The construction of the iconic Twin Towers

One of the most remarkable architectural accomplishments of the 20th-century was the building of the World Trade Center in Lower Manhattan, New York City. The seven-building complex would feature at its heart the two tallest structures in the world—the twin 110-story towers that were to symbolize the economic might of the USA.

Construction efforts on the site began in March 1965 and concluded by 1973 when the towers opened. It was one of the most challenging building projects ever to take place on the island of Manhattan. Traditional skyscrapers built before the 1960s had an entirely different support structure that would prove useless for high-rise buildings such as the twin towers. For instance, the Empire State Building, at the time the world’s tallest structure, integrated a system of large vertical columns passing through each floor, each at 15-30 feet (5-9m) distance. The exterior walls of this tower have a minor role in structural integrity and stability.

If the twin towers of the World Trade Center were to become the tallest skyscrapers in the world, the building concept seen in structures such as Empire State Building had to be abandoned. Japanese-American architect Minoru Yamasaki who was chosen to do the design of the trade center complex, sought inspiration in nature. The structure of the towers to a certain degree mimicked the anatomy of bamboo. The bamboo’s inside is hollow, and each culm segment begins and ends with a solid joint called a node.

To open up broad patches of rentable office space that the plan for the twin towers demanded, engineers opted for rigid tubes of hefty steel to crust the buildings’ exterior. The steel tubes essentially provided the stability of the towers from the outside, while using minimal columns on the inside. Even inside, support columns were centered in the middle to maximize available office space for each floor. The entire complex with all seven buildings of the World Trade Center was required to accommodate roughly 15 million square feet of office space. The most significant chunk of that office space would be in the matching tower structures.

Setting up the foundation of the towers was another big challenge. The buildings were to be erected on an artificial patch of land created into the Hudson over the centuries as Manhattan grew. Construction workers dug 70 feet (20m) to the bedrock, excavating vast amounts of dirt.

Engineers used a ‘slurry trench’ method to secure the foundation from floods by the nearby Hudson River. A huge trench was dug around the site’s perimeter, 3,500 ft (1,066m) long and 3 ft (1m) wide, then filled with a type of absorbent clay, denser than the surrounding dirt. The slurry mix prevented the dirt from sipping into the trench. Steel cages up to seven stories high and weighing 25 tones each were at the end lowered inside the trench panels and poured around with concrete, forcing the lighter slurry up and out. The completed foundation became known as “the bathtub.”

A great deal of attention also went onto how strong winds may affect the twinned buildings. For this, engineers incorporated over 10,000 “viscoelastic dampers” into each tower, containing viscose, metal, epoxy, and polyacrylic glue. With such an innovative solution in place, the towers could lean up to three feet (90cm) in either direction when winds would hit.

It is estimated that approximately 200,000 tons of steel were used to construct the twin towers. A particular type of cranes, known as “kangaroo cranes,” invented in Australia, were used by builders to hoist the pieces of steel up in the heights. The kangaroo crane could hover upward as many as three levels at a time as steel was stiched to the structure. It was the only solution to build such a colossal, steel-cast skyscraper back in the 1960s. The height of the towers, each 110-stories high, topped all the tallest cranes available.

Engineers also looked for innovative solutions on how to install elevators without taking too much space. The design of the elevator system drew inspiration from New York’s subway system. Each tower was split into three zones, and each zone got an express elevator. The system allowed people to get off at one of the tower’s “sky lobbies” then switch to “local” elevators to get to their designated floor. The elevator solution saved on shaft space, again adding up to each tower’s available office space. As did the choice to have the building’s staircases in the middle of each tower.

When the World Trade Center was built, city building codes required six staircases for high-rise structures such as the twin towers, however, in the case of the towers, an exemption was made. A newer building code was favored, which required only three instead of six staircases. It’s believed that had the towers had more stairways within their construction, at least hundreds more people would have made it alive on the day two planes crashed in the north and south towers into what turned out the deadliest terrorist attack in history.

The twin towers were reduced to rubble during the horrendous September 11 attacks in 2001, killing 2,977 people. Unable to withstand the impact of the plane hits, the tower’s floors pancaked to the ground and covered the entire Lower Manhattan in ashes and papers, which survivors at the scene described, fell like snow.

At the 9/11 Memorial and Museum, visitors can read the victims’ names in the attacks and view authentic artifacts dug out from the piles of steel and concrete. The memorial center opened on the 10th anniversary of the attack in 2011.

During its existence, the twin towers of the World Trade Center were an icon of New York City. According to some estimates, the skyline with the towers appears as a backdrop in 472 films. After the towers perished, mentions of the complex in various media were altered or sometimes erased. One World Trade Center, now the tallest structure in the northern hemisphere, currently serves as the principal building of the rebuilt World Trade Center complex, rising just next to the memorial center.

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On the cover: A view of Manhattan, New York City, with the twin towers of the World Trade Center back in the day. Photo credit: Gerd Eichmann, CC BY-SA 4.0

Tags: Architecture, High Rise Architecure, New York, Twin Towers, World Trade Center