The Twin Towers and the Twentieth Century

Through several past exhibitions, The Skyscraper Museum has examined the history of the World Trade Center complex in its conception, design, and construction from the 1960s through the mid-1970s-- and their destruction on the morning of 9/11. A special section devoted to the Word Trade Center and rebuilding at Ground Zero occupies a portion of the Museum's galleries.

Upon their completion in 1971 and 1973, the Twin Towers of the World Trade Center were the tallest and largest skyscrapers in the world. Innovative engineering carried the structures to 110 stories- 1368 and 1362 feet (417 and 415 meters) -creating floors an acre in size, with more than 4 million square feet per building. Except for the contemporary Sears Tower in Chicago, nearly 100 feet taller, but slightly smaller in total area, no skyscraper has ever matched their scale.

To be both big and tall was a phenomenon of the 1960s and 1970s, the climax in the evolution of skyscraper size. The World Trade Center epitomized the ambitions of an era when faith in technology and a fascination with monumentality spurred designs for megastructures and urban master plans. New York's skyline was on the rise, and modernity seemed to matter more than history. Still, considerable conflict surrounded the towers. Writing in The New York Times in May 1966, architecture critic Ada Louise Huxtable noted, "Who's afraid of the big, bad buildings? Everyone, because there are so many things about gigantism that we just don't know."

September 11, 2001 defines our memory of the Twin Towers, and the profound proportions of that tragedy continue to reverberate in New York and beyond. The question of size in the urban scheme remains a complex issue for the future of tall buildings everywhere. As new spires around the world exceed the sheer height of the supertalls of the seventies, none have surpassed the overall scale of the giants of the twentieth century, nor likely ever will.

World Trade Center at The Skyscraper Museum
The World Trade Center was the first skyscraper design to be tested in a boundary layer wind tunnel, which replicates natural wind. Air flow around tall buildings creates turbulence and eddies that can cause vibration or sway. These effects can be accurately predicted by three kinds of tests. Special highly accurate models are built to represent the building's form and nearby surroundings, often at a scale of 1:400.

A "pressure tap" model, as in the lower large photograph in this case, measures local pressures on the facade using hundreds of tiny tubes, or taps, connected to a computer.

A "force balance" model measures overturning of the entire building as wind gusts strike randomly.

An "aeroelastic" model, as in the upper large photograph in this case, also measures overturning, but includes the effect of the natural rate of rhythmic rocking back and forth, known as the building's period.

World Trade Center at The Skyscraper Museum
From these studies, the engineers were able to analyze and predict the definition of both the steady-state and the fluctuating wind pressures on the facade; the steady-state and dynamic behavior of a tall building in turbulent wind; and a quantification of the increase in street-level winds associated with the construction of a high-rise building. They also developed a new theory of the prediction of the breakage rate of glass subjected to the natural winds of the atmosphere.

While now standard practice in the technology of high-rise design, the pioneering effort for the World Trade Center required significant reconstruction of laboratory facilities. Ultimately, three different wind tunnels at three different labs were used: one in the United States (Colorado State University), one in the United Kingdom (National Physical Laboratory, Teddington), and one in Canada (The University of Western Ontario).