In the early 1960s structural engineer Fazlur Rahman Khan, considered the "father of tubular designs" for high-rises, discovered that the dominating rigid steel frame structure was not the only system apt for tall buildings, marking a new era of skyscraper construction in terms of multiple structural systems. His central innovation in skyscraper design and construction was the concept of the "tube" structural system, including the "framed tube", "trussed tube", and "bundled tube". His "tube concept", using all the exterior wall perimeter structure of a building to simulate a thin-walled tube, revolutionized tall building design. These systems allow greater economic efficiency, and also allow skyscrapers to take on various shapes, no longer needing to be rectangular and box-shaped. The first building to employ the tube structure was the Chestnut De-Witt apartment building, this building is considered to be a major development in modern architecture. These new designs opened an economic door for contractors, engineers, architects, and investors, providing vast amounts of real estate space on minimal plots of land. Over the next fifteen years, many towers were built by Fazlur Rahman Khan and the "Second Chicago School", including the hundred story John Hancock Center and the massive 442 m (1,450 ft) Willis Tower. Other pioneers of this field include Hal Iyengar and William LeMessurier.