Willis Tower vs Aon Center

Willis Tower
Aon Center

Comparing the Willis Tower and the Aon Center is interesting because they both stand in Chicago, IL, and were completed just one year apart, but they were designed by different architects.

This offers a unique glimpse at how rival designers approached projects in the same city during the same era.

Height
442m
Floors
108

Height & Size

Height
346m
Floors
83

The Willis Tower is clearly the larger tower of the two, both in terms of height and number of floors. It rises to 1450ft (442m) with 108 floors above ground, while the Aon Center reaches 1135ft (346m) with 83 floors above ground.

Willis Tower also offers more total built-up area, a total fo 4,499,999 sqf (418,064m2), which is about 900,034 sqf (83,616m2) more than what the Aon Center offers.

Of course, each project may have faced different briefs or regulatory constraints, which we don't really know about and could also explain the outcome.

Style
International Style

Architectural Style

Style
International Style

Both the Willis Tower and the Aon Center were designed in line with the aesthetic conventions of the International Style style.

Both buildings were completed when the International Style style was already past its peak. This makes them feel like late echoes of the movement, more reflective of continuity or nostalgia than of cutting-edge design at the time.

Main use
Commercial

Uses

Main use
Commercial

Both the Willis Tower and the Aon Center were designed to serve as commercial towers, and that has remained their main use since their completion, serving similar roles in the urban fabric.

The Aon Center also provides 679 parking spaces.

Structure
Frame
Facade
Curtain Wall

Structure & Facade

Structure
Framed Tube In Tube
Facade
Curtain Wall

The two towers rely on different structural systems, reflecting distinct engineering strategies.

The Willis Tower uses a Frame structural system, which relies on a regular grid of columns and beams to sustain its weight, while the Aon Center uses a Framed Tube In Tube system, that combines a strong central core with a perimeter tube of columns.

Yet, when it comes to their facade, they both employed the same solution, a Curtain Wall facade.

A curtain wall is a non-load-bearing facade hung from the structural frame. It is anchored to floor slabs and transfers only its own weight and wind loads, allowing for sleek, glassy exteriors.

Willis Tower Aon Center
Skidmore, Owings & Merrill Architect Edward Durell Stone
1970 Construction Started 1970
1974 Year Completed 1973
International Style Architectural Style International Style
Commercial Current Use Commercial
108 Floors Above Ground 83
4 Floors Below Ground 5
413 Last Floor Height 328
442 m Height (m) 346 m
527 Tip Height 362
418064 Built-up Area (m²) 334448
104 Number of Elevators 50
Frame Structure Type Framed Tube In Tube
Steel Vertical Structure Material Steel
Concrete Horizontal Structure Material Concrete
No Facade Structural? Yes
Glass, Steel, Aluminum Main Facade Material [
Morse Diesel International Main Contractor Turner Construction
Sears, Roebuck & Company Developer Standard Oil Company Of Indiana
Westinghouse, Schindler Group And Otis Elevator Company Otis
Fazlur R. Khan, Srinivasa Iyengar Structural Engineer Perkins & Will
IL State IL
Chicago City Chicago
233 S. Wacker Drive Address 200 E.Randolph Street