The official story posits that on 9/11, the twin towers came dow due to a combination of damage from the planes, and the subsequent fires they initiated. 9/11 researcher C. Thurston counters this notion in his article "Explosions or Collapse?". I have taken and modified an excerpt of his article, with links where people have disagreed with his view or more information on particular points here.
I have countered other arguments against
the Controlled Demolition Hypothesis here.
The Controlled Demolitions of the Twin Towers Theory
The above link has lists and links with the evidence on the controlled demolition theory. The lists were originally taken from Architects and Engineers for 9/11 Truth, which I quoted in sciforums. The additions were counters to posters on sciforums.com.