The whole "how could they build a sky scraper that couldn't survive a direct plane hit" argument is flawed when you see this.
http://99percentinvisible.org/episode/structural-integrity/
If a tower can be designed, built and occupied without it being tested for certain wind conditions that happen once every 55 years then its plausible one could be put up and not be tested for the events that happened.
Leslie Robertson, the lead WTC structural engineer claimed that even at the time of design the buildings were only tested for an aircraft strike at 180mph and that no additional testing was done for after effects such as fires, secondary explosions and wind effects once there was a hole in the structure.
The argument is interesting on both sides though and all things should be questioned when an attack of that scale can happen.
Is the Dyatlov pass incident a conspiracy or a mystery?
I remember reading something shortly after the attack, saying that one of the reasons that fire spread so quickly and the internal damage was so extensive, was due to alterations made to the offices over recent years. Originally, the WTC was built with lots of small offices, but over recent years, most floors had been altered to make them almost completely open plan and this dramatically reduced the fire protection offered by all the internal walls.