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One of the most dramatic events of New York City in the 1960s was the construction of the World Trade Center. Design and construction would take years and the efforts of thousands of people. A project of this size created enormous challenges, challenges that demanded the use of dramatic new engineering concepts. A wide variety of designs was considered; final plans called for a complex of low-level buildings surrounding two 1,350-foot towers, the tallest ever built. Their great height was made possible by the use of load-bearing walls.
Extremely tall buildings were traditionally inefficient since huge amounts of interior space were taken up by structural supports and elevators. The Trade Center towers have overcome this problem. The exterior walls were designed to bear much of the weight of the towers, as well as all of the wind loads. The only internal supports would be in a central core of columns. Elevators would be placed in the shafts formed by the core columns. To further conserve space, the top of the towers would be organized into three zones served by express elevators; local elevators would run within each zone. These engineering considerations determined the towers basic design: sheer symmetrical walls rising without setbacks.
The site selected for the Trade Center was an old section of Manhattan's Lower West Side. The area housed a great number of small businesses, but the predominance of electronics stores caused it to be known as "Radio Row." In 1966 demolition began. In all, 164 buildings had to be torn down, and generations of power, telephone, gas, steam and water lines had to be rerouted.
The site actually consisted of waterlogged landfill, which had accumulated over two centuries out of old wharves and debris. To support the great weight of the towers, foundations would have to be dug down 70 feet to bedrock. But the removal of water from this huge area would have caused a dangerous lowering of the surrounding water level, undermining nearby buildings. The solution was to construct an underground concrete retaining wall to surround the site. This was built with the slurry trench method, used for the first time in this country. A trench was dug down to bedrock and a thick bentonite slurry was pumped in. The slurry was denser than the surrounding mud and dirt, and thus kept the trench walls from collapsing.
As each section was completed, 25-ton cages of metal reinforcement rods were lowered into the slurry-filled trench. With the cage in place, concrete was poured in. Since the concrete had a greater density than the slurry, the slurry was forced up, out of the trench, and could be used for the next section. In this way, an underground wall was built completely sealing the site. Excavation began.
As each section of the slurry wall was revealed, workers drilled holes through the wall and casings were pushed through down to bedrock on the far side. Steel tendons were then inserted through these holes and socketed into bedrock bracing the wall against external pressure.
More than a million cubic yards of dirt had to be removed to make way for the Trade Center's foundations. The excavated earth was placed in the Hudson River adjacent to the site, to create more than 23 acres of new land, land which was donated to the city of New York. The site presented another major challenge; the tubes containing the PATH commuter rail lines lay underground within the excavation area. The fragile tubes had to be supported in protective cradles while excavation continued around them. Throughout construction the PATH trains carried 130,000 commuters daily. At no point was service interrupted.
Seven stories down, bedrock was finally reached. Foundations for the towers could now be prepared. Concrete footings were formed and poured into bedrock. Massive assemblies of steel beams called grillages were laid on these footings. Each grillage would anchor one of the load-bearing tower columns.
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