For nearly three decades, the Twin Towers were among the most recognizable architectural symbols of New York City. They were not just two huge office skyscrapers. When they appeared, they represented a radical engineering experiment, changing the way supertall buildings could be designed, and creating a new visual center in Lower Manhattan.
On September 11, 2001, both were destroyed in the terrorist attacks on the United States. From a purely architectural perspective, this event left an unusual problem: how do you rebuild a site that is both an active part of one of the world's most expensive business districts and a space of memory?
The answer that New York gave over the next two decades is an architectural complex in which empty space is as important as new skyscrapers.
Two skyscrapers that change the skyline of Manhattan
The design for the original World Trade Center was the work of the Japanese-American architect Minoru Yamasaki, with Emory Roth & Sons as the project architect, and the structural engineering was entrusted to Skilling, Helle, Christiansen & Robertson.
Construction of the complex began in 1966. The North Tower, 1 World Trade Center, was completed in 1972, and the South Tower, 2 World Trade Center, in 1973. The complex officially opened on April 4, 1973. Both towers have 110 floors above ground.
The North Tower stands at 1,368 feet (417 m) tall, while the South Tower stands at 1,362 feet (415 m). Upon completion, they were the tallest buildings in the world, although they held the record for a relatively short time. But their architectural significance does not end with their height.
The revolution is in the construction
To raise 110 floors without the interior turning into a forest of massive columns, engineers used a system of framed tubes, which was innovative for its time.
The outer facade of each tower is both an architectural shell and an important part of the supporting structure. Along the perimeter, dense steel columns are located, interconnected. Together with the inner core, they absorb vertical loads and enormous horizontal forces from the wind.
This allows a large part of the office floors to remain practically free of internal supporting columns.
Another important innovation is the organization of the elevators. Instead of all the elevators passing through the entire height of the building, the towers use a system of express elevators and the so-called sky lobbies, where passengers transfer to other elevators.
According to NIST, this arrangement increases the usable area of a typical floor from about 62% to 75%. The principle has since become a standard solution in many of the world's tallest buildings.
Thus, the Twin Towers are important not only as a visual symbol, but also as a laboratory for technologies that will make the next generation of supertall skyscrapers possible.
Almost 10 million square feet of office space
The two towers provide a total of nearly 10 million square feet of office space, approximately 929,000 square meters. The entire World Trade Center complex covers about 16 acres and includes seven buildings.
As of 2001, the complex employed about 50,000 people and housed more than 430 companies from 28 countries. Tens of thousands of visitors and travelers pass through it every day. Therefore, these are not just two office buildings. The World Trade Center functions as a vertical city. In addition to offices, the complex has shops, restaurants, transportation links and public spaces.
One of the most famous places is the Windows on the World restaurant, located on the 106th and 107th floors of the North Tower. Opened in 1976, the complex includes a restaurant with about 300 seats, a bar, a wine cellar, smaller dining areas, banquet and conference rooms. The South Tower has an observation deck, which has become one of the tourist attractions of New York.
How much did they cost to build?
Historical sources give different numbers here depending on whether only the construction of the towers or the entire complex and infrastructure is taken into account.
PBS indicates a value of about 400 million USD for the Twin Towers. Broader historical estimates place the Port Authority's investment in the entire World Trade Center, including infrastructure, at around $900 million at current prices.
Therefore, the straightforward statement that "the Twin Towers cost $900 million" may be misleading. It is more accurate to view the amount as an investment in the broader World Trade Center project.
September 11 interrupts architectural history
On September 11, 2001, the North and South Towers were hit by two hijacked Boeing 767s.
According to the final technical investigation by the US National Institute of Standards and Technology, NIST, the aircraft strikes damaged load-bearing columns and removed some of the fireproof insulation of the steel elements. The subsequent multi-story fires heated and weakened the structure. The deformation of the floor systems gradually pulled inward some of the external columns, which led to the beginning of the progressive collapse. The South Tower collapsed at 9:58 a.m., and the North Tower at 10:28 a.m.
From an architectural point of view, New York lost in less than two hours two buildings that had defined the composition of the Lower Manhattan skyline for almost three decades. And the far more difficult question begins: what should be built in their place?
