The Fall of Skyscrapers - Pushing the Limits: New Adventures in Engineering - page 113

month after the terrorist attack, it was estimated that only 15 percent of the debris had been re-
moved, and it was thought that it would take a year to clear the site. However, with respect, dedic-
ation, and resolve, the army of workers at Ground Zero, as the site came to be called, cleared it out
to bedrock months sooner.
Among the concerns engineers had about the cleanup operation was how the removal of debris
might affect the stability of the ground around the site. Because some of the land on which the
World Trade Center was built had originally been part of the Hudson River, an innovative barrier
had been developed at the time of construction to prevent river water from flowing into the base-
ment of the structures. This was done with the construction of a slurry wall, which began as a deep
trench that was filled with a mudlike mixture until a hardened reinforced-concrete barrier was in
place. The completed structure provided a watertight enclosure, which came to be known as the
“bathtub,” within which the World Trade Center towers were built. The basement floors of the twin
towers acted to stabilize the bathtub walls, but these floors were crushed when the towers broke
up and collapsed into the enclosure. Early indications were that the bathtub remained intact, but in
order to be sure its walls did not collapse when the last of the debris and thus all internal support
was removed, vulnerable sections of the concrete wall were tied back to the bedrock surrounding
the site even as the debris removal was proceeding.
Atop the pile of debris, the steel beams and columns were the largest and most recognizable parts
in the wreckage. The concrete, sheetrock, and fireproofing that were in the building were largely
pulverized by the collapsing structure, as evidenced by the ubiquitous dust present in the after-
math. (A significant amount of asbestos was used only in the lower floors of one of the towers, bad
publicity about the material having accelerated during the construction of the World Trade Center.
Nevertheless, in the days after the collapse, the once-intolerant Environmental Protection Agency
declared the air safe.) The grillelike remains of the buildings' facades, towering precariously over
Ground Zero, became a most eerie image. Though many argued for leaving these cathedral wall-
like skeletons standing as memorials to the dead, they posed a hazard to rescue workers and were in
time torn down and carted away for possible future reuse in a reconstructed memorial. As is often
the case following such a tragedy, there was also some disagreement about how to treat the wreck-
age generally. Early on, there was clearly a need to remove as much of it from the site as quickly as
possible so that any survivors there could be uncovered. This necessitated cutting up steel columns
into sections that could fit on large flatbed trucks. Even the relocation of so much wreckage presen-
ted a problem. Much of the steel was marked for immediate recycling, but forensic engineers wor-
ried that valuable clues to exactly how the structures collapsed would be lost.
All of the speculations of engineers about the mechanism of the collapse were and are in fact hy-
potheses, theories of what might have happened. While computer models can be constructed to test
hypotheses and theories, actual pieces of the wreckage may provide the most convincing confirm-
ation that the collapse of the structures did in fact progress as hypothesized. Though the wreckage
may appear to have been hopelessly jumbled and crushed, telltale clues could have survived among
the debris. Pieces of partially melted steel, for example, could provide the means for establishing
how hot the fire burned and where the collapse might have initiated. Badly bent columns can give
evidence of buckling before and during collapse. Even the scratches, gouges, and scars on large
pieces of steel can be useful in determining the sequence of collapse. This was to be the task of