posto alcune informazioni prese a questo indirizzo:www.skyscraper.org/TALLEST_TOWERS/t_wtc.htm
"Yamasaki and engineers John Skilling and Les Robertson worked closely, and the relationship between the towers' design and structure was clear. Faced with the difficulties of building to unprecedented heights, the engineers employed an innovative structural model: a rigid "hollow tube" of closely spaced steel columns with floor trusses extended across to a central core. The columns, finished with a silver-colored aluminum alloy, were 18 3/4" wide and set only 22" apart, making the towers appear from afar to have no windows at all.
Also unique to the engineering design were its core and elevator system. The twin towers were the first supertall buildings designed without any masonry. Worried that the intense air pressure created by the buildingsā high speed elevators might buckle conventional shafts, engineers designed a solution using a drywall system fixed to the reinforced steel core.
For the elevators, to serve 110 stories with a traditional configuration would have required half the area of the lower stories be used for shaftways. Otis Elevators developed an express and local system, whereby passengers would change at "sky lobbies" on the 44th and 78th floors, halving the number of shaftways."
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