The Aon Center
Originally the Standard Oil Building and later the Amoco Building, the Aon Center – at 83 floors – is Chicago’s third tallest building. Stylistically, it reflects the Classically-inspired Modernism that East Coast architects like Durell Stone and Philip Johnson embraced in the 1960s and 70s. Structurally, however, the building is pure Chicago, being a framed tube, a structural method first introduced in Chicago in the early 1960s and characterized by close-set perimeter columns tied together with spandrel beams that allow for column-free floorplans. (Framed tubes were the precursor to the bundled tubes used for the Sears Tower and other super tall structures.) Originally clad in thin slabs of gleaming white Carrara marble that eventually buckled and cracked, the building was re-clad in granite in the early 1990s.