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.
RELATED: WORLD TRADE CENTER TRANSPORTATION HUB SKYROCKETS OVER BUDGET
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.