Aon Center (Chicago)

The Standard Oil Building was constructed as the headquarters of the Standard Oil Company of Indiana, which had previously been housed at South Michigan Avenue and East 9th Street. When it was completed in 1973, it was the tallest building in Chicago and the fourth-tallest in the world, earning it the nickname "Big Stan".[10] (A year later, the Sears Tower took the title as Chicago's and world's tallest.) 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. This construction method was also used for the former World Trade Center towers in New York City.

When completed, it was the world's tallest marble-clad building, sheathed entirely with 43,000 slabs of Italian Carrara marble. The marble used was thinner than previously attempted in cladding a building, which soon proved to be a mistake. On December 25, 1973, during construction a 350-pound marble slab detached from the façade and penetrated the roof of the nearby Prudential Center.[11] In 1985, inspection found numerous cracks and bowing in the marble cladding of the building. To alleviate the problem, stainless steel straps were added to hold the marble in place.[10] Later, from 1990 to 1992, the entire building was refaced with Mount Airy white granite at an estimated cost of over $80 million.[5][12] Amoco was reluctant to divulge the actual amount, but it was well over half the original price of the building, without adjustment for inflation. Two-thirds of the discarded marble was crushed and used as landscaping decoration at Amoco's refinery in Whiting, Indiana, one-sixth was donated to Governors State University, in University Park, and one-sixth donated to Regalo, a division of Lashcon Inc. Under a grant from the Illinois Department of Rehabilitative Services, Regalo's 25 handicapped workers carved the discarded marble into a variety of specialty items such as corporate gifts and mementos including desk clocks and pen holders.[5][13] The building's facade somewhat resembles that of the North and South tower of the former World Trade Center Complex due to the upward flow of the columns.

The Standard Oil Building was renamed the Amoco Building when the company changed names in 1985. In 1998, Amoco sold the building to The Blackstone Group for an undisclosed amount, estimated to be between $430 and $440 million.[5][10] It was renamed as the Aon Center on December 30, 1999, although the Aon Corporation would not become the building's primary tenant until September 2001.[14] In May 2003, Wells Real Estate Investment Trust, Inc. acquired the building for between $465 and $475 million.[5][15] On August 10, 2007, Wells Real Estate Investment Trust, Inc. changed its name to Piedmont Office Realty Trust, Inc.)[16]

Real estate investors Mark Karasick and Victor Gerstein acquired the building from Piedmont in 2015 for $713 million.[1]

On May 14, 2018 the owners unveiled $185 million proposal for an observatory featuring a thrill ride on the roof called the Sky Summit, the world's tallest exterior elevator, and new entrance pavilion. The observatory was supposed to be completed in 2022, but the COVID-19 pandemic has delayed construction plans by about a year.[17][18]