In the 911 incident, only the top of the Twin Towers was hit. Why did the entire building collapse?

The main reason was the fire caused by the plane crashing into the Twin Towers. The impact of the plane itself cannot shake it at all. This behemoth, even if the plane produced an impact at high speed, could not destroy the entire building. The culprit was the explosion caused by the impact of the plane, which in turn caused a fire. The plane itself carried a large amount of The gasoline fueled the fire, and many decoration materials and office supplies in the Twin Towers were flammable, so the fire spread quite quickly.

The high temperature generated by the fire softened the main steel structure of the Twin Towers, and because the weight of the entire building was so great, the foundation of the building began to become unstable and swayed continuously. The floor hit by the plane began to soften first, and the superstructure fell downwards like dominoes. The impact force continued to accumulate and grow, and the entire building collapsed downwards.

There are three main reasons why the North Tower collapsed later than the South Tower: the impact point was higher, the aircraft was slower, and the fire protection system of the affected floors had been partially updated; the first tower collapsed It changed the balance of forces connecting the underground parts together, just like a child's seesaw, so the second one fell down one after another. In short, the plane impact is only the triggering factor, and the fire is the sufficient condition. If there is only an impact but no fire, the Twin Towers will not fall down.

What fell from the World Trade Center hit the floor of the floor below, and the floor was supported by steel bars, so it could not undergo plastic deformation. Therefore, the pressure it can absorb is extremely limited. Therefore, the floors of the World Trade Center are smashed down layer by layer, with increasing weight, faster and faster speed, and greater and greater kinetic energy. As long as the building above hits the first floor, it indicates that the entire building must completely collapse. Moreover, floor after floor fell down, leaving no room for survival, so the possibility of survival was zero.