3.5.1 Design

The Twin Towers were built with a 'tube-in-a-tube' design.

47 massive vertical steel columns, each at least 3 feet by 16 inches in size and with steel 2 inches thick, and latticed together with horizontal and diagonal steel beams, made up the central core. The central core held up the building, extending 9 floors underground.













Around the perimeter were 240 smaller vertical columns, measuring 14 inches by 13.5 inches, built in sets of three.













The floors were grids of interlocking steel trusses that were welded and bolted to the core and perimeter columns, and covered in a 10cm layer of concrete. Both Towers were identical in design and contained 110 floors.



The buildings were specifically designed to survive multiple impacts from the biggest jetliner around at the time, the Boeing 707. The planes used on 9/11 were hardly any bigger than these planes.





The World Trade Center consisted of seven buildings. WTC1 and WTC2 were the North and South Towers. Buildings 3, 4, 5 and 6 were all less than 10 floors and were at the base of the main Towers. Building 7 was constructed later (in 1986) across the street, 330 feet from WTC1, and contained 47 floors. Building 7 was designed differently to the Twin Towers: it was 100% steel (no concrete) and had a trapezium shaped. Like the Towers, Building 7 was supported by its core columns (25), with the perimeter columns (58) mostly to hold up the floors.
























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