The tests strongly suggest the airplane strikes did not bring the towers down. And it wasn't the thousands of gallons of jet fuel the planes carried.
http://www.cbsnews.com/stories/2004/06/17/eveningnews/main624595.shtml

So the popular belief that all of this fuel made the building collapse inevitable, is not true, says Sunder

"Steel tends to soften or weaken and become more like Playdough when you heat it up," says Sunder.