Construction of the World Trade Center

In honor of the tragedy that took place 21 years ago, we want to dedicate this blog to those who were lost too soon, those who lost loved ones, and everyone affected. September 11th was a day where our nation came together as one and mourned those we never met and would never know. The World Trade Center (WTC) at the time was the tallest buildings in New York City, and briefly were the tallest buildings in the world! There are some amazing features these towers had that greatly impacted the construction industry, so we’d like to share some interesting facts and statistics about the construction of the Twin Towers.

The Numbers Behind the World Trade Center

The 16 acre complex was home to 7 buildings and the Austin J Tobin Plaza (5 acres).

There was almost 10 million square feet of rentable office space. This size is equivalent to nearly 3 Disneyland Parks.

The World Trade Center had its own zip code, 10048, and the governmental offices inside were 10047.

On windy days, the towers swung up to a foot on either side.

It took 20 days to clean all the windows on the towers.

Construction Facts

1. The foundation construction used a ‘Slurry Trench’ method.

Due to the towers being near the Hudson River, the foundations needed to resist the possibility of flooding. The site originally was a landfill meaning the excavation for the foundation would require digging 70 feet into the bedrock, resulting in more than 1 million cubic yards of dirt being brought up. For the flooding issue, a 3,500-foot long, 3-foot wide trench was dug around the perimeter of the building site and filled with a mixture of water and bentonite creating an absorbent clay, known as slurry. Steel cages at 7-stories high, weighing 25 tons, were placed into the trench and enclosed in concrete. The foundation was known as ‘the bathtub’ and proved to be extremely successful!

2. The building is supported from the outside.

Typically, skyscrapers are supported with large interior beams running vertically through each level. However, the office spaces were designed to be open and airy in the WTC. With this, engineers used stiff steel tubes on the outside of the building to place the strength around the office rather than inside.

3. Kangaroo cranes were used to construct the towers.

In total, an estimated 200,000 tons of steel was used to construct the towers. However, as the building grew taller, the gravitational force on all this steel as grew as the towers planned height maxed out any possible crane. Instead, engineers used an Australian-designed crane named the ‘kangaroo crane’. These cranes used hydraulics to simulate a ‘jump’ that would increase their reach by 3 stories, allowing the towers to be built to their planned height.

Ericshawwhite, CC BY-SA 3.0, via Wikimedia Commons
4. The elevator design was inspired by the New York City subway system.

With each tower standing at 110 floors, engineers wanted to minimize the amount of space used for these to maximize office space. To do so, they used the subway system for a guide. They split the building into three zones, each with their own elevator. One was specifically for visitors wanting to see the sky deck, and the others were for office staff. This allowed for visitors and staff to easily access their desired areas, while also maximizing the rented office space.

5. There was an exemption in the amount of stairwells used compared to building code standards.

One of the most unfortunate construction facts about the WTC are the stairwells. At the time, building codes in New York required that buildings with the towers height must have 6 staircases. However, the Port Authority was exempt from this code, thus the towers only having 4 staircases. The stairs were also all located in the middle of the building to, again, help maximize the amount of office spaces, which in turn proved to be a devastating decision when tragedy struck in 2001.

This day changed America forever and will always be the event that brought America together for the first time in a long time. While the construction of these amazing skyscrapers is impressive, we’ll never forget the lives that were taken far too soon from an act of hate. We send our deepest condolences to the families and friends who lost loved ones on September 11th.

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