I read, right after 9/11, that the towers' steel frames were built strong enough to withstand a hit from a Boeing 707.  This is why they didn't collapse immediately - it took the intense heat from the fires to melt the vertical steelï»¿ beams.  Once the beams on a single floor collapsed, that was the end of the building.  A civil engineer explained to me in detail how this happened.The implication here is that without the fires, the towers would not have fallen.