Although little appreciated at the time, the design and construction of the twin towers of the World Trade Center marked a dramatic break with nearly a century of skyscraper tradition.
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 Wind Challenge
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.
Exterior Steel Columns
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.
The Elevator Limit
The tube structure was just the first in a series of engineering breakthroughs at the trade center. The very height of the buildings, nearly 30 stories taller than their nearest rival, the Empire State Building, would have been impossible to achieve were it not for another innovation, placed deep within the towers. For years, a height of 80 or 85 stories (about that of the Empire State) had been held as the maximum feasible for tall buildings. Anything taller required too many banks of elevators to reach its upper floors, decreasing the amount of rentable space in the lower portions of the building. Adding more floor space above simply meant subtracting it below, thus canceling out any advantage.
As planning for the towers proceeded, a Port Authority engineer named Herb Tessler devised a way of overcoming that historic limitation. His ingenious system of local and express elevators, linked by "sky lobbies" at the 44th and 78th floors, allowed the local elevators on the upper floors to be "stacked" directly above those on the lower floors, thus preserving valuable floor space to be rented.
As the buildings began to rise, another innovation made its appearance. Unlike earlier skyscrapers, assembled one column or beam at a time, the twin towers were erected in huge prefabricated components. Exterior wall sections three stories tall and floor decks 60 feet long could be lifted into place at once. The daring new technique dramatically speeded the construction process.
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.
My American Experience
How did you first learn of the September 11 attacks? Do you remember what you were doing when the Twin Towers fell? What did it mean to you? Do you have specific memories of that day?