Theories Gravity Caused the Twin Towers to Crush Themselves
The official story of the collapse of the Twin Towers is that gravity caused them to crush themselves. The impacts and fires weakened the buildings near the the crash zones but it was gravity that did the work of demolishing them. There are principally two official theories:
Both theories are frequently called pancake theories because the pancake metaphor subtly suggests the buildings consisted primarily of heavy floor slabs, not vertical pillars of steel. Variants of the column failure theory were the first out of the block. They ask us to believe that all of the columns on one level suddenly failed, causing the overhanging portion of the tower to fall at least a floor, thereby acquiring enough momentum to crush the intact portion of the tower, floor-by-floor.
The column failure theory would suffice for scientific papers and insurance claims, but the American public would require something less obviously inconsistent with the reality that the columns of entire floors were not glowing red-hot. Hence the truss failure theory was presented on popular science shows, and endorsed by FEMA's official report. It asks us to believe that the fires weakened floor trusses, causing a whole floor diaphragm to fall, initiating a chain reaction of floor failures. That is supposed to lead to the collapse of the perimeter walls and core structures, an idea made more palatable by hiding the towers' core structures, or misrepresenting them as mechanically unsubstantial.
The experts gave us the cool-sounding progressive collapse to help sell the collapse theories, whether they emphasize falling floors or buckling columns. But does it apply to steel structures? Some structures do fall apart under their own weight if sufficiently weakened or disturbed, such as burned wood-frame houses, or earthquake-fractured masonry buildings. Steel structures are not known to, outside of rare cases, but perhaps they could be designed to. Getting a steel structure to crumble from the top down -- as in the alleged progressive collapse phenomenon -- is an even greater challenge.
Not only is top-down progressive collapse completely unknown in other steel structures, we know of no documented case of it using any materials. Hence we have created the progressive collapse challenge. We believe the first two challenges are possible. Some variant of the following construction may work.
If such a structure were disturbed near the top, it could come tumbling down, and the collapse just might start at the zone of the disturbance and progress downward.
In addition to the officially endorsed column failure and truss failure theories, other theories have been volunteered to explain the explosive disintegration of the towers, while also ignoring or denying demolition. Some of these are identified in a taxonomy of collapse theories.