Rising Steel: The Construction of the World Trade Center from 1966-1979 in Photos

Getting the project approved wasn’t simple. Because the Port Authority operated in both New York and New Jersey, it needed agreement from both state governors. The initial plan for an East River location met with resistance from New Jersey’s governor. A compromise was reached. The Port Authority agreed to take over the failing Hudson and Manhattan Railroad, a commuter line important to New Jersey residents, and rename it PATH (Port Authority Trans-Hudson). In return, the World Trade Center project would move to the railroad’s Hudson Terminal site on the West Side of Lower Manhattan, a location much easier for New Jersey commuters to reach. This political deal was essential in determining the final location of the complex. New York City also had to approve the plans, leading to negotiations about tax payments.  

Designing the Giants

Minoru Yamasaki was chosen as the main architect in September 1962, working alongside the firm Emery Roth & Sons. Yamasaki was known for designs that focused on human experience, like his popular Federal Science Pavilion at the Seattle World’s Fair. He aimed to create buildings with “visual delight,” incorporating decorative details and light, a contrast to the plain glass boxes common at the time.

The Port Authority had a huge requirement: 10 million square feet of office space. Yamasaki first thought of several smaller buildings but decided twin towers would look better and meet the space needs. The Port Authority also insisted the towers become the world’s tallest, overtaking the Empire State Building. Yamasaki wasn’t initially keen on such extreme height, feeling it clashed with his focus on human scale, but the demand shaped the final plan for two 110-story towers.  

To make these immense towers possible, Yamasaki collaborated with the structural engineering firm Worthington, Skilling, Helle & Jackson (John Skilling and Leslie Robertson were key figures) and benefited from the work of engineer Fazlur Rahman Khan. They employed an innovative “framed-tube” structural system. This design was a direct response to the multiple challenges: achieving record height, providing the vast required office space, resisting wind forces efficiently, and satisfying Yamasaki’s aesthetic goals.  

The tube-frame design concentrated the building’s strength on its outer walls and a strong central core. The exterior walls were made of many steel columns spaced very closely together – 59 columns on each side of the tower above the lobby level. These columns, connected by strong horizontal steel beams called spandrel plates at every floor, formed a stiff, hollow tube. This outer tube was designed to handle almost all the wind forces pushing against the building and also carried a large part of the building’s weight. This efficient design used significantly less steel – about 40 percent less – than traditional skyscraper construction methods.  

Inside each tower, a large rectangular core (about 87 feet by 135 feet) contained 47 heavy steel columns that ran from the foundation bedrock all the way to the top. This core supported the rest of the building’s weight and housed the elevators, stairwells, and building utilities.  

A major benefit of the tube-frame system was that it allowed for huge open office areas on each floor, nearly an acre in size, without any interior columns blocking the space. This met a key demand from the Port Authority.  

Yamasaki also made distinct aesthetic choices. He designed very narrow windows, only 18 inches wide. This was partly because he personally had a fear of heights and wanted people inside to feel secure while still having a view. The building exteriors were covered in aluminum alloy skin. At the base of the towers, he used tall, pointed arches, drawing inspiration from Gothic and Arabic architecture. He envisioned the large plaza surrounding the towers as a welcoming public space, a “Mecca” for people to gather. Despite Yamasaki’s intentions to create human-centered, uplifting buildings, the sheer size and unique look of the towers led to harsh criticism from some architectural critics when the design was revealed; they saw the towers as too large, plain, or out of place in the city.

Preparing the Ground

The chosen location for the World Trade Center was on the West Side of Lower Manhattan, centered on the old Hudson Terminal building. This 16-acre area was mostly occupied by a neighborhood known as “Radio Row”.  

Radio Row wasn’t just a few shops; it was a bustling 13-block district that had thrived since the 1920s. It was famous for its dense collection of small, family-owned electronics stores selling radios, parts, and related gear, but it also included restaurants, other businesses, and some apartments.  

The Port Authority used its power of eminent domain – the right of the government to take private property for public use – to acquire all the properties needed for the World Trade Center. This plan met fierce resistance from the Radio Row merchants and residents. Led by shop owner Oscar Nadel, they formed a committee, organized protests (including a mock funeral for the small businessman), and fought the decision in court. They argued the project would destroy their livelihoods and a unique neighborhood. Despite their efforts, the courts sided with the Port Authority. Demolition of Radio Row began in March 1966 and wiped the neighborhood off the map by the year’s end. The Port Authority offered businesses $3,000 each as compensation, an amount many felt was far too low for the loss of their established shops and community. This process highlighted the human cost often associated with large “urban renewal” projects.  

