[crazytony]

As far as weights go, the 707 and the 767-200s that hit the towers were fairly close in size, weight and fuel capacity and that is demonstrated by both towers surviving their impacts. The problem was the fire and specifically the inability to fight the scale of the fire effectively.

[iwontberude]

And insufficient modelling for the heat of jet fuel mixed with office supplies which severely weakened the structure allowing it to pancake in a cascading fashion from the first support columns to buckle.

[wat10000]

The designers also assumed that the airplane would be moving pretty slowly if it was so close to the ground, either departing from or landing at a nearby airport, not coming in at near top speed.