There are numerous ways to wire the inner core columns so they won't be heat blasted with debris and jet fuel, which burned up in the initial explosion anyway. There is footage of a women standing where the plane hit in the North Tower. The areas were cold and much of the fires were not hot enough to weaken the steel at all. There was however severing of beams on these floors, possibly nano-Thermite was used in order to weaken the structure, hence the ailvery molten metal pouring from the towers and the white smoke after 35mins. The actual explosives that streamlined down the building likely had radio transceivers and were several floors below impact.