Khan's central innovation in skyscraper design and construction was the idea of the "tube" structural system for tall buildings, including the framed tube, trussed tube, and bundled tube variants. His "tube concept", using all the exterior wall perimeter structure of a building to simulate a thin-walled tube, revolutionized tall building design. Most buildings over 40 stories constructed since the 1960s now use a tube design derived from Khan's structural engineering principles.  Lateral loads (horizontal forces) such as wind forces, seismic forces, etc., begin to dominate the structural system and take on increasing importance in the overall building system as the building height increases. Wind forces become very substantial, and forces caused by earthquakes, etc. are important as well. The tubular designs resist such forces for tall buildings. Tube structures are stiff and have significant advantages over other framing systems. They not only make the buildings structurally stronger and more efficient, but also significantly reduce the structural material requirements. The reduction of material makes the buildings economically more efficient and reduces environmental impact. The tubular designs enable buildings to reach even greater heights. Tubular systems allow greater interior space and further enable buildings to take on various shapes, offering added freedom to architects. 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. Khan was among a group of engineers who encouraged a rebirth in skyscraper construction after a hiatus of over thirty years.  The tubular systems have yet to reach their limit when it comes to height. Another important feature of the tubular systems is that buildings can be constructed using steel or reinforced concrete, or a composite of the two, to reach greater heights. Khan pioneered the use of lightweight concrete for high-rise buildings, at a time when reinforced concrete was used for mostly low-rise construction of only a few stories in height. Most of Khan's designs were conceived considering pre-fabrication and repetition of components so projects could be quickly built with minimal errors.  The population explosion, starting with the baby boom of the 1950s, created widespread concern about the amount of available living space, which Khan solved by building upward. More than any other 20th-century engineer, Fazlur Rahman Khan made it possible for people to live and work in "cities in the sky." Mark Sarkisian (Director of Structural and Seismic Engineering at Skidmore, Owings & Merrill) said, "Khan was a visionary who transformed skyscrapers into sky cities while staying firmly grounded in the fundamentals of engineering."

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Khan's central innovation in skyscraper design and construction was the idea of the "tube" structural system for tall buildings,