I really question the idea that jet fuel could not burn hot enough to weaken steel beams. I'm no expert, and I don't know how jet fuel compares with diesel fuel, but I know of potters who have fired kilns to cone 10 (2300ºF) using diesel fuel.

There are a lot of variables involved in how hot a fuel burns. A piece of wood in my woodstove burns at around 500ºF, but I can fire a wood-fired kiln up to over 2300ºF. It's all a matter of surface area, ventilation, and time.