[JamesP81]

This guy is a kook, steel or any metal gets to a weakened state long before reaching any melting temperature

My dad is a machinist, he works on heavy industrial and marine gearboxes and other large-scale machines, so I have over time picked up a bit of metallurgy from him.

When a part is made of steel, they do something called heat tempering, which is done after heat treatment. Tempering reduces the brittleness of the steel and increases it's durability. I couldn't find data on tempering for structural steel, but 280C-300C is sufficient for cold forging tools and springs, both of which require very fine quality steels. To temper, you heat the steel up to the required temperature, then cool it quickly (called quenching). This is done in several different media, but water is a common one. The thing is, if you heat the steel and don't cool it quickly after tempering, the tempering is lost. The steel becomes very brittle, if it doesn't melt in the heat. 300C temperatures are credible to me in a fire like that. And any loss in the heat tempering in a portion of the steel structure is almost certain to cause a collapse.

[Zuriel]

I remember the photo of what was left of the failed attempt to free our people in Iran during the Carter term.
There was C-130 turbine engines lying on the desert and not much else. I realize that aircraft use a minimum of steel.

As a steel fabricator, I'm well aware of getting the right mix of fuel at the torch tip for maximum heat. The C-130's fuel (and the copter's) fuel did a pretty good job of almost incinerating everything. And that was with an inefficient burn (lots of fuel burning rapidly, so lots went up without really directing it's heat.

The WTC jets left holes to fan the flames in a more controlled, 'blast furnace', fashion. Very little fuel was wasted in developing a very hot fire that could easily weaken the center of the buildings.