Shattering of Structure
The Twin Towers' Frames Disintegrated Before Falling
|This photograph shows the top of the South Tower disintegrating as it has only just begun to fall. Note the curves in the edges of the walls above the zone of collapse.|
There is clear visual evidence that the structural frames of many stories above the impact zones in both towers were shattered before the tops had fallen significant distances. The evidence is particularly strong in the case of the South Tower.
A movie taken from the east gives one of the most complete records of the South Tower collapse. The motion of the top revealed by the movie has some very strange features. At first the motion consists of a tipping of the approximately 30 stories above the impact zone as a unit, about a fulcrum in or around the impact zone. The tipping motion accelerates for about 2.5 seconds. Then, at about the time the first large ejections of dust start at the impact zone, the motion of the top changes: It begins to fall precipitously, and its rotation (imparted by the tipping) rapidly decelerates and virtually ceases after a second.
The rapid downward acceleration indicates that the fulcrum has been destroyed. This is difficult to reconcile with a gravity-driven collapse. Since the top had already tipped about 15 degrees, the downward force on the building's structure below the fulcrum was already decreasing. One would expect the tipping to continue, eventually resulting in the top falling like a tree.
Disappearing Angular Momentum
The deceleration of the top's rotation is even more discrediting to the idea of a gravity-driven collapse, which cannot explain the documented changes in angular momentum. Conservation of angular momentum is the tendency of a rotating solid object to continue rotating at the same rate in the absence of torque. Initially the block consisting of the top 30 stories of the tower acted as a solid object, and rotated about a fulcrum near the impact zone. Although the fulcrum was the axis of rotation, the block had two types of momentum: the angular momentum of the block around its center of gravity, and the linear momentum of its center of gravity tilting away from the tower's vertical axis. When the portion of the building below the collapse zone disintegrated, the block would preserve its angular momentum by continuing to rotate at the same rate (but the acceleration of the rotation would cease due to the removal of the torque that was being applied by intact columns at the fulcrum). But in reality, the rotation of the block rapidly decelerated as the downward plunge began. Once the fall started, any resistance it encountered from parts of the building would have imparted torque on the block in the same direction as the original fulcrum, and would have accelerated its rotation.
Given the apparent absence of any torque to counter the rotation of the block, the slowing of its rotation can only be explained by the breakup of most of the block, which would have destroyed its moment of inertia.