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Controlled Demolition

Buildings do fall vertically like Building 7, when destroyed by controlled demolition.

The controlled demolition of large structures is a well developed art and science. Removing a tall building from an urban landscape without damaging adjacent structures -- a considerable engineering feat -- is a task that only a handful of controlled demolitions companies specialize in. One such company is Controlled Demolition Inc., which, incidentally, was subcontracted by Tully Construction to coordinate the removal of rubble from Ground Zero and the disposal of the structural steel in the months following the attack.

The steel skeletons of buildings like WTC 7 are extremely robust. They are designed to withstand earthquakes and hurricanes, and are over-engineered to handle several times the maximum loads anticipated during their lifetimes. Such steel skeletons have local structural integrity. An event that destroyed one portion of the structure would not cause distant portions to shatter. If some force obliterated the load-bearing columns well below the top of a 600-foot tall skyscraper, the top of the building would topple like a tree, not smash its way down through intact floors and into its foundation.

Controlled demolition destroys vertical steel structures while overcoming their tendency to topple onto adjacent real-estate. It does so by shattering the steel skeleton through the precisely timed detonation of explosive charges.

Demolitions are large undertakings with high stakes. The number of charges required is at least the number of columns times some fraction of the number of floors. An error in timing of the detonations could cause expensive collateral damage.

Most demolitions seek to implode the building, causing the mass to move toward the center, resulting in a tidy rubble pile. In tall buildings this is typically done by shattering the interior structures of the building first or ahead of the exterior structures. That causes the interior mass to fall first, pulling outer structures toward the center. Pieces of the outer walls end up on top of the rubble pile. 1  

Building 7's documented vertical plunge and tidy rubble pile with exterior wall fragments on top are exactly the kinds of results that controlled demolitions achieve through careful engineering.


1. How Building Implosions Work, howstuffworks.com, [cached]

page last modified: 2007-07-29
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