The design of the Twin Towers is often called a "tube within a tube," referring to the fact that all of the weight of the building was supported by the external walls and an internal column. Previously, the exterior walls of a skyscraper were called curtain walls -- they weren't relied upon for strength, so it wasn't imperative that super-sturdy materials were used for them.
But for towers one and two, the external walls would not only bear the weight of the interior floors, but they would also have to withstand tremendous pressure from the wind. Because the external "tube" of each tower was perforated with openings for windows, the entire web of steel could shift in strong winds, transferring the load from the windward side to the leeward side of the buildings through something known as Vierendeel action [source: FEMA].
For the columns that comprised the walls, a mixture of 12 different types of steel with yield points between 42,000 pound per square inch (psi) and 100,000 psi were used, while the interior columns consisted of a steel known as A36, a designation which meant it had a yield strength of 36,000 psi. The thickness of these columns also varied -- from as thin as 0.25 inch (6.35 millimeters) at the top of the building to as thick as 4 inches (10.16 centimeters) at the base [source: FEMA]. In all, 200,000 tons of super-strong steel (which had just recently become available in 1968) were used to create the two towers [source: Gayle].
Just inside the walls, at approximately 10,000 locations throughout each tower, visco elastic dampers were installed [source: FEMA]. These were basically large shock absorbers that could bend with wind pressure and then return to their original form. Because the towers were designed to sway and adjust in the wind, these dampers helped reduce the impact of this movement on occupants. It was the first time this technology had ever been used in a high-rise [source: FEMA].
The floors that flowed between the supporting walls and interior columns were made from 0.5 inch (1.27 centimeter) thick steel slabs covered in 4 inches (10.2 centimeters) of lightweight concrete.
Overall, 425,000 cubic yards (324935.8 cubic meters) of concrete were poured, 43,600 windows were installed, 12,000 miles (19312.1 kilometers) of electrical cables were laid and 198 miles (318.6 kilometers)of heating ducts were installed [source: Ross] to create the two majestic towers that helped define Manhattan's skyline for 30 years.