The lead structural engineer for the towers, Leslie Robertson, did an analysis that showed the buildings could survive the structural damage from a plane strike, and in fact they did. But it was the combination or fire and structural damage that brought them down. Regardless of Skillings' assertions that the building would stand after the fire, there was no analysis of the effects of fire when the building was designed because, according to Robertson, there weren't any tools then to do that kind of analysis, and no available systems that would withstand that kind of disaster, anyway:
The two towers were the first structures outside of the military and nuclear industries designed to resist the impact of a jet airliner, the Boeing 707. It was assumed that the jetliner would be lost in the fog, seeking to land at JFK or at Newark. To the best of our knowledge, little was known about the effects of a fire from such an aircraft, and no designs were prepared for that circumstance. Indeed, at that time, no fireproofing systems were available to control the effects of such fires.
http://www.nae.edu/Publications/Bridge/EngineeringandHomelandSecurity/ReflectionsontheWorldTradeCenter.aspx
And the unequivocal proof of that is that the fire mitigation systems were just "to code" -- intended for normal office fires with nothing at all special to handle a disaster like what happened -- and it is clear now that they were inadequate.
Once again, the "truth movement" gives us half-truths and fuzzy thinking instead of truth.