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Meanwhile, orders were placed for the 200,000 tons of steel which would be needed for construction. Individual pieces were prefabricated and rarely interchangeable. Moreover, there would be no room to stockpile materials at the construction site. This created a tremendous logistical problem. Steel sections would have to arrive at the site in the exact order and at the exact time needed. To meet this challenge Port Authority engineers used a computerized system known as Critical Path Method, or CPM. CPM would coordinate every aspect of production, track the flow of materials and minimize any delays.
In August, 1968 actual steel construction began. Kangaroo cranes imported from Australia were used for the first time in the United States. The cranes were assembled on top of the core columns. Each could lift 60 tons at a time. They would be the driving force behind the towers' construction. The cranes had the ability to jack themselves up 36 feet at a jump. As the walls grew to the height of a crane, the crane would hoist itself up, a neighboring crane would swing core columns into place beneath it, and construction would continue.
Seventy feet up, at street level, steel trees were put into place around the perimeter of the towers. These trees would transfer the load of the exterior wall to the more widely-spaced columns now anchored to bedrock.
Construction of the towers could now proceed with great speed. Three basic elements were used: two- or three- story sections for the load-bearing wall, massive steel beams for the core columns and floor sections to span the sixty feet between core and wall. Through the complex orchestration of these structural elements, three floors could be constructed in ten days.
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