The pile-driver theory takes up where the column failure theory leaves off, explaining how the top each tower crushed the intact portion into oblivion.

The pile-driver theory existed, at least implicitly, since the day of the attack. The paper by Bazant and Zhou contains the essential idea.

Ronald Hamburger, who co-authored Chapters 1 and 2 of FEMA's Report has given public appearances supporting the official story.

Structural engineer Ronald Hamburger articulated the pile-driver theory by name, stating to an audience on November 29, 2001:

Think of the impact of dropping a 25-story building straight down ... It was like a pile driver, which is why it collapsed as it did. ¹

The pile-driver theory receded with the ascendency of the truss-failure theory, which abandoned the idea of the entire top sections of the Towers falling as intact blocks. Instead, FEMA told us, the unsupported heights of the freestanding columns, lacking the lateral support of the pancaked floor diaphragms, buckled and collapsed:

As the floors collapsed, this left tall freestanding portions of the exterior wall and possibly central core columns. As the unsupported height of these freestanding exterior wall elements increased, they buckled at the bolted column splice connections, and also collapsed. Perimeter walls of the building seem to have peeled off and fallen directly away from the building face, while portions of the core fell in a somewhat random manner. The perimeter walls broke apart at the bolted connections, allowing individual prefabricated units that formed the wall or, in some cases, large assemblies of these units to fall to the street and onto neighboring buildings below.

Thus, FEMA never invokes the pile-driver theory, preferring to have us believe that the Towers simply fell apart as a consequence of the alleged floor pancaking.

In early 2005, with the release of some of the preliminary reports from NIST's investigation, it became apparent that the pile-driver theory was poised for a come-back. The nearly 300-page Draft of NIST's Final Report on the Twin Towers has only one paragraph describing the start of each Tower's collapse, but the pile-driver theory is unmistakable. The following passage describing the South Tower is almost identical to the one describing the North Tower.

The building section above the impact zone began tilting to the south as the columns on the east and west walls rapidly became unable to carry the increased loads. This further increased the gravity loads on the core columns. Once the upper building section began to move downwards, the weakened structure in the impact and fire zone was not able to absorb the tremendous energy of the falling building section and global collapse ensued. ²

Apparently in response to criticisms that the Draft Report "does not actually include the structural behavior of the tower after the conditions for collapse initiation were reached and collapse became inevitable", the Final Report added a new section making the pile-driver theory more explicit. It reads, in part:

Failure of the south wall in WTC 1 and east wall in WTC2 caused the portion of the building above to tilt in the direction of the failed wall. The tilting was accompanied by a downward movement. The story immediately below the stories in which the columns failed was not able to arrest this initial movement as evidenced by videos from several vantage points.

The structure below the level of the 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 structures below to absorb that through energy of deformation. ³

References

1. Structural engineer describes collapse of the World Trade Center towers, news-service.stanford.edu,
2. Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Towers, NIST.gov, , page 146
3. Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Towers, NIST.gov, , page 146

page last modified: 2006-12-29