of Illinois at Urbana–Champaign. In three years Khan earned two master's degrees – one in structural engineering and one in theoretical and applied mechanics – and a PhD in structural engineering with thesis titled Analytical Study of Relations Among Various Design Criteria for Rectangular Prestressed Concrete Beams. His hometown in Dhaka did not have any buildings taller than three stories. He did not view 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. Career In 1955, employed by the architectural firm Skidmore, Owings & Merrill (SOM), he began working in Chicago. He was made a partner in 1966. He worked the rest of his life side by side with fellow architect Bruce Graham. Khan introduced design methods and concepts for efficient use of material in building architecture. His first building to employ the tube structure was the Chestnut De-Witt apartment building. During the 1960s and 1970s, he became noted for his designs for Chicago's 100-story John Hancock Center and 110-story Sears Tower, since renamed Willis Tower, the tallest building in the world from 1973 until 1998. He believed that engineers needed a broader perspective on life, saying, "The technical man must not be lost in his own technology; he must be able to appreciate life, and life is art, drama, music, and most importantly, people." Khan's personal papers, most of which were in his office at the time of his death, are held by the Ryerson & Burnham Libraries at the Art Institute of Chicago. The Fazlur Khan Collection includes manuscripts, sketches, audio cassette tapes, slides and other materials regarding his work. Personal life For enjoyment, Khan loved singing Rabindranath Tagore's poetic songs in Bengali. He and his wife, Liselotte, who immigrated from Austria, had one daughter who was born in 1960. In 1967, he elected to become a United States citizen. Innovations Khan discovered that the rigid steel frame structure that had long dominated tall building design was not the only system fitting for tall buildings, marking the start of a new era of skyscraper construction.