the weld fills a seam
the cast part had no seam originally and that they needed a new housing
i told them before i picked up my first wrench that i give it 24 hours before it WILL break again after the repair
so i take it out, they send it to Ct. for repair (V the crack and fill it in)
it comes back i put it in
and 3 hours later the crack opened back up
broken welds are a big part of my job
i see it a lot, a weld isnt as strong as you seem to think it is