The world watched in astonishment 20 years ago today as one of the most iconic skyscrapers in US history and the greatest symbol of New York City was demolished

The Twin Towers were part of the World Trade Center. It was a complex of buildings located in Lower Manhattan, New York City, United States, inaugurated on April 4, 1973.

Today marks 20 years since its collapse and the world remembers the horror experienced by the attacks of September 11, 2001, which destroyed the most representative symbol of New York.

When the North Tower (1970) and South Tower (1971) were completed, they were known as the Twin Towers and were the tallest buildings in New York City at the time. The rest of the complex consisted of five corporate buildings housing various companies.

Their story

The building was constructed between 1966 and 1985 at a total cost of $400 million and was located in the heart of the Financial District in Lower Manhattan. It was designed by Minoru Yamasaki and Associates in the early 1960s. Each tower had 110 floors. The cornerstone was laid on August 5, 1966.

Attempt

The Twin Towers had suffered minor attacks, but the one that occurred on the morning of Tuesday, September 11, 2001, left the world in a state of shock, when a group of Al-Qaeda hijackers managed to crash two Boeing 767 planes into them, carrying out one of the worst terrorist attacks on the United States.

The first plane struck the North Tower at 8:45 AM. The building burned for 102 minutes and collapsed at 10:28 AM in just 11 seconds. At 9:03 AM, the second plane crashed into the South Tower. This building withstood the flames for 56 minutes and collapsed at 9:59 AM in 9 seconds; the death toll was 2,763. Building 7 collapsed that same day, and the rest had to be demolished because the damage was irreparable. The process of clearing the debris and recovering the site took eight months.

Its construction

In March 1965, the Port Authority began acquiring the land. The site was located on a landfill 20 meters below bedrock. To complete the project, a retaining wall was constructed along the west side of the site to keep out water from the Hudson River. It took fourteen months to finish the retaining wall, which was necessary before excavating the interior of the site could begin.

Tishman Realty & Construction was commissioned to oversee the construction of the project. Work began on the north tower in August 1968; work on the south tower began in January 1969

Architecture

Minoru Yamasaki was the lead architect for the project and appointed Emery Roth & Sons as associate architects. He designed a plan to incorporate two identical towers into the complex; he originally planned for each tower to be 80 stories tall. However, to meet the Port Authority's requirement of 930,000 square meters of office space, each tower would have to be 110 stories tall.

The Design

A major limitation in this type of construction is related to elevators; there is a rule that the taller a building is, the more elevators are required to serve it, and this consumes too much space. Yamasaki and the other engineers agreed to use a new system that featured two special lobbies, allowing users to transfer from a high-capacity express elevator to a local elevator, which could stop on any floor of a section. In total, there were 104 elevators in each tower.

Structure

The structural engineering firm Worthington, Skilling, Helle & Jackson was responsible for implementing Yamasaki's design. The Port Authority's engineering department served as the founding engineers, Joseph R. Loring & Associates as the electrical engineers, and Jaros, Baum & Bolles as the mechanical engineers for this enormous project. One of the lead engineers, Leslie Robertson, worked closely with Canadian engineer Alan G. Davenport to develop the viscoelastic dampers, capable of absorbing much of the swaying

Height

When its construction was completed in 1970, the North Tower became the tallest building in the world for two years, surpassing the Empire State Building, which had held the title for 40 years. It stood 417 meters tall and had a 110-meter-high telecommunications antenna or mast added to its roof in 1978. The highest point reached 527 meters.

The south tower consequently became the second tallest building in the world when it was completed in 1971. The observation deck on the roof was 415 m high and the internal one 400 m.

The Twin Towers held the title of tallest building in the world until 1973, when the Willis Tower in Chicago reached a height of 440 m to the roof. During their existence, they had the most floors (110), a number only surpassed by the construction of the Burj Khalifa, inaugurated in Dubai in 2010.

The entire complex, comprised of buildings, had 1,040,000 square meters of space. Initially conceived for companies and organizations directly involved in commerce, it did not initially achieve the expected success in attracting those types of clients. In the early years, several government organizations were key tenants of the World Trade Center, including the State of New York.

It wasn't until the 1980s that the city's financial situation began to improve, and private companies, primarily financial firms linked to Wall Street, became tenants. By the 1990s, around 500 companies were operating from within the complex.

The 110th floor of the north tower was where the radio and television transmission equipment for most of the NY channels was located.

The visits

Visitors had to pass through strict security checks. They could take two short trips via stairs, weather permitting, from the observation deck on the 107th floor to another external one located on the 110th floor. On a clear day, they could enjoy views of more than 80 km away. As an emblematic figure in the world of American finance, it was a popular spot for tourists who wanted to go to the top and take photos of the panoramic view.

Now Ground Zero

One World Trade Center is the main building of the new complex built on the site, with 104 floors. It cost $3.9 billion and was inaugurated on November 3, 2014. It stands 417 meters tall to its roof, reaching a total height of 541 meters. Currently, it is the tallest building in the Western Hemisphere and ranks sixth among the tallest buildings in the world.

The National 9/11 Memorial is the site of the former Twin Towers. There, two identical reflecting pools, known as the Forest of Trees, stand on the same grounds where the Twin Towers once stood. The names of all those who died in the attacks are inscribed in bronze on the upper walls of the pools. The 9/11 Memorial Museum displays artifacts from the buildings, photographs of the attack, and other items.

Nearby is the Oculus, a structure designed by the Spanish artist Santiago Calatrava that connects up to 11 different subway lines, provides underground access to the main towers of the World Trade Center, and houses a shopping mall and restaurants. It was inaugurated on March 4, 2016, at a cost of $4 billion. Its design symbolizes a dove released by a child's small hands and has been listed as the most expensive subway station in the world.

Sources: Knowing USA/Wikipedia.