In October 1970 the Trade Center’s north tower eclipsed the Empire State to become the tallest building in the world. But that distinction was short-lived. For just last spring the Sears, Roebuck & Co. building in Chicago topped out at 1,450 feet, a hundred feet higher than the twin towers.

Improvements in building technology also are a major factor in the skyscraper boom. More efficient elevator systems save on interior space. Use of lightweight cement and prefabrication methods have been important. Stiffer steel frames provide greater rigidity and strength. The Trade Center’s twin towers, for instance, have a maximum sway in high winds of eleven inches, compared to fifteen inches for older skyscrapers.

Owing to such improvements, it is reportedly feasible to put up skyscrapers 200 or more stories high. “We could start erecting a mile-high structure next year,” says L.E. Robertson, partner in the New York engineering firm that put up the World Trade Center.