Free fall
From Debunk911myths
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Free fall?
Theory
Conspiracy theorists say that the collapse of the World Trade Center towers occurred in 9 and 11 seconds respectively. They say that this is free fall speed, and indicates controlled demolition.
Fact
A video taken of the collapse shows at 11 seconds that the collapse of the South Tower is still under way. The cameraman began running at that point, but the sound of the building collapsing continues for many more seconds.
The elapsed times for the first exterior panels to strike the ground after the collapse initiated was 11 seconds for 1 WTC (North Tower) and 9 seconds for 2 WTC (South Tower). NIST also points out that significant portions of the cores of both buildings (roughly 60 stories of 1 WTC and 40 stories of 2 WTC) are known to have stood 15 to 25 seconds after collapse initiation before they, too, began to collapse.
"The buildings are 70 percent just air in volume and all of the columns are not solid steel, they are steel boxes in which the thickness of the steel varies from 1/4 inch (at the top) to 1 1/2 inches (at the bottom). But they were properly designed to carry the weight of the steel itself, the weight of the partitions, the occupants, the furnishings... those are all things that the structure can withstand very well, but they are not designed to accommodate the failure of a 20 floor section in a dynamic impact on the structure below. If it's moving down in a dynamic fashion, the magnitude of the energy unleashed is so large that no structure can withstand that kind of force that is applied, and the building came down in essentially free fall." -- S. Shyam Sunder
Bazant and Zhou explain that once the collapse initiated on the one floor, the kinetic energy of the top part of the tower impacting on the floor beneath was 8.4 times larger than the plastic energy absorption capability. At that point, the subsequent progressive collapse was inevitable. The amount of resistance or absorption capability that each floor beneath could provide was minuscule in comparison to the amount of kinetic energy from the above structure collapsing. This explains why the collapse happened as quick as it did -- not free fall speed, but perhaps twice the amount of time than free fall.[1]
- NIST FAQ regarding "free fall"
- Freefall video evidence
- 9/11 Conspiracy Theories - PBS video, with S. Shyam Sunder