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Total Progressive Collapse: Why Buildings Sometimes Crumble

h3. (Image: "David Carriel":http://www.flickr.com/photos/15765624@N08/) _ODDITY examines strange and esoteric phenomena and events from the remote, uncanny corners of technology, science and history._ There’s no mystery, no great and shadowy...

There's no mystery, no great and shadowy cabal behind the force that brought the Twin Towers tumbling down. In fact, total progressive collapse, an engineering phenomenon, is to blame. First recognized in the late 1960s at Ronan Point, a 22-story apartment block in Newham, east London, where an 18th floor gas explosion obliterated a load-bearing wall, TPC triggered a partial collapse that brought an entire corner of the complex buckling to the ground.

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Via The Daily Telegraph, 1968

The idea is simple enough: Any buildings bearing severe, concentrated blows that cripple significant structural components (support beams, columns, etc.) can crumble if the damages aren't contained and if components elsewhere blow out. According to Mechanics of Progressive Collapse (PDF), when upper floors crash down on those below, sandwiching layers of rubble and debris, the initial height h of a story reduces to λ h, where λ expresses the compaction ratio. (In finite-strain theory, λ is called "stretch.") From here, "the load can increase without bounds."

Blink and you'll miss the fall. But however quick and dirty, TPC is really a two-phase process: Crush-down, whereby gravity and the immense downward kinetic thrust of floors upon mashed floors successively crushes everything below, gives way to crush-up, the free-falling top floors now piling on top of the wreckage. This second phase further pulverizes the rubble, as witnessed in the 1995 bombing of the Murrah Federal Building in Oklahoma City.

Or the collapse of the New World Hotel in Singapore in 1986.

Or the fall of the 2000 Commonwealth Ave. tower in Boston in 1971.

Or the collapse of the World Trade Center towers, described most thoroughly in the "NOVA documentary How the Towers Fell.

Total progressive collapse is a potential "failure mode" of extreme concern, but in certain cases it can clear the air of any suspicions of charged demolition.

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h5. Top image:

David Carriel