With the site cleared, engineers faced a major challenge: the ground itself. The World Trade Center site was built on landfill, not solid ground. Bedrock, the solid rock layer needed to support the massive towers, was about 65 to 70 feet below street level. Furthermore, the site was very close to the Hudson River, and the water level in the ground was high. Digging a huge hole this deep would normally cause groundwater to flood the excavation. Pumping the water out could also cause the ground under nearby buildings to settle and potentially damage them.  

The solution was an innovative engineering technique called the “slurry wall” method, used to create what became known as the “bathtub”. This technique was relatively new in the United States, especially on such a large scale, making its use essential for the project’s success on this difficult site.  

Here’s how it worked: Construction crews dug a deep, narrow trench around the perimeter of the main site area. The trench was dug in sections, typically 3 feet wide and 22 feet long, going all the way down to bedrock, about 70 feet deep. As they dug each section, they filled the trench with slurry – a thick, heavy mixture of bentonite clay and water. This dense liquid did two things: it held the sides of the trench open, preventing collapse, and it acted as a barrier, keeping groundwater from flowing into the trench.  

Once a trench section reached bedrock, workers lowered a large steel cage inside it for reinforcement. Then, using a special pipe called a Tremie pipe, they pumped concrete into the bottom of the slurry-filled trench. Because concrete is heavier than the slurry, it sank to the bottom and gradually filled the trench, pushing the lighter slurry mixture out the top where it could be collected and reused for the next section.  

This process was repeated panel by panel, over 150 times, eventually forming a continuous, underground concrete wall nearly 3,400 feet long that completely enclosed the core area of the site. Building the slurry wall was a major project, taking 14 months from early 1967 to early 1968. Construction faced difficulties, including digging through old debris like shipwrecks and timbers buried in the landfill, hitting unexpected boulders, and managing the slurry mixture. Another significant challenge was keeping the existing PATH commuter train tunnels, which ran through the site, operating safely throughout the foundation construction.  

Setting the Foundation

After the massive underground slurry wall – the “bathtub” – was finished and sealed into the bedrock, the huge job of digging out the inside could start. Workers removed more than one million cubic yards of soil, rock, and landfill debris from within the bathtub’s perimeter. Much of this excavated material wasn’t wasted; it was transported across West Street and used to create new land along the Hudson River shoreline, which eventually became Battery Park City.  

As the excavation inside the bathtub got deeper, a new problem arose. The soil and water pressure from outside the walls pushed inward with tremendous force. Without the soil inside to support them, the tall concrete slurry walls could have buckled or collapsed. To prevent this, engineers installed hundreds of “tie-back” anchors.  

The installation process for these anchors was critical and had to happen in stages as the digging went down. Crews drilled holes diagonally outward through the completed slurry wall, continuing through the earth outside the bathtub and deep into the solid bedrock beyond. Strong steel cables, called tendons, were threaded through these holes and securely anchored (grouted) into the bedrock. Then, using hydraulic jacks, the tendons were pulled tight (tensioned) and locked into place against the wall. Each anchor acted like a powerful brace, pulling the wall back against the external pressure and holding it firmly in place. This anchoring process was repeated at different depths, typically every 10 to 15 feet of excavation, ensuring the wall remained stable as the hole deepened. Some difficulties with installing these tiebacks led to the use of concrete buttresses at the base of the wall for additional support in some areas. This careful sequence—completing the wall first, then excavating while simultaneously installing anchors—was essential for safely creating the deep foundation space.  

The excavation continued until it reached the hard bedrock known as Manhattan schist, about 70 feet (21 meters) below street level. This provided the solid, stable base needed for the enormous weight of the Twin Towers. The foundations for the towers, including the heavy steel columns of the core and the perimeter walls, were built directly onto this bedrock, using components like structural steel grillages to distribute the load. The scale of this underground work alone—the massive excavation, the innovative slurry wall, and the extensive anchoring system—demonstrated the project’s immense size and complexity even before the towers began to rise.  

Reaching for the Sky: Building the Towers

With the deep foundation secure, the steel skeletons of the Twin Towers began their climb. Work on the North Tower’s superstructure started in August 1968, and the South Tower followed in January 1969. The process began with erecting the massive steel columns forming the central core of each tower.  