Instead of two new towers on old foundations: empty space
One of the most important decisions in the reconstruction was not to build new skyscrapers on the exact sites of the two destroyed towers. Their footprints have been preserved. There today are two huge square memorial pools, each almost an acre in size. They are the centerpiece of the National September 11 Memorial.
The memorial was designed by architect Michael Arad, in collaboration with landscape architect Peter Walker. The project is called Reflecting Absence, or “Reflecting Absence“.
The water cascades down the walls of the two pools and disappears into smaller square openings in the center, the bottom of which is not visible from the level of the square.
Arad himself defines the idea as “absence made visible“. This is perhaps the most significant architectural change compared to the old World Trade Center. Before 2001, the site demonstrated height, scale and engineering power. The new memorial uses the opposite: emptiness, negative volume and downward movement.
Where before the architecture rose more than 400 meters, today it symbolically sinks into the ground. The memorial opened on September 11, 2011, exactly ten years after the attacks.
Underground Museum
The National September 11 Memorial Museum was built next to the memorial, which opened to visitors in May 2014. The architectural solution is again unusual: a significant part of the museum is located below street level, in the very archaeological context of the old World Trade Center.
Visitors descend to the foundation level and can see original structural elements. Among them is part of the so-called slurry wall, the huge underground wall built in the 1960s to keep the waters of the Hudson River out of the construction pit. It survived the demolition of the complex and is now part of the museum's architecture. The memorial and museum cost a total of about $700 million.
The New Vertical: One World Trade Center
The new skyscraper is not placed on the footprints of the destroyed towers. It rises in the northwest part of the complex. One World Trade Center was designed by the team of Skidmore, Owings & Merrill, led by architect David Childs. Construction began in 2006, and the building was completed in 2014. Its height is no coincidence. One World Trade Center reaches exactly 1,776 feet, or 541.3 meters, including the architectural peak. The number is a direct reference to 1776, the year of the American Declaration of Independence. The building has 104 floors according to SOM's architectural documentation and a total floor area of about 3.5 million square feet, approximately 325,000 square meters. The main function is again office, but the tower includes public spaces and an observation area.
The architectural image is different from that of the old towers. Instead of Yamasaki's repeating vertical grid, One World Trade Center uses large glass surfaces and a geometrically transforming volume. From a square base, the facades rise through beveled planes that create the tower's characteristic polyhedral shape.
The new tower costs nearly 4 billion USD
The price also shows how different the era is. The value of One World Trade Center reaches about 3.9 billion USD, which makes it one of the most expensive skyscrapers ever built.
However, comparisons with the original towers should be made with caution. The costs were incurred in different decades, with different dollar values, construction standards, technologies, and security requirements. In addition, the amount of about $900 million associated with the original World Trade Center includes a broader investment in the complex and infrastructure, while $3.9 billion is related to One World Trade Center.
To this must be added $700 million for the memorial and museum. This still does not represent the total cost of restoring Ground Zero, because the new World Trade Center also includes other office buildings, transportation infrastructure, and public spaces.
From an architectural symbol to an entire urban ensemble
Instead of the old World Trade Center, not a single building was actually built to “replace” the Twin Towers. A new urban system was built. The new complex includes office skyscrapers, the memorial and museum, transportation infrastructure, and commercial and public spaces. Thus, the function of the place remains economic and urban, but a permanent memorial function has been added to it.
And here lies the greatest architectural difference between the old and new World Trade Centers. Yamasaki's original complex is an architecture of repetition. Two almost identical rectangular monoliths create power through their duality and scale. They dominate the city and can be recognized from miles away.
The new complex refuses to copy them. The vertical symbol is only one, One World Trade Center. There are no new skyscrapers in the places of the two destroyed towers. There are two gaps. Thus, architecture solves an unusual task: to simultaneously restore the Manhattan skyline and leave part of it forever missing.
A quarter century after 9/11, this is perhaps the strongest feature of the new World Trade Center. The old towers demonstrated what engineering can raise upwards. The new architecture shows that sometimes the most important space in a city is the one that is deliberately left undeveloped.
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