Aon Center, Chicago

Aon Center is a striking feature of the city's skyline, distinguished by its clean, white stone facade that sets it apart from the surrounding steel-and-glass towers. Completed in 1973, this 1,136-foot-tall skyscraper was originally conceived as the Standard Oil Building, designed by architect Edward Durell Stone, known for his unique take on Modernism. The building's monumental presence was intended to reflect the stature of Standard Oil of Indiana, which sought a new headquarters at the prominent site at the head of Grant Park, previously occupied by industrial structures.

The architectural approach for Aon Center was innovative for its time, utilizing a structural form that combined an inner core with perimeter columns to create a robust, open floor plan. This method, which also influenced the design of the Willis (Sears) Tower, featured a tube-like arrangement where the inner core housed elevators and services, while the outer columns and trusses provided structural support. This design allowed for large, open office spaces and a distinctive architectural profile.

The building's original cladding, made of white Carrara marble, was a nod to classical elegance but proved problematic. The marble could not withstand the harsh weather conditions of Chicago, leading to cracks and deterioration. As a result, the entire facade was re-faced with durable granite in the early 1990s, at significant expense, to restore the building's appearance and functionality.

In 1985, Standard Oil became Amoco, and the building was known as the Amoco Building until its sale in 1998. The new owner rebranded the skyscraper as Aon Center after one of its major tenants, reflecting the building's ongoing role as a central fixture in Chicago's commercial landscape. Despite the changes over the years, Aon Center remains a testament to both architectural ambition and the challenges of maintaining iconic structures in a dynamic urban environment.