On September 11, 2001, the largest attack on the United States occurred in New York City when two planes plunged into the both world trade towers. The towers, shortly after the collisions, collapsed and fell 1,353 feet to the streets of Lower Manhattan. Many believe that the impact of the airplanes hindered the fatalities of the towers. However, from an engineering aspect, this was not the only conflict that corrupted the enormous landmarks.
It is believed that the initial force of the plane caused several stories to buckle on top of one another. This presented a tremendous amount of weight on the structure below. The stress of this alone, normally, would not suffice as a reason for the tower to fall down. The planes, because they just took off, were full of jet fuel. This allowed for a much longer fire at a more intense temperature. As the fuel burned, it engulfed all of the contents of the office; hence, the fire intensified.
So, now, with 7 or more stories stacked atop each other, all of the office supplies, and the internal masonry blazing, the steel structure throughout the tower began to weaken. The weakened structure, due to its uniqueness, collapsed internally.
The world trade towers had steel columns tightly spaced along the perimeter of the building and the same inside the buildings core arrangement. Fires burn hottest in the center. As this raging fire burned and as it imprisoned more floors of the tower, the heart of the building grew to such extreme temperatures that the center supports melted. The center supports started to collapse pulling down the steel trusses that span between the perimeter and the center supports. This is also the explanation of the building completely vertical tumble.
It can be obviously seen, after some investigation, that the impacts of these planes were not responsible for the crumbling of the twin towers.