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Nearly two decades ago, the September 11th attacks rocked the U.S. and the world. Following the destruction, New York was left without world trade centers. It was in this aftermath and rebuilding effort that the One World Trade Center was conceived – and construction began.
The skyscraper now stands as the tallest building in the entire western hemisphere and the tallest outside of Asia. It has become the iconic building of the modern NYC skyline and serves as a symbol of the city's resilience following the fateful attacks in 2001.
The structure and design of One WTC
The footprint of One World Trade Center is symbolic in its own right as it is a 200-foot square footprint, the exact same as the original towers. It's perfectly rectangular for the first several floors, at which point it begins to taper, all the way to the top floor, the 104th.
Due to the way that it twists, in the middle of the structure, its floors make a perfect octagon.
Another notable feature designed into the structure is the fact that from the base to the parapet, or top of the roof, the structure is the same height as the twin towers, 1,368 feet (417 meters). Engineers designed the total height with the tower on top to be 1,776 feet (541 meters). That number should ring a bell to any American watching, as 1776 is the year of the signing of the Declaration of Independence.
Safety was paramount in the engineering process
The entire engineering and construction process of the towers was based on the lessons drawn from the collapse of the two world trade centers prior. Coming out of that disaster, it was clear that the building codes that were in place weren't going to cut it to create a perfectly safe skyscraper, so the engineers worked to make it as safe as possible.
The team behind the project had to figure out how to increase safety while still staying within a reasonable budget, which meant a complex analysis of the world's best practices in the industry, as well as even consulting with first responders to determine what would help them do their jobs better.
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The original WTCs were built using external steel columns, but the new tower utilizes a hybrid concrete and steel structure. The structural components have a concrete core to take the compression loading with a strong steel exterior, to support tension, and normal wear. This hybridized steel and concrete structure rises to the top stories of the structure.
The floors of the building are made up of concrete, though massive steel beams were cast into the floors to allow for as few columns as possible on the inside. This is a notable design element of the internal aesthetics, as anyone who visits will notice the significant lack of internal supports.
As far as a new safety design that was implemented in the structure, its concrete core now has a dedicated first-responders staircase, which allows firefighters to respond to dangerous situations while inhabitants flee through the normal staircase. This has since become standard building code for new structures in New York.
The "core" that houses these specialized staircases is made of high-strength 14,000 PSI concrete. If you know anything about normal concrete compression ratings, that number should shock you. It's the strongest concrete ever poured in New York. This increased strength per square inch also meant that the walls of the center core could be thinner than normal and still support increased loads.
In the concrete mix, the engineer also replaced 50% of the cement content with industrial by-products to cut the carbon footprint of the building. Think of that process as recycling of industrial waste.
Going along with that concern for the environment, 95% of the steel in the structure was recycled, and the building overall got a Gold environmental rating from LEED, or Leadership in Energy and Environmental Design.
In total, the construction of the One World Trade Center took 8 years and 6 months, finally being completed in November of 2014 – costing an incredible 3.9 Billion USD.
Simple yet powerful aesthetics in design
The architects who designed the world trade center specifically created the tapering of the sides to catch light from the sky in a way that would exemplify the towering nature of the building. This ability to catch light wasn't just a geometric feature, but also thanks to the 1 million square feet of specially designed glass that coats the exterior.
This glass has an extremely high U-Value, or thermal transmittance. Essentially, this means that the glass does not insulate very well, by design, so that much of the thermal energy from the sun can be used to heat the inside of the building.
Glass plays a pivotal role in the look and feel of the building as the geometric architecture is rather simple – yet striking.
Upon entrance to the building, you'll be met with a 50-foot high glass-walled atrium followed by stories 20-91 being used for office space. The remaining floors are used for mechanical components and the top three floors are a public observation deck.
If you'd like to watch an entire documentary on the construction of this building – as engineers like to do – we'd recommend watching the following.