From Salon,

Why did the buildings collapse? 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.

The buildings were architecturally interesting in many ways. Each structure is based on a central steel core, which is surrounded by the outside wall, a 209-foot by 209-foot cube of 18-inch tubular steel columns, set 22 inches apart. The cores and "tube walls" share the enormous physical weight of the structures and protect them against the extraordinary wind forces of buildings that tall. There are trusses that support each floor, but no other columns between the cores and outside walls. Some floors contain nearly 40,000 square feet of open office space.

News reports said the planes were jetliners, a 757 and a 767. The 757 has a 124-foot wingspan, is 155 feet long and can weigh 100 tons. A 767 is bigger, with a 156-foot wingspan and 159-foot length and can weigh a maximum of 200 tons. (A 747 is more than 200 feet long and can weigh 400 tons.) The planes hit the buildings near the 70th or 80th floors. Their impact severely damaged the tube walls, which carried a large proportion of the buildings' weight. CNN footage of the second plane hitting a tower appeared to show that a large part of the jetliner went all the way through the building, suggesting that the interior core was also damaged.

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.

"It's a very rugged system," he said. "It takes a long time for the collapse mechanism to develop. It's not like kicking the leg out from underneath a chair. The building is 200-foot square and there's a lot of structural system there."

But once the upper floors began to give way, terrible force was set in motion. Each floor of a building that big might weigh 6 million pounds, he said. Once impact is factored in as well, he said, the force becomes irresistible.