The failure in the WTC towers was not in load bearing structural steal. The Towers were internally stressed by the equivalent of guy wires, very light steel members reinforcing the concrete floors. When these lightweight members sagged there was nothing to keep the load bearing members stable.
But more specifically, there was no heavy steel supporting the floors. When the lightweight steel failed, tons of concrete pancaked onto the lower floors.
Check on an analysis at www.wtc7.net/articles/FEMA/wtc_ch5.htm and see how that squares with some of the other info on this........some interesting viewpoints.