Engineers shocked by towers: “The structural engineer who designed the towers said as recently as last week that their steel columns could remain standing if they were hit by a 707.” Chicago Tribune However, a Salon feature discusses why the towers collapsed:

According to Gregory Fenves, a professor of Civil Engineering at the University of California at Berkeley, the planes weakened the buildings’ structures at key points. Fenves, working on information gleaned from preliminary TV reports, stressed that he was speculating. He said that if the planes had hit the structures higher, they could have merely damaged their tops; if they had hit lower, they would have been up against the enormous weight and resistance of the base of the buildings…

Once a building like a World Trade Center tower loses some of its support, the building in effect goes to work, Fenves said. “The loads are trying to redistribute,” he said. “The loads are figuring out how to get back down to the ground.” At the same time, he noted, the fires are deforming the physical properties of the support steel.