Title: Lower Manhattan 1970
Subtitle: December 14, 1970
Text: In December 1970, the North Tower was topped out and the South Tower had achieved about two-thirds of its 1362-foot height. At this peak of construction activity, some 5,000 workers were employed on the site.
The Twin Towers were the first supertall buildings to use "tube" construction, in which the outside walls were load bearing and acted as a thin shell, eliminating the conventional grid of columns used in frame construction. The prefabricated wall units spanned three stories and weighed up to 22 tons. They were hoisted into place up the outside the building by the four "kangaroo" cranes that sat atop each tower. As a section of floors rose above the height of the cranes, the 200-ton machines were "jumped" upward to the next elevated position by diesel-driven hydraulic jacks. The process was repeated over and over as the towers rose.
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