Brit TV did a docu a few years ago about the collapse of the WTC towers with diagrams, computer sims, interviews with architects and construction industry experts and so on, and concluded that the burning jet fuel melted the internal steel truss framework, causing the collapses.
That was just their guess of course; I wonder what temperature steel melts at?
The show also said there's a saying in the building industry- "Never trust a truss", but apparently trusses are cheaper than some other methods.
But in defence of the builders, they never knew terrorists would one day fly airliners into the towers..