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A demolition was actually planned out in detail for the twin towers in the 1980's. The planners engaged architects who developed estimates for a complete take-down and rebuild and drafted conceptual drawings. The prominent firm of Emory Roth then took over as project architects and occupied a tower office.
The demolition of such gigantic steel structures, with their thick concrete floors, if lawfully performed in conformance with New York City codes, would have been an immensely arduous and expensive task and was estimated back then at $5.6 billion.
The costs included the slow and laborious task of cutting, with oxy-acetylene torches, the giant hardened steel members of the high-rise structures. (In those days one could not so easily melt steel at low temperatures, as for example by burning kerosene, the new official physics allowing for this process having not been established until a few weeks after September 11, 2001.)
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http://portland.indymedia.org/en/2006/01/331483.shtml