Aon Center

Aon, CNS 2010

200 East Randolph Street

At a time when glass and steel buildings were the norm, architect Edward Durell Stone believed in masonry. He thought masonry buildings conveyed a feeling of strength and permanence that glass buildings did not. When he clad the Aon Building in marble, creating the tallest marble faced building in the world, he hailed the material as costing “no more than crumpled up aluminum.” If only that were really the case. Today the Aon Building is remembered as the biggest building blunder in Chicago history.

Completed in 1973, the tower was originally built for the Standard Oil Company (its name later changed to the Amoco Building and eventually Aon Center) to consolidate their twelve downtown offices into one central building to be shared with some of their major subsidiary companies.  Standard Oil didn’t want just anyone designing their headquarters, and Stone had the reputation they were looking for. He was regarded as one of the country’s greatest living modernist architects, and was famous for designing such buildings as the Kennedy Center in Washington D.C., and the Museum of Modern Art in New York. Stone was charged with the building’s overall design, and the Chicago based architecture firm of Perkins + Will handled the structural design and details.

CNS 2010

At 1136 ft. Stone was aiming for the Aon tower to be Chicago’s tallest building, but the Willis Tower took over that title before the Aon Building was completed. However, today it remains a defining skyscraper in our skyline as the third tallest building in Chicago (behind the Willis and Trump towers). Its structure is essentially a vertical square tube. The mechanicals (elevators, bathrooms etc.) are located in the central core of the building leaving the surrounding area on each of its 82 floors as clear unobstructed and workable space. Beneath each floor, trusses connect the core of the building to the many vertical piers that make up the tower’s four sides. This system allows for a column-free interior, but consequently a great number of piers are needed to support the weight of the building. That is why there is such a high stone to glass ratio in the Aon Building. That, and we can’t forget Stone’s love for, well, stone.

It’s simple in form, and its simplicity is what makes it so striking. Its massiveness and height give the building strength. Its vertical piers, one after another, give the building rhythm. From a distance there’s more to these vertical piers than what meets the eye – they’re triangular. And originally they were clad in Cararra marble (the same marble Michelangelo favored in his prized sculptures). But today it’s clad in white North Carolina granite.

Aon Plaza, CNS 2010

What happened? Chicago’s weather happened. The marble was cut to an unprecedented width of only 1.25 inches. It didn’t take long to discover that the marble was cut too thin for Chicago’s extreme temperature changes. Heat causes permanent expansion in marble. While the exterior side of the marble was expanding, the side of the marble facing the building was not, and it began to bow. As it would eventually become a safety hazard, the marble was removed and replaced with granite 2 inches thick. The original cost of the building was $120 million. Replacing the stone cost somewhere between $60 and $80 million – more than half the building’s original price.

It still looks good though. In fact, today the Aon Building is one of Chicago’s greatest treasures, albeit underappreciated. The tower occupies only one-quarter of its entire lot. The rest is reserved for a grand bi-level plaza that doubles as a water park. Waterfalls and fountains cool the plaza. Rows of honey locust tress provide welcome shade on hot summer days and warm bright colors during the fall. Nestled in one corner of the plaza is Harry Bertoia’s “sounding sculpture” which sings in the wind.  Made up of a series of thin metal rods, it mimics and puts music to the Aon Building’s soaring piers of stone.

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Edward Durell Stone designed a number of noteworthy buildings in his lifetime. To take a tour of his architecture click here. And though Larry Perkins and Phillip Will are no longer around, their firm still is. Click here to visit the site of Perkins + Will.

To the left notice how the rods of Bertoia’s “Sound Sculpture” relate to the lines in the Aon Building. Click on the image to ENLARGE it!