Dome

Close side view of the upper shaft; The second tallest tower in Chicago, the Standard Oil Building is a stand-out due to its monumental proportions and gleaming white facade. Measuring 194 feet square in plan and boasting generous 30,000 square-foot floor plates, the tower was originally clad in Italian Carrara marble, and later (1990-1992) replaced with 44,000 pieces of 2-inch thick granite. The building employs a tubular steel-framed structural system with V-shaped perimeter columns to resist earthquakes, reduce sway, minimize column bending, and maximize column-free space. To further expand rentable area, 40 of the building's 50 elevators are double-deckers, an often preferred solution to ensure efficient vertical circulation while reducing the space consumed by the central service core. The foundation features 56 caissons consisting of steel-encased vertical shafts filled with reinforced concrete and socketed several feet into solid bedrock more than 100 feet below ground. The caissons contain 50 million pounds of concrete and 2 million of steel. It was renamed as the Aon Center on December 30, 1999. Source: The Skyscraper Museum [website]; http://www.skyscraper.org/ (accessed 12/9/2007)