What really happened on Sept.11, 2001?

From 2001 through most of 2007, I didn't think much about this subject. I had casually wondered if the fires could actually be responsible for bringing the towers down, thinking about the heat and thermodynamics necessary to weaken the steel columns. I have an undergraduate degree in chemistry, and find thermodynamics very interesting. The idea that fires fueled by office materials and jet fuel (enhanced kerosene) could weaken steel beams, resulting in total failure of the entire structure seemed vaguely unlikely. After all, there have been other large steel-structured office building fires that had not resulted in complete, catastrophic collapse, but I didn’t dwell on my vague doubts.