I couldn't edit my post (thanks to the new limit), but here is a little food for thought:
Here is an experiment you can do right now to replicate the kind of structure that the WTC had. Stand on an undamaged and empty soda can. Do so by stepping up on it with the weight of your foot completely distributed across the top. Now, have someone quickly flick the side of the can with their finger to put a tiny dent in it. They had better be quick or risk getting themselves crushed by your falling weight. The can, with a tiny dent in it, will not be able to stand up to your weight because the forces have shifted and exponentially exceeded the capacity of the can as a solid member. With the way the WTC was designed, you have a more complex but similar effect.
Here is an experiment you can do right now to replicate the kind of structure that the WTC had. Stand on an undamaged and empty soda can. Do so by stepping up on it with the weight of your foot completely distributed across the top. Now, have someone quickly flick the side of the can with their finger to put a tiny dent in it. They had better be quick or risk getting themselves crushed by your falling weight. The can, with a tiny dent in it, will not be able to stand up to your weight because the forces have shifted and exponentially exceeded the capacity of the can as a solid member. With the way the WTC was designed, you have a more complex but similar effect.