A key strategy for building the towers quickly and efficiently was the extensive use of prefabricated parts. Instead of assembling everything piece by piece high above the ground, large sections were built in factories, shipped to the site, and lifted into place. This approach saved time, reduced costs, and allowed for better quality control.  

The exterior walls were a prime example of prefabrication. They were assembled from large panels, each typically containing three of the square, hollow steel columns welded to the heavy spandrel plates spanning between them for two or three floor levels. These panels, often 36 feet tall, 10 feet wide, and weighing as much as 22 tons, were manufactured off-site. They were delivered to the construction site in the precise order needed and then bolted to the adjacent panels already in place.  

The floor system that spanned the wide-open space between the central core and the exterior walls also used prefabricated units. These were large sections, often 60 feet long and 20 feet wide (or 30×20 ft depending on location), built around a series of lightweight steel trusses (bar joists). These panels included the metal decking on which concrete would be poured and sometimes even had channels for utilities like air ducts pre-installed. Cranes lifted these large floor sections into position, where they were connected to girders on the core and special seats on the exterior wall spandrels. After the panels were secured, workers poured a 4-inch layer of lightweight concrete over the metal decking to create the finished floor slab.  

Lifting these heavy prefabricated steel sections – core columns, wall panels weighing up to 22 tons, and floor units – required powerful cranes. For the main tower erection, four specialized cranes called “kangaroo cranes” were used on each tower. These cranes, officially Favco Standard 2700 models made in Australia, had a unique ability: they could raise themselves up inside the building’s core using powerful hydraulics as the tower grew taller. This self-climbing feature allowed them to keep pace with construction without needing constant disassembly and reassembly on the outside. They could lift components weighing up to 50 tons.  

The general building sequence involved erecting the steel core columns first, slightly ahead of the exterior walls. The kangaroo cranes, positioned within the core, would lift and place the core steel. Then, the cranes would lift the prefabricated exterior wall panels into place. Once sections of the core and perimeter walls were standing, the large prefabricated floor panels were hoisted and fitted between them, tying the structure together. This systematic approach, combining prefabrication, specialized cranes, and a planned sequence, allowed construction to proceed rapidly, with crews often completing about three floors per week.  

At the very bottom of the towers, in the lobby area covering the first seven floors, a different type of column was used for the exterior. These were massive, branching steel structures called “tree columns” or “tridents”. Each trident took the load from three of the standard perimeter columns above it and channeled that weight down to a single, large foundation point on the bedrock. This design feature was crucial for creating the tall, open lobby spaces with large windows that architect Yamasaki wanted. The design itself, with its repeating modular components, was perfectly suited for these efficient, system-based construction methods.  

By the Numbers: Scale and Materials

Height and Size: The North Tower (WTC 1) stood 1,368 feet (417 meters) tall, making it, for a short time, the tallest building in the world. The South Tower (WTC 2) was slightly shorter at 1,362 feet (415 meters). Each tower contained 110 stories. The sheer size extended horizontally as well; each floor within the towers covered approximately one acre. The entire World Trade Center complex spanned 16 acres and eventually consisted of seven distinct buildings centered around a large plaza. Together, the buildings offered nearly 10 million square feet of rentable office space. 

Materials Used: Constructing buildings of this magnitude required staggering quantities of materials, implying a massive logistical effort to source and deliver everything to the busy Lower Manhattan site.

Steel: More than 200,000 tons of structural steel formed the skeletons of the complex. To meet this demand, the Port Authority sourced steel from at least seven different companies. Reflecting the precise engineering needed, the towers’ design incorporated 14 different grades of steel, chosen for their specific strength properties in different parts of the structure.  

Concrete: Over 425,000 cubic yards of concrete were poured, primarily for the foundations, core elements, and the lightweight floor slabs. 

Glass: The facades of the Twin Towers featured 43,600 individual windows, covering a total area of more than 600,000 square feet. 

People Power: Building the World Trade Center was a monumental human effort. More than 10,000 construction workers were involved throughout the project’s duration. It was also dangerous work; records indicate that 60 workers lost their lives during the construction process. Once operational, the complex became a city within a city, accommodating around 50,000 people who worked there daily, plus tens of thousands of visitors. 

The project’s unprecedented scale was not just a feature, but a driving force behind many of the innovative design choices and construction techniques employed. Achieving the desired height, floor space, and efficiency demanded new approaches to structure, materials, and logistics.

