Imagine standing at the edge of a massive, 16-acre hole in the middle of Lower Manhattan in 1966. It was basically a giant pit of mud and old radio shops. People called it "Radio Row," and it had to go to make room for something the world had never seen. When we talk about the twin towers being built, we aren't just talking about a construction project. It was a massive, $400 million gamble that broke every rule in the architecture handbook.
The Port Authority of New York and New Jersey wanted something big. Huge, actually. They hired Minoru Yamasaki, an architect who was actually afraid of heights, which is kind of ironic if you think about it. He teamed up with the engineering firm Worthington, Skilling, Helle & Jackson to figure out how to stack 110 stories of steel without the whole thing collapsing under its own weight or swaying so much that office workers got seasick.
The bathtub that saved Lower Manhattan
Before a single piece of steel could go up, they had to deal with the water. See, the site was right next to the Hudson River. If you just start digging a 70-foot deep hole in Manhattan, the river is going to fill it up pretty fast. They couldn't just pump the water out because it would undermine the foundations of all the nearby buildings.
The solution was the "slurry wall." It’s basically a massive underground bathtub. They used giant machinery to dig a trench, then filled it with a mix of water and bentonite clay (the slurry) to keep the walls from caving in. Then, they lowered 22-ton steel cages into the trench and poured concrete in from the bottom. The concrete pushed the slurry out, and once it hardened, they had a three-foot-thick waterproof wall. This was a massive deal. Without that bathtub, the twin towers being built would have been physically impossible.
Steel skin and the "hollow tube" design
If you look at old photos of the Empire State Building being built, you see a forest of internal columns. That’s how buildings were made back then—a grid of steel inside to hold everything up. But Yamasaki and his lead engineer, Leslie Robertson, wanted open floor plans. They didn't want people bumping into pillars every five feet while they were trying to type on their Selectric typewriters.
So they flipped the design inside out.
They created a "tube-frame" system. Instead of the support being in the middle, they moved it to the outside. The exterior walls were actually the load-bearing structure. They used 59 perimeter columns on each side, spaced very closely together—only 39 inches apart. This is why the windows in the original World Trade Center were so narrow. Yamasaki wanted people to feel secure, and those narrow windows acted like a cage. It worked. The buildings were incredibly stiff.
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Moving people through the sky
Elevators were the next nightmare. If you had 110 floors and used traditional elevators, the elevator shafts would have taken up half the floor space on the lower levels. You’d have no room for desks.
The engineers looked at the New York City Subway for inspiration. They created "sky lobbies" on the 44th and 78th floors. You’d take a massive express elevator to a sky lobby, then hop off and take a local elevator to your specific floor. It was genius. It cut the number of shafts needed by nearly half, which meant more rentable square footage. This "local-express" system is now the standard for almost every supertall building on the planet, from the Burj Khalifa to the Shanghai Tower.
The day the steel arrived
By 1968, the first pieces of steel were moving into place. The North Tower (One World Trade Center) started rising first. They used these massive "Kangaroo Cranes" from Australia that could actually sit on top of the building and lift themselves up as the tower grew.
It was a logistical mess.
Every single piece of steel was prefabricated in factories across the country. They’d bring the steel to a staging area in New Jersey, then truck it across the Holland Tunnel at night. Each piece was numbered and had a specific "home" in the structure. If one truck was late, the whole assembly line slowed down. At the height of construction, they were adding about two floors every week. It was fast. It was loud. And honestly, it was dangerous. Despite the scale, only 60 people died during the entire construction process, which, while tragic, was statistically lower than many other projects of that size at the time.
Why the floor system was weird
Most people don't realize that the floors weren't just solid slabs of concrete poured over beams. They were actually lightweight steel trusses. These trusses spanned the distance between the central core (where the elevators were) and the exterior "tube" walls.
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They were basically big metal zig-zags.
They’d hoist these pre-assembled floor sections—which already had the corrugated steel deck and some utility lines attached—and bolt them into place. Then they’d pour a thin layer of concrete on top. This made the floors incredibly light, which was great for the foundation but would later become a point of intense study by structural engineers looking at the buildings' performance under extreme stress.
Controversy and the "monsters" of Manhattan
Not everyone loved the twin towers being built. In fact, a lot of people hated them. Famous architectural critics like Ada Louise Huxtable were pretty brutal, calling them "gigantic clichés" and "boring." There was also a huge lawsuit from the Empire State Building owners. Why? Because they were terrified that all that new office space would drive down rents across the city.
And then there was the television problem.
In the late 60s, everyone got their TV through antennas. The massive steel structures of the Twin Towers started reflecting signals, causing "ghosting" on TV screens all over the city. The Port Authority eventually had to move the main broadcast antennas from the Empire State Building to the North Tower to fix it.
A city within a city
When the North Tower was finished in 1970 and the South Tower in 1971, the numbers were just staggering.
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- Over 200,000 tons of steel.
- 43,000 windows.
- 1.2 million cubic yards of earth removed.
- 6 acres of marble.
The towers had their own zip code (10048). They housed 50,000 workers and saw 200,000 visitors a day. They weren't just buildings; they were a vertical neighborhood. The Windows on the World restaurant on the 106th and 107th floors of the North Tower became one of the highest-grossing restaurants in the United States.
The engineering legacy
What we learned from the twin towers being built changed engineering forever. We learned that steel doesn't have to be a solid wall to be strong. We learned that wind is the real enemy of a skyscraper, not gravity. The towers were designed to sway about three feet in the wind, and they used special viscoelastic dampers—basically giant shock absorbers—to soak up that energy so people inside wouldn't feel the movement.
It’s easy to look back now and focus only on how they came down, but from a purely technical standpoint, their construction was a masterclass in problem-solving. They proved that you could build "the impossible" on a piece of land that used to be underwater.
Putting the facts into practice
If you're researching the history of urban planning or structural engineering, these towers represent the "high-water mark" of the 20th-century skyscraper. Understanding how they were put together helps you understand why modern buildings like One World Trade Center (the new one) are built so differently.
Next Steps for Research:
Check out the "Slurry Wall" remnants. If you visit the 9/11 Memorial & Museum, you can actually see a section of the original slurry wall that held back the Hudson River. It’s a massive piece of engineering history that is still doing its job today.
Look into the wind tunnel testing. The Twin Towers were among the first buildings to undergo extensive wind tunnel testing at Colorado State University. Researching the work of Jack Cermak will give you a deeper look into how they prevented these giant boxes from vibrating themselves apart in a hurricane.
Compare the "Tube" vs. "Core-and-Outrigger." Modern skyscrapers (like the ones in Dubai) often use a central concrete core with "outrigger" arms for stability. Comparing this to the Twin Towers' "hollow tube" design shows exactly how engineering philosophy has shifted away from external steel skeletons toward reinforced concrete cores.