Today, variations on tube systems developed by Khan have become all but standard in skyscraper design around the world, including the 80-story Capital Market Authority headquarters building, a joint venture by HOK / Omrania, currently under construction in Riyadh. Khan later used a variation of this system, the “bundled tube,” for the Sears Tower (now the Willis Tower), the tallest building in the world from 1973 to 1998. The tower’s 344-meter “trussed tube” structure topped out on May 6, 1968, signaling not only a close collaboration between architects and engineers, but also a paradigm shift in skyscraper design. The Hancock or “Big John” was instantly recognizable by its monolithic tapering profile and multistory diagonal X-bracing carrying lateral wind loads to the corners. This innovative “tube” system was first employed in Chicago’s 43-story Dewitt-Chestnut apartment building (1963-1966), with its rigid screen of closely spaced exterior columns and open floorplates.īut it was the 100-level John Hancock Center (1965-1969) that showed the world that the future of skyscraper design was tubular. Instead of relying upon a conventional steel frame system, he pushed most of the structural elements to the perimeter, so that the entire building would behave structurally as a hollow tube - reducing the amount of steel by nearly 50%, lowering building costs, and creating more open interiors. Khan, working for Skidmore, Owings & Merrill (SOM), proposed a new way for tall buildings to support themselves. Posted on The Hancock Center reflects close collaboration between architects and engineers: Bruce Graham and Fazlur Khan of SOM. Its observation deck is host to over a million tourists each year and is another reason why the Sears/Willis Tower holds such a place in the Chicago cultural zeitgeist.How the Hancock Center and Tube Structures Changed Skyscraper History It is clad in black anodized aluminum panels and bronze-tinted glass, a façade that has become emblematic of American architecture in general and Chicago’s skyline in particular. This system allowed for large open office spaces on the lower levels (where the Sears offices would be located) and smaller floors as the building soared in height with unobstructed views of the city. This innovation made Fazlur Khan one of the most influential structural engineers of the 20 th century, and it continues to this day. Khan from SOM, and these nine tubes formed the skyscraper’s basic structure. The “bundled-tube” configuration was innovated by engineer Fazlur R. The different heights allow for the building to step back, meeting setback regulations and creating the iconic staggering effect that the building is known for. SOM proposed a superstructure of nine interlocking tubes of varying heights, divided in 75′ x 75′ squares that are separate buildings joined together as one. The company analyzed their current spacial needs, as well as the space needed for growth up to the year 2003 being as meticulous as determining the number of desks for personnel. The height of the building is due to spacial needs. The building is 108 stories tall, rising 1,450 feet above the city. The design also had to incorporate extra office space for the anticipated future growth of the company. Architect Bruce Graham led the team alongside structural engineer Fazlur Rehman Khan. Sears, Roebuck, & Company commissioned the architecture firm Skidmore, Owings, & Merrill to design an office building that would house their headquarters and the many offices they had scattered around Chicago in one building. Beyond these records, the tower is also notable for its tube frame structural design which has revolutionized skyscraper design and made possible the structures we see today. Known as the Sears Tower for most of its existence, it held the record for the world’s tallest building from 1974 to 1998, almost 25 years, and held the record for the tallest building in the western hemisphere for about 41 years. Willis tower is today the most iconic presence in Chicago’s famous skyline.
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