projects after World War II typically rejected the classical designs of the early skyscrapers, instead embracing the uniform international style; many older skyscrapers were redesigned to suit contemporary tastes or even demolished—such as New York's Singer Building, once the world's tallest skyscraper. German-American architect Ludwig Mies van der Rohe became one of the world's most renowned architects in the second half of the 20th century. He conceived the glass façade skyscraper and, along with Norwegian Fred Severud, designed the Seagram Building in 1958, a skyscraper that is often regarded as the pinnacle of modernist high-rise architecture. Skyscraper construction surged throughout the 1960s. The impetus behind the upswing was a series of transformative innovations which made it possible for people to live and work in "cities in the sky". In the early 1960s Bangladeshi-American 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, 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-storey John Hancock Center and the massive Willis Tower. Other pioneers of this field include Hal Iyengar, William LeMessurier, and Minoru Yamasaki, the architect of the World Trade Center. Many buildings designed in the 70s lacked a particular style and recalled ornamentation from earlier buildings designed before the 50s. These design plans ignored the environment and loaded structures with decorative elements and extravagant finishes. This approach to design was opposed by Fazlur Khan and he considered the designs to be whimsical rather than rational. Moreover, he