Both buildings collapsed symmetrically and more or less straight down, though there was some tilting of the tops of the towers and a significant amount of fallout to the sides. In both cases, the portion of the building that had been damaged by the airplanes failed, which allowed the section above the airplane impacts to fall onto the undamaged structure below. As the collapse progressed, dust and debris could be seen shooting out of the windows several floors below the advancing destruction. The first fragments of the outer walls of the collapsed North Tower struck the ground 11 seconds after the collapse started, and parts of the South Tower after 9 seconds. The lower portions of both buildings' cores (60 stories of WTC 1 and 40 stories of WTC 2) remained standing for up to 25 seconds after the start of the initial collapse before they too collapsed.
[12]
While they were designed to support enormous static loads, they provided little resistance to the moving mass of the sections above the floors where the collapses initiated. Structural systems respond very differently to static and dynamic loads, and since the motion of the falling portion began as a free fall through the height of at least one story (roughly three meters or 10 feet), the structure beneath them was unable to stop the collapses once they began. Indeed, a fall of only half a meter (about 20 inches) would have been enough to release the necessary energy to begin an unstoppable collapse.
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Collapse initiation
After the planes hit the buildings, but before they collapsed, the cores of both towers consisted of three distinct sections. Above and below the impact floors, the cores consisted of what were essentially two rigid boxes; the steel in these sections was undamaged and had undergone no significant heating. The section between them, however, had sustained significant damage and, though they were not hot enough to melt it, the fires were weakening the structural steel. As a result, the core columns were slowly being crushed, sustaining
plastic and
creep deformation from the weight of higher floors. As the top section tried to move downward, however, the hat truss redistributed the load to the perimeter columns. Meanwhile, the perimeter columns and floors were also being weakened by the heat of the fires, and as the floors began to sag they pulled the exterior walls inwards. In the case of 2 WTC, this caused the eastern face to buckle, transferring its loads back to the failing core through the hat truss and initiating the collapse. In the case of 1 WTC, the south wall later buckled in the same way, and with similar consequences.
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Total progressive collapse
The collapse of the World Trade Center has been called "the most infamous paradigm" of
progressive collapse.
[32] In the case of both towers, the top section tilted towards the face that had buckled, behaving largely as a solid block separate from the rest of the building. It fell at least one story in freefall and impacted the lower sections with a force equivalent to over thirty times its own weight. This was sufficient to buckle the columns of the story immediately below it; the block then fell freely through the distance of another story. Total collapse was now unavoidable as the process repeated through the entire height of the lower sections. The force of each impact was also much greater than the horizontal momentum of the section, which kept the tilt from increasing significantly before the falling section reached the ground. It remained intact throughout the collapse, with its center of gravity within the building's footprint. After crushing the lower section of the building, it was itself crushed when it hit the ground.
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