Realizing the design for unprecedented height and scale of the Twin Towers depended on the structural engineer. In April 1962, at the same time that Yamasaki signed a contract with the Port Authority, the Seattle based firm of Worthington, Skilling, Helle, Christiansen, and Jackson won their commission, and the young soon-to-be partner Leslie E. Robertson became the lead engineer on the project.

Supertall towers, a term that describes buildings of more than 80 stories, posed new engineering problems. The world’s tallest building from 1931 until the completion of the World Trade Center in 1972 was the Empire State Building, an 85-story office tower of 1050 feet, topped by a slender metal mooring mast rising to 1250 feet.

The height of the Twin Towers was described as 110 stories, measured from the plaza. From the perspective of design and construction, though, the towers rose more than 1,450 feet above their foundations in bedrock. The North Tower measured 1368 feet and the South Tower 1362 feet to their flat rooftops, surpassing the tip of the Empire State's spire by more than 100 feet and its top floor by more than 300 feet. Each building had 111 floors, with 116 framed levels including below grade services. On the North Tower, the TV mast raised the height to 1,815 feet. With 59 closely spaced columns per side, the towers were 209 feet square, and each floor was nearly an acre.