At the time, the tallest skyscraper in the world was New York's Empire State Building, at less than a quarter of the height suggested for the Illinois.

It probably would have been possible to erect a self-supporting steel structure of the required height,[citation needed] but there are a number of problems that occur when a building is that tall.

This also could have been solved by placing a tuned mass damper somewhere within the tower as was done in the Citigroup Center and Taipei 101, although this design innovation was not well-known until decades later.

[citation needed] The late 1990s and early 2000s saw substantial increases in the load-bearing strength of concrete, making it possible to build entirely in this stiff material.

[1] The elevator banks extend beyond the sloping exterior walls at various points, giving the building the appearance of an elongated pyramid with protruding parapets.