The towers were build to resist a Boing. The number of stories are not of importance. Structures are build according to the number of stories, so what's your point? The firefighters went in because they knew there would be no danger of collapse. There is no place for heroic suicide among firefighters. If you wanna be useful, you have to stay alive. If steel supports would have been the reason for a sideways collapse we wouldn't have seen a controlled demolition collapse anyway.