Skyscrapers arised in the 1880s, these were supported by an interior skeleton made of steel columns placed every 20 feet or the like. The exterior walls of the building, known as "curtain walls" because they hung from the steel frame, specifically serve to enclose the structure and also provide protection from the elements.
The twin towers' unprecedented height and size posed a new style of structural challenge. The framework needed not only to support the weight of this 1,360-foot-tall buildings, but to overcome much greater loads resulting from the high winds of New York Harbor pushing against the broad, flat sides of the buildings, particularly their uppermost floors.
The scale model of the Twin Towers, January, 18, 1964
Preparing the construction site, 1967.
The area of future construction site is marked with a white line, 1967.
In the mid-1960s, the plan generated by the structural engineers John Skilling and Leslie Robertson to re-conceive the basic structure of tall buildings was the solution. A super-strong lattice of exterior steel columns at the World Trade Center was positioned less than two feet apart and closely locked together at every floor. This would convert each tower into a giant "tube." The remarkably rigid outer structure could easily resist the force of 150-mile-per-hour winds -- by far the highest recorded in the region. For the first time, known in the century-long history of skyscrapers, this exterior wall was restored to structural duty.
Tower 1, 1969.
The next three illustrations are from various articles from the Engineering News Record reproduced here. They show a good deal of detail about the towers' core structures.
The edge of a core structure of one of the towers.
At the height of construction, structural steel more than 800 tons were being raised into the sky everyday by four Australian-built "kangaroo cranes." The steel was latched into place by about 3,600 construction workers. Among those who labored were Carl Furillo, who had once played as right field for the Brooklyn Dodgers, another is a New Jersey man George Nelson, who 50 years ago had helped with construction of the Empire State Building, and dismissed the World Trade Center saying "it's just another building." A group of Mohawk ironworkers, whose legend of fearlessness had made them a regular personality on the steel frames among New York skyscrapers as early as the 20th century, was among the workers who helped raise the towers into the clouds.
A huge portion of a core structure together with stubs of core columns rising a few feet above the decking.
The structural top of one of the towers, including diagonal beams that connected the core structure to the perimeter walls.