> The four sides of each building formed a tube with window slots This tube was designed to carry the weight of the building and its contents directly downward to the foundations. The walls acted as vertical beams, wide and tall as the buildings, that bent to the side to absorb wind loads.
> Prior to the construction of the Twin Towers, skyscrapers were designed to support themselves through large internal columns spaced about 30 feet (9 meters) apart, which interrupted the flow of interior space. For this project however, the engineers came up with a different solution -- the exterior walls themselves would support the bulk of the structure, and they would get a boost from one single column of beams in the center.
Make a big hole in the primary support structure and things go badly.
(And before parroting the "jet fuel doesn't melt steel beams" thing as a follow-up, watch this excellent and short video from a blacksmith: https://www.youtube.com/watch?v=FzF1KySHmUA)
