as a whole. About half the exterior surface is available for windows. Framed tubes require fewer interior columns, and so allow more usable floor space. Where larger openings like garage doors are needed, the tube frame must be interrupted, with transfer girders used to maintain structural integrity. Khan's tube concept was inspired by his hometown in Dhaka, Bangladesh. His hometown did not have any buildings taller than three stories. He also did not see his first skyscraper in person until the age of 21 years old, and he had not stepped inside a mid-rise building until he moved to the United States for graduate school. Despite this, the environment of his hometown in Dhaka later influenced his tube building concept, which was inspired by the bamboo that sprouted around Dhaka. He found that a hollow tube, like the bamboo in Dhaka, lent a high-rise vertical durability. The first building to apply the tube-frame construction was the DeWitt-Chestnut Apartment Building which Khan designed and which was finished in Chicago by 1963. This laid the foundations for the tube structural design of many later skyscrapers, including his own John Hancock Center and Willis Tower, and the construction of the World Trade Center, the Petronas Towers, the Jin Mao Building, and most other tall skyscrapers since the 1960s, including the world's tallest building , the Burj Khalifa. Variants From its conception, the tube has been varied to suit different structural needs. Framed This is the simplest incarnation of the tube. It can appear in a variety of floor plan shapes, including square, rectangular, circular, and freeform. This design was first used in Chicago's DeWitt-Chestnut Apartment Building, designed by Khan and finished in 1965, but the most notable examples are the Aon Center and the original World Trade Center towers. Trussed or braced The trussed tube, also termed braced tube, is similar to the simple tube but with comparatively fewer and farther-spaced exterior columns. Steel bracings or concrete shear walls are introduced along the exterior walls to compensate for the fewer columns by tying them together. The most notable examples incorporating steel bracing are the John Hancock Center, the Citigroup Center, and the Bank of China Tower. Hull and core These structures have a core tube inside the structure, holding the elevator and other services, and another tube around the exterior. Most of the gravity and lateral loads are normally taken by the outer