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In post 4 of the WTC Collapses thread, shaman_ argued that the towers did not fall on their own, but were aided in their collapse by planes hitting them beforehand. In post 5, I make clear that the planes were of negligible effect on the towers. 9/11 Research, in its article Towers' Design Parameters, explains:

Frank A. Demartini, on-site construction manager for the World Trade Center, spoke of the resilience of the towers in an interview recorded on January 25, 2001 [He stated:]
The building was designed to have a fully loaded 707 crash into it. That was the largest plane at the time. I believe that the building probably could sustain multiple impacts of jetliners because this structure is like the mosquito netting on your screen door -- this intense grid -- and the jet plane is just a pencil puncturing that screen netting. It really does nothing to the screen netting.
Demartini, who had an office on the 88th floor of the North Tower, has been missing since the 9/11/01 attack, having remained in the North Tower to assist in the evacuation. 6 Demartini had first worked at World Trade Center when Leslie E. Robertson Associates hired him to assess damage from the truck bombing in 1993.

In post 25, shaman_ challenged this argument by quoting Leslie Robertson. A NOVA video credited Leslie Robertson with being the structural engineer responsible for designing the towers. Leslie Robertson states in Reflections on the World Trade Center:
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.

However, the fact is that the original designer of the towers was John Skilling. Leslie Robertson was only a junior member of the firm [Worthington, Skilling, Helle and Jackson], and Skilling was known at the time to be the engineer in charge. In 1993, five years before his death, Skilling said that he had performed an analysis on jet plane crashes and the ensuing fires and that "the building structure would still be there."

One begins to wonder, was it true that little was known about the effects of a fire from such an aircraft or was Leslie Robertson twisting the truth? In any case, Kevin Ryan makes it clear that the fires should have had a negligible effect on the buildings, as he made clear in his letter to NIST.

In post 29, shaman_ argued that Mr. Ryan bases that letter on the belief that the steel didn't reach temperatures over 250C and that that view is mistaken. My response to this is here.