The Role of Steel in the World Trade Centers: The Engineering of the Twin Towers

Steel factory floor, stacked with a lot of steelIn September of 2020, the country looked back to what was a history-altering event that happened in New York but shook the entire nation. The day of September 11th is never easy to revisit, but remembering our history is an important part of understanding our present. In the last few blogs, we have focused on the role of steel in the 20th centuries most far-reaching global conflicts. This month’s post, we brought it back home. We thought we’d take a look at how the world trade centers were a feat of engineering and the role that steel played in those historic buildings that came down almost twenty years ago. 

Here at Steel Specialties, we have contributed to building everything from schools to car dealership lots, to baseball stadiums, churches, and more. It goes to show just how much steel construction is part of our daily lives. When we go on vacation, we might not pay much attention to the fact that we’re likely traveling on incredibly well-structured and engineered metal birds. 

Architectural History of the World Trade Centers

The building of the World Trade Center was a long haul project. The concept really began after World War II, when New York City wanted to make itself an epicenter of transatlantic trade. David Rockefeller and others worked on the project but ran into considerable obstacles along the way. It wasn’t until the 1960s that the project gained traction and the New York Port Authority became involved. Architect Minoru Yamasaki (who interestingly had a fear of heights) was finally chosen to spearhead the architecture of the buildings and work continued. The final design was released to the public in 1964. Structural engineers, foundation engineers, and mechanical engineers got to work in hashing out the project and working out the logistics. 

When the Twin Towers were first erected, they had a mixed reception. Bird lovers protested that it might harm migrating fowl and get in the way of nature’s high flying creatures. The architectural community of New York deemed them the two filing cabinets, others exclaimed that they looked like the boxes the Chrysler Building and Empire State Building came in, folks in the broadcasting field said it would interfere with television reception signals.  

Steel Numbers —  A Look at Steel’s Presence in the Iconic Buildings 

Engineering feats and challenges were aplenty during the construction of the World Trade Centers. As always, there must be a balance between functionality and safety, and an inspiring design that satisfies the eye. Famously, one of the biggest logistical hurdles faced by the architect was the question of elevators that occupied a tremendous amount of space. This problem was solved by the use of sky lobbies, and having people moving to the higher floors of the building switch off on localized elevators. 

Digging In

The ground in lower Manhattan was largely landfill, the engineers would have to dig down 70 feet to reach bedrock. Once they did, they would remove rocks and dirt and pour water and bentonite, to plug up any crevices. Then, workers would lower a 22 ton, seven-story high steel cage into the trench and stuffed it with concrete that would then displace the bentonite mixture and serve as a very powerful foundation and help seal the basements and keep water from the Hudson River away from the base.  

Towards the Sky

For the columns that comprised the walls, there was a mixture of 12 different types of steel. The yield points on these were 42,000 pounds per square inch and 100,000 psi were used. The yield point refers to the amount of load the steel can take before its behavior starts to shift a little. The engineers had to carefully plan that the interior steel columns would not take the brunt of the weight. The interior columns were constructed out of a special kind of steel called A36—with a yield strength of 36,000 psi. The thickness of the column also varied throughout the structure. In areas, the thickness ranged from .25 inches at the very top of the building to 4 inches at the base.  Part of the revolutionary design created by Yamasaki and his engineers was the idea of the two hollow tubes, which were mostly supported by closely spaced steel columns encased in aluminum. The exterior steel lattice would be bound and connected by robust floor trusses that would connect it to the interior steel core of the building. This helped the exterior or ‘skin’ of the building to help with support and not put all the weight on the interior steel column. 

The towers stood at 110 stories high and soared into New York’s skyline for almost thirty years.  The floors between the supporting walls and interior columns were constructed from slabs of steel of .5 inches thick of lighter weight steel. Overall, once you step back, the structure utilized approximately 200,000 tons of super-strong steel.

The project also used about 425,000 cubic yards of concrete, 43,600 windows, 12,000 miles of electrical cables, and much more!

Not a small undertaking by any stretch. And despite the many criticisms and doubts that the architectural and engineering community had about the buildings—not to mention local residents— the World Trade Center soon became symbolic and a quintessential part of the New York skyline. 

All of this would change, of course, on September 11th of 2001. And although it seems like just yesterday, it was almost 20 years ago that thousands of innocent Americans would perish in horrific and unforgettable ways. It was that day that the incredible structure, a feat of engineering, and American innovation and construction would collapse in flames. We will always mourn those losses and commemorate the heroes that for the duty of the job and inherent bravery went into those melting steel buildings to rescue their fellow countrymen and women.

The memorial that stands today where that incredible structure was erected is partly made of steel. The Port Authority of New York and New Jersey has given away more than 2,600 pieces of steel left from the rubble as historical artifacts. As it turns out, even steel can fall to forces of evil but like the country itself and the people that inhabit it, it is resilient and always comes back stronger. One might even say that America has a heart of steel!