Design
A key aspect of the innovative engineering of the World Trade Center was the detailed design of the new thin-shell or "tube" construction, which used many different strengths of steel to respond to the varying loads and external forces on the buildings' structure.

A family of steels—from plain carbon to quenched and tempered steels—were used to meet the required strength, both by the type of component, such as wall panels or floor trusses, and often by the location within the structure based on analysis and wind-tunnel testing. This approach resulted in both structural efficiency and significant savings in the purchasing of hundreds of tons of structural steel. The design allowed for column-free space of 60 feet clear span, with less steel and a lower story height than is found in most 50-story buildings.

Prefabrication
Another area of innovation included the prefabrication of structural components developed to a point never before achieved in high-rise buildings. There were nearly 6,000 prefabricated wall panels, generally three stories high and three columns in width, 36 x 10 feet, with the heaviest units weighing 22 tons. On the lower floors, some units measured 56 x 10 feet and weighed as much as 54 tons. There were also 5,716 prefabricated floor panels, generally 20 x 60 feet, including the steel deck and electrical raceways; this design allowed for the floor-to-ceiling height to be kept to 12 feet.

Digital Format
For the first time, the detailed design for most of the steel work was presented in digital form with IBM cards, rather than in conventional drawings. Using only data cards from the design as input, 55,000 tons of structural steel was detailed by computer and checked with the fabricators through the exchange of data cards.

Contracts
When the Port Authority rejected the bids for the overall project from the dominant steel companies US Steel and Bethlehem Steel, the engineers’ detailed specifications for the different steel components made it possible to divide the work into multiple contracts to be bid by smaller, more specialized companies. As John L. Tishman and Leslie E. Robertson explain in the audio clips here, ultimately there were eleven basic steel contracts and thirty-nine different fabricators.