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Old 05-07-2013, 12:49 PM   #9
Rohirrim
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Join Date: Jan 2003
Location: Twixt Hell & Highwater
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Malik Jackson
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Interesting that even Wiki tells the story of WTC7. Not difficult to figure out:

The building was constructed above a Con Edison substation that had been on the site since 1967.[6] The substation had a caisson foundation designed to carry the weight of a future building of 25 stories containing 600,000 sq ft (56,000 m2).[7] The final design for 7 World Trade Center was for a much larger building than originally planned when the substation was built.[8] The structural design of 7 World Trade Center therefore included a system of gravity column transfer trusses and girders, located between floors 5 and 7, to transfer loads to the smaller foundation.[9] Existing caissons installed in 1967 were used, along with new ones, to accommodate the building. The 5th floor functioned as a structural diaphragm, providing lateral stability and distribution of loads between the new and old caissons. Above the 7th floor, the building's structure was a typical tube-frame design, with columns in the core and on the perimeter, and lateral loads resisted by perimeter moment frames.[7]
http://en.wikipedia.org/wiki/7_World_Trade_Center
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