Modern versus classic in Chicago's Loop (© Randall Krause)
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Originally designed in the late 60's by Bruce Graham of Skidmore, Owings, and Merrill for Chicago-based Sears, Roebuck and Co, Sears Tower was topped out in May of 1973 — just over 35 years ago.
During the skyscraper's initial construction, which began in August of 1970, two other prominent office tower projects vied for the "world's tallest" title: the Standard Oil of Indiana Building (now the Aon Center) located in downtown Chicago and the World Trade Center high-rise complex formerly located in Manhattan. Both were surpassed within their first year of completion by Sears Tower.
In 1984, SOM was commissioned to overhaul many of the tower's lower-levels, including the addition of a vaulted atrium alongside Wacker Drive. Following Sears' departure, the building underwent a comprehensive renovation by DeStefano + Assoc of all public and Skydeck lobbies and elevators. This also included the construction of two glass-and-steel canopies over the Franklin Street and Jackson Street entrances. By the fall of 1995, a seasonal rooftop illumination program was also initiated.
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 Sears Tower was the last supertall building constructed during the International architecture period, and SOM's interpretation of the style is remarkably bold and awe-inspiring.
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The superstructure consists of nine interlocking tubes that terminate at different heights, creating the iconic stepped-back appearance of the tower. Each tube is a rigid steel frame that performs in tandem with its neighbor to efficiently counteract all lateral and gravity loads. (This is in contrast to the popular tube-in-tube system, in which a rigid network of floor diaphrams and closely-spaced exterior columns work in unison to resist lateral loads whereas a centralized core functions to carry vertical loads exclusively.)
This "bundled-tube" configuration was a revolutionary engineering concept at the time, pioneered by SOM's very own Fazlur R. Khan. It allowed for large open office spaces on the lower levels, where Sears, Roebuck and Company would reside, and smaller floor plates on the upper levels with unobstructed views of the cityscape. To aid in occupant comfort, belt trusses were rigged on the upper mechanical floors thereby further reducing shear forces (i.e. wind-induced sway).
Building automation was a primary design consideration as well. For fire, security, and HVAC monitoring and control, engineers opted for a Honeywell computer system, one of the largest such networks ever installed at the time. Another technological innovation was the robotic window washers that could be programmed to descend along tracks built into the curtain wall. Currently, they operate eight times per year.
Sears Tower also provides facilities for broadcasting to the Chicago metropolitan area. Twin 85-foot cylindrical supports, both 12-feet in diameter, project from inside the roof to provide a rigid steel base for 159-foot triangular communication towers each of which is encased in white fiberglass radome, extending the full building height. The 101st floor is leased to broadcasters for housing of transmitter equipment.
Vertical transporation was also crucial. Designers had to ensure efficient flow of pedestrian traffic throughout the building — which, on any given day, could number in the tens of thousands. Westinghouse Electric Corp. employed "skylobbies" for this purpose, a system which Otis Elevator Co. had originally implemented in Chicago's own John Hancock Center. Designated transfer-floors would be served by banks of double-deck express elevators. Separate banks of local-service elevators would carry passengers to their final destination.
