The Twin Towers began construction in 1966 and were completed in 1973. In all, the structure was built with 47 steel core columns, 236 steel exterior columns, and countless steel trusses and I-beams that held all of it together. The steel core columns occupied approximately 40% of the space of the towers, making up the central support structure for each building. These center columns were connected to one another through a series of I-steel beams, and were then connected to the exterior columns with that same web of steel trusses and beams. The exterior column assemblies were constructed from a framework of 1.5-foot thick solid steel beams that were prefabricated in triplicate, then staggered for strength and bolted together to create a massive mesh of steel--one which was essentially over-engineered for its time.
from the film: World Trade