A nice demonstration of how the twin towers collapsed on 9-11 happened quite by accident several years ago in Oakland, CA. The freeway overpass pictured below failed and came down one night. This overpass's structure is built entirely out of steel. Notice how it has a limp noodle kind of look to it.
Why did this bridge come down? One night a truck driver, hauling a full load of gasoline went around the curve too fast and went off the bridge. The resulting fire burned beyond anybody's ability to put it out. Later that evening, the bridge collapsed, not due to any explosion, but
just because of the heat.![](https://static01.nyt.com/images/2007/04/29/us/30collapse1.jpg)
Here's a video but I think is is after it had already collapsed. It happened at 3:45 in the morning and this was in the days before everyone carried a video camera around with them, so I don't think there is any actual footage of the collapse.
Gasoline can weaken, soften and even melt steel. An airliner full of fuel can take down a building.