My architect friend said that the buildings were strong enough to survive the initial impact of the aircraft, but it was primarily the resultant fire spread by the burst fuel tanks (the planes had taken off from a nearby airport when hijacked, and had full fuel tanks). When the rivets holding the girders together melted and let go, the floors of the buildings started "pancaking," and once that started, it was like a chain reaction.

Tall buildings tend to fall straight down, essentially into their own "footprint."

Some people who are enamored with the conspiracy theory claim that the buildings should have fallen sideways like felled trees, but tall buildings simply don't do that.

My friend isn't into conspiracy theories, he just knows a lot about architecture and the design and construction of tall buildings.

Don Firth