Aon Center (Chicago)

Design and characteristics

Designed in a modernist idiom, the tower presents a simple, rectilinear profile with a uniform light exterior. Its structure is a steel and concrete framed office tower with large, regularly spaced windows and minimal ornament. Typical floorplates and centralized mechanical systems reflect its role as a corporate office building.

• Height: about 346 m (1,136 ft)
• Floors: 83
• Completion: 1973
• Primary use: offices and corporate headquarters

History and name changes

When it opened in the early 1970s it was built to house a major oil company's regional headquarters and carried that firm's name. The tower later received a corporate rebranding in 1985 and changed hands again at the end of the decade and into the 1990s. After a subsequent sale it adopted the current name following acquisition by a global services firm in 1999. Over the decades it has been counted among the city's tallest structures and remains a prominent element of the skyline (tallest building context).

Material issues and renovation

One of the building's most notable episodes involved its exterior cladding. The tower was originally faced with a light natural stone that, though visually striking, proved vulnerable to weathering and cracking on a tall façade. In response, a major recladding program was carried out to replace the original panels with a more durable stone finish, a large‑scale engineering and conservation effort that extended the building's service life and altered its outward appearance.

Function and significance

Throughout its life the Aon Center has served primarily as commercial office space, hosting insurance, financial and professional services tenants. It is referenced frequently in discussions of Chicago's postwar skyline and is used as a case study in high‑rise façade engineering and maintenance. While it has no public observation deck like some other towers, its ground‑level presence and plaza spaces contribute to the pedestrian fabric of the Loop.

Notable facts

The building is often cited for the uncommon choice of its original cladding material on a skyscraper and for the large refurbishment that followed. Its straightforward massing and bright exterior make it readily identifiable in photographs and maps of downtown Chicago.

For further architectural and technical details see sources linked in the article: skyscraper profile, Chicago resources, and historical timelines at national archives and specialized building databases (1973 completion, 1985 rebrand, 1999 sale, height comparisons).