Twin Towers Under Construction (19 Photos)
Since they had first risen in the 1880s, skyscrapers had been supported by an interior skeleton of steel columns placed every 20 feet or so. The exterior walls of the building, which were hung from the steel frame (and thus known as “curtain walls”), served only to enclose the structure and provide protection from the elements.
The unprecedented height and size of the twin towers, however, posed a new kind of structural challenge. The structures needed not only to support the sheer weight of the 1,360-foot-tall buildings, but to overcome the even greater loads caused by the high winds of New York Harbor pushing against the wide, flat sides of the buildings, especially along their uppermost floors.
Scale model of the Twin Towers, January, 18, 1964
Preparing a construction site, 1967.
Future construction site area is marked with a white line, 1967.
The solution, developed by the structural engineers John Skilling and Leslie Robertson in the mid-1960s, was to re-conceive the basic structure of tall buildings. At the World Trade Center, a super-strong lattice of exterior steel columns, placed less than two feet apart and locked tightly together at every floor, would transform each tower into a giant “tube.” The remarkably stiff outer structure could readily resist the force of 150-mile-per-hour winds — far higher than any ever recorded in the region. For almost the first time in the century-long history of skyscrapers, the exterior wall was returned 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 peak of construction, more than 800 tons of structural steel were being raised into the sky each day by four Australian-built “kangaroo cranes.” The steel was bolted into place by an army of 3,600 construction workers. Among them were Carl Furillo, who had once played right field for the Brooklyn Dodgers, and a New Jersey man named George Nelson, who 50 years earlier had helped build the Empire State Building, and who dismissed the World Trade Center as “just another building.” A group of Mohawk ironworkers, whose legendary fearlessness had made them a regular presence on the steel frames of New York skyscrapers since the early 20th century, was among the men who raised the towers into the clouds.
A large portion of a core structure 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.