<P> Hat trusses (or "outrigger truss") located from the 107th floor to the top of the buildings were designed to support a tall communication antenna on top of each building . Only 1 WTC (north tower) actually had an antenna fitted; it was added in 1978 . The truss system consisted of six trusses along the long axis of the core and four along the short axis . This truss system allowed some load redistribution between the perimeter and core columns and supported the transmission tower . </P> <P> The tube frame design, using steel core and perimeter columns protected with sprayed - on fire resistant material, created a relatively lightweight structure that would sway more in response to the wind compared to traditional structures, such as the Empire State Building that have thick, heavy masonry for fireproofing of steel structural elements . During the design process, wind tunnel tests were done to establish design wind pressures that the World Trade Center towers could be subjected to and structural response to those forces . Experiments also were done to evaluate how much sway occupants could comfortably tolerate; however, many subjects experienced dizziness and other ill effects . One of the chief engineers Leslie Robertson worked with Canadian engineer Alan G. Davenport to develop viscoelastic dampers to absorb some of the sway . These viscoelastic dampers, used throughout the structures at the joints between floor trusses and perimeter columns along with some other structural modifications, reduced the building sway to an acceptable level . </P> <P> In March 1965, the Port Authority began acquiring property at the World Trade Center site . Demolition work began on March 21, 1966, to clear thirteen square blocks of low rise buildings in Radio Row for construction of the World Trade Center . Groundbreaking for the construction of the World Trade Center took place on August 5, 1966 . </P> <P> The site of the World Trade Center was located on landfill with the bedrock located 65 feet (20 m) below . To construct the World Trade Center, it was necessary to build a "bathtub" with a slurry wall around the West Street side of the site, to keep water from the Hudson River out . The slurry method selected by Port Authority's chief engineer, John M. Kyle, Jr., involved digging a trench, and as excavation proceeded, filling the space with a "slurry" mixture composed of bentonite and water, which plugged holes and kept groundwater out . When the trench was dug out, a steel cage was inserted and concrete was poured in, forcing the "slurry" out . It took fourteen months for the slurry wall to be completed . It was necessary before excavation of material from the interior of the site could begin . The 1,200,000 cubic yards (920,000 m) of material excavated were used (along with other fill and dredge material) to expand the Manhattan shoreline across West Street to form Battery Park City . </P>

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