This ^^^
Since the first time I heard this bullshit, I understood that to fail it doesn't have to "melt" to fail, only reach a temperature that compromises it strength.
Structural steel begins to soften around 425 °C (800 °F) and loses about half of its strength at 650 °C (1200 °F).
The thing is, the steel did melt though at least in some cases.
There were multiple eyewitness accounts of molten metal at ground zero. Some of that may have been aluminum and copper, but some was certainly steel or components of it.
The whole conspiracy theory started from images like this:
https://en.m.wikipedia.org/wiki/File:9_11_underground_fires_Hotslag-300x215.jpg#mw-jump-to-license
Now despite crusty 200px resolution images that may or may not show molten steel, damage to some of the steel beams was consistent with melting. There are many witness accounts of slag and I believe more photos.
This is why the conspiracy nuts hooked onto "It must be thermite!!!!" Since jet fuel without an oxidizer probably can't liquidify steel. Not that I'm defending the conspiracy nuts but it is a bit puzzling. I didn't realize there were even dumber people that hooked onto the phrase and thought it implied why the buildings couldn't have collapsed.
Anyway the answer seems to be FEMA posted an interesting report showing exactly how the fires DID "liquify" the steel at lower temperatures than normally thought possible, and not due to a traditional "melting" process
Here's a FEMA link
https://www.google.com/url?sa=t&sou...wQFnoECAkQAQ&usg=AOvVaw0rXzD2s_HmHe3liPhbR0tB
It's very interesting to me. Mat sci is not my strong suit but my attempt at converting that into laymens terms would be it seems to me that long exposure to corrosive combustion gasses began to separate and expose the alloying elements of the steel (such as iron nickel and copper) which have a lower melting point. Which then melted into a mixture which penetrated further into the steel and became a viscous cycle of sorts
Most of the molten metal was probably from Wtc7. Wtc7 burned unrestricted. It was moreso a normal office fire than fueled by jet fuel, but the collapse of the other Wtc towers knocked out the water supply to the sprinklers so the temps were extreme. It's believed the building collapsed due to the tremendous thermal expansion which strained the joints between floors.