The main integrity of 1 and 2 was in the cores which hosed the lifts and obviously lift shafts. Once you remove a ceiling panel you can get at all the exposed structural beams and columns. Extensive lift renovations were done 10 months before 9/11. See around 11.00.
So the building was deliberately weakened, so when the planes would hit they could initiate a collapse? But not explosives?
What rules out the planes causing the collapse without premeditated weakening of the structure?