Essay: Collapse of World Trade Center
There were recorded seismic spikes on the ground before the WTC collapsed which were caused by falling debris from the burning building hitting the ground. The time it took for the two towers to collapse was about 11 seconds for WTC1 and nearly 9 seconds in the case of WTC2. These times are nearly the same for a falling ball in a vacuum where resistance is zero. This can be explained by the fact that the floors below the hit floors offered no resistance to the falling building since they were already very weak when disintegration started. According to NIST the times taken by the collapsing building are because
“the structure below the level of collapse initiation offered minimal resistance to the falling building mass at and above the impact zone. The potential energy released by the downward movement of the large building mass far exceeded the capacity of the intact structure below to absorb that energy through energy of deformation”. National Institute of Standards and Technology (NIST) Federal Building and Fire Safety Investigation of the World Trade Center Disaster (2006) available online).