Did an unconventional structural design contribute to the World Trade Center collapse?

On this first anniversary of the World Trade Center attacks, we are inundated with retrospectives of 9/11’s horrible events, almost to the point where one becomes inured to the familiar images of the planes, the explosions, and the subsequent events. We all know that it was the planes colliding with the buildings that precipitated the towers’ collapses, but what happened to the buildings then that eventually resulted in their failure? Could they in some way have sustained the initial impacts and remained standing? These, of course, are technical questions that can be hard to deal with when the spectre of the dead remains so closely with us. Nevertheless, that’s what The Learning Channel (TLC) documentary World Trade Center: Anatomy of a Collapse has as its subject matter, and if you are in the mood for that sort of an assessment, you will be amply rewarded by this slim 52-minute disc.

The content of the program is fascinating. It provides a short historical background covering the decision to build, the criteria for the size of the buildings (key was the requirement for large amounts of open office space as well as the high total office space square footage), and the actual construction process. The heart of the program is a detailed exploration of the structural engineering design decisions and the new design approach that was needed to allow the buildings to extend beyond 80 stories. The conventional engineering approach for skyscrapers has normally been a lattice-like structure composed of steel posts and beams. Normally a concrete encased core contains the elevators, stairwells, and other plant systems. The office space surrounds this core, but is characterized by posts that can obstruct large open spaces to a greater or lesser extent. The World Trade Center design avoided the latter by having an outer shell of steel and an inner concrete core connected to each other by horizontal steel trusses for each floor. When the planes collided with the buildings, they essentially severed the concrete core, thus cutting off all the elevator shafts and stairwell accesses from the floors above to those below (with the exception of one stairwell in one of the towers). The fires that resulted, and further fed by the full loads of jet fuel on the planes, caused the floor trusses to bend and eventually separate from where they were connected to the central core and the outer shell. As one floor failed, it fell on the one below, resulting in a cascade of failed floors internally. Without this support between the steel shells and internal cores, the shells soon buckled and the entire buildings collapsed. Helping to explain how this all happened are interviews with the structural engineer charged with the original design and the overall project manager, along with CGI. Interspersed with these are the recollections of survivors initially trapped in the buildings. It all adds up to an extremely informative yet still moving account of what did happen and what likely took place to allow the rapid, complete collapses. Interestingly, the structural engineer reveals that the design process took into account the possibility of the towers being hit by an airplane, but accidentally and slowly, due to the plane perhaps being lost in fog. No consideration was ever apparently given to any jet fuel that may have contributed to any subsequent fire. At one point, the suggestion is made that had the towers been of a conventional structural design, they may not have collapsed subsequent to the plane collisions. One can only imagine what legal ramifications there may be to such a finding.

This program originally aired on The Learning Channel and is now presented on DVD full frame in accord with its original presentation. The new interview and CGI footage shot specifically for the program looks sharp and clean, as it should. Stock footage covering various events in the 30-odd year life of the buildings is variable in quality depending on the state of the source material. The sound is adequate for the program with dialogue being generally clear and understandable. There is no supplementary material included on the disc.

THE VERDICT

For those looking for technical answers about the WTC collapse, this disc offers an informative blend of historical information and new insight that is well worth viewing.

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