Overcoming Challenges

Building the tallest structures in the world presented unique engineering hurdles that required groundbreaking solutions. Two of the most significant were managing wind forces and designing an efficient elevator system.

Fighting the Wind: Skyscrapers act like giant sails, catching the wind. For towers reaching over 1,300 feet, resisting these wind forces was a primary design challenge. The main structural defense was the framed-tube design itself. The closely spaced exterior columns and deep spandrel beams created a very stiff outer shell that acted like a hollow box beam, or cantilever, anchored in the ground. This tube efficiently transferred wind loads from the side facing the wind to the sides parallel to the wind and the side opposite the wind.  

To fully understand how the towers would behave in high winds, engineers conducted extensive wind tunnel testing on models of the buildings. This was one of the first times such detailed wind studies were performed for a skyscraper project. These tests helped predict wind pressures and how much the towers might sway. Researchers even conducted tests, disguised as eye exams, to determine how much building movement people could tolerate without feeling uncomfortable. 

The tests showed that the towers’ sway might be noticeable to occupants. To solve this, engineers implemented a pioneering system of about 10,000 viscoelastic dampers in each tower. These devices, made of steel plates sandwiching layers of a special sticky, elastic polymer material, were installed between the ends of the floor trusses and the exterior columns. When the building swayed, these dampers would deform slightly, absorbing energy and reducing the motion felt by people inside. This was the first major use of such damping technology in a high-rise building. 

Revolutionary Elevators: Moving tens of thousands of people efficiently up and down 110 floors posed another huge challenge. A traditional elevator system, with shafts running the full height of the building for every car, would have taken up an enormous amount of valuable core space, making the towers economically impractical. 

The engineers devised an innovative solution modeled after express and local subway lines: the sky lobby system. The towers were divided vertically into three zones. The main lobby served the lower zone directly. Two intermediate floors, the 44th and 78th, were designated as “sky lobbies”. Large, high-capacity express elevators ran non-stop from the ground floor only to these sky lobbies. Once at a sky lobby, passengers would walk across the floor to separate banks of local elevators that served only the floors within that specific zone (e.g., floors 44-77 or 78-110). 

This system drastically cut down on the number of elevator shafts needed to run the full height of the building. Estimates suggest it saved as much as 70% of the shaft space compared to a conventional layout, freeing up much more floor area for offices. The Otis Elevator Company supplied the majority of the 198 elevators and 49 escalators in the complex. The express elevators were technological marvels for their time, capable of speeds up to 1,700 feet per minute. While the towers appeared externally as simple forms, these internal systems revealed a high degree of complexity and advanced technology needed to make their height and function possible. 

Urban Logistics: Beyond the technical challenges of height and wind, simply building the WTC in the heart of Lower Manhattan was a massive logistical operation. Delivering the immense quantities of steel, concrete, glass, and other materials required careful planning to avoid paralyzing city traffic. Steel deliveries, for instance, were often scheduled for the early morning hours, using the Holland Tunnel. Coordinating the large workforce of over 10,000 people added another layer of complexity. Furthermore, the site was riddled with existing underground utilities – water mains, sewer lines, electrical cables, steam pipes – many poorly documented, which had to be carefully relocated or worked around.

Marking Milestones

The construction of the World Trade Center complex was a long process marked by several key dates. The project didn’t appear overnight; it involved years of planning, site preparation, and phased construction before the iconic towers dominated the skyline.

The official start came with the groundbreaking ceremony on August 5, 1966. The first couple of years focused heavily on the complex below-ground work, including the construction of the slurry wall and foundation excavation. 

The steel structures of the towers began their vertical ascent in 1968. Erection of the North Tower’s steel started in August 1968, with the South Tower following in January 1969. 

A significant symbolic milestone occurred in September 1970, when steel erection on the North Tower reached the 103rd floor, making it taller than the Empire State Building and thus the new tallest building in the world. The North Tower (WTC 1) officially “topped out” – meaning the highest structural element was put in place – in December 1970. The South Tower (WTC 2) topped out in July 1971. 

Even before the towers were fully finished or officially opened, they began to be occupied. The first tenants moved into the North Tower in December 1970 and into the South Tower in January 1972. This phased completion and occupancy allowed parts of the complex to become operational while other areas were still under construction. 

The entire World Trade Center complex, including the Twin Towers and surrounding low-rise buildings, was formally dedicated and officially opened for business on April 4, 1973. This date marked the culmination of nearly seven years of intensive construction work, following more than a decade of planning and political maneuvering.