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Even if the steel was weakened by the jet fuel this would not have dropped the entire building! Sky scrapers don't just get thrown together and collapse in free fall, they have a very strict building code. One of which relies on the build being able to take impacts from flying objects. When the steel at the point of impact was heated (btw jet fuel burns at 980C in a controlled environment) it may have been heated enough for enough amount of time to change it's crystal structure but that would not have weaken the massive steel pillars below or above it. Also, to mention the pools of melted metal, STILL glowing red hot weeks after the initial collapse. Are you trying to say that jet fuel burns hot enough and long enough to melt enough metal to create pools of lava like liquids to last at least a week? These were found by fire fighters when they were removing ruble searching for bodies. .....
None of all that conjecture is necessary, I asked a simple question regarding whether you believe that the jet fuel burning was insufficient to weaken the structure and cause the collapse. You've answered it.
I must beg to disagree. Here are the reasons.
"Standard burning heat" is all it takes to turn iron red hot allowing it to have no more structural strength than mud. Acetylene cutting torches melt steel everyday, by simply adding oxygen. But that is not what is required to bring the structural framework down. It is required to bring it up to about 1000F. That's all.
http://www.steelconstruction.info/Fire_damage_assessment_of_hot_rolled_structural_steelworkI'm feeling a bit lazy at the moment but my opinion is that 600F would be enough. The next part of this is to determine how much steel was there, and how many joules would it take to bring it to this temperature, and whether a fraction of the jet fuel and energy of impact was sufficient to allow that temperature to be reached.