How was the construction of the World Trade Center unique?

There were two main factors that greatly distinguished the two main towers of the World Trade Center from anything that had been built before them: their walls and their elevators.

Prior to the construction of the Twin Towers, skyscrapers were designed to support themselves through large internal columns spaced about 30 feet (9 meters) apart, which interrupted the flow of interior space. For this project however, the engineers came up with a different solution -- the exterior walls themselves would support the bulk of the structure, and they would get a boost from one single column of beams in the center.

This allowed for a much more open plan on every floor of the building, which not only had aesthetic value but had financial worth as well -- the more floor space, the higher the rent the buildings' owners could collect.

Adding to the creation of open floor plans was the design of the elevators. A classic problem in skyscrapers is that as buildings grow taller, the number of residents increases. With more residents, more elevator shafts are needed. But the more elevator shafts there are, the less floor space there is for tenants.

This issue was solved in the construction of the twin towers through the use of express and local elevators. In much the same way the New York City subway system worked, express elevators would take passengers to "sky lobbies" placed on various floors throughout the building where they would then disembark and switch to local elevators to get to their required floor. The use of this system cut the number of required elevator shafts in half, thus preserving valuable floor space.

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.