The tubular systems are fundamental to tall building design. Most buildings over 40-stories constructed since the 1960s now use a tube design derived from Khan's structural engineering principles,[1][8] examples including the construction of the World Trade Center, Aon Center, Petronas Towers, Jin Mao Building, and most other supertall skyscrapers since the 1960s.[9] The strong influence of tube structure design is also evident in the construction of the current tallest skyscraper, the Burj Khalifa.[7]