Willis Tower

The Willis Tower is an International Style skyscraper designed by Skidmore, Owings & Merrill, with Bruce Graham as lead architect, and built between 1970 and 1974, for a reported $175 million dollars, in Chicago, IL.

Willis Tower is not the only name you might know this building by though. It is common for companies to want to attach their names to iconic buildings when they move in, or for the general public to come up with nicknames, and this one is no exception. The building has changed names several times over the years, and is also known as:

• Sears Tower between 1974 and 2009.
• Willis Tower from 2009 until this day.

Its precise street address is 233 S. Wacker Drive, Chicago, IL. You can also find it on the map here.

Architect Bruce Graham and engineer Fazlur Khan used an innovative structural system to address wind loads on a building of this height. The structural steel frame was pre-assembled in sections and then bolted into place. The structure is made up of 9 square steel tubes, each 23 x 23 meters wide, interconnected to act as a single unit, with no columns between the core and the perimeter. Two of the tubes are 50 stories high, two are 66 stories, three are 90 stories, and two are 108 stories.

Some people say the building has 110 floors instead of 108. That is including the elevator shaft and its roof, which are not normally counted as floors.

When it was completed in 1974 it was the tallest building in the world, a title it held until either (and this depends on how building heights are measured, which was not standardized at the time) 1998, when the Petronas Towers in Kuala Lumpur were completed, or 2004, when the Taipei 101 was..

The building has been restored 3 times over the years to ensure its conservation and adaptation to the pass of time. The main restoration works happened in 1980, 2009 and 2019.

Skidmore, Owings & Merrill, with Bruce Graham as the lead architect, was the architecture firm in charge of the architectural design.

That being said, architecture is a complex discipline involving many professionals from different fields, without whom this building would have not been possible. We will surely be leaving out a lot of names here, but here is a list of the people we do know also played their part in making the Willis Tower a reality:

• Fazlur R. Khan, Srinivasa Iyengar in charge of Structural Engineering
• Morse Diesel International as the Main Contractor
• Enclos Corp in charge of Facade Consultancy
• Westinghouse, Schindler Group and Otis as the company in charge of the elevators system
• Sears, Roebuck & Company as the Main Developer
• Jaros, Baum & Bolles in charge of MEP Engineering

The Willis Tower can be categorized as an International Style building.

The Willis Tower uses a frame structure made of steel columns and concrete slabs.

A frame structure uses a combination of beams and columns to sustain the building's weight. The walls in this case are non-load bearing, which allows for more flexibility when distributing the interior spaces.

The facade uses a non-load bearing curtain wall system. This means the curtain wall modules are anchored to the building's structural frame, typically by being attached to the edge of the floor slabs. The curtain wall system connects to the slabs using brackets, anchors, and mullions, which transfer the loads imposed by wind and temperature changes, to the building's primary structural elements.

This setup allows the curtain wall to accommodate differential movement between the facade and the structural frame, such as thermal expansion, floor deflection, or sway from wind forces. This system's integration with the slab edges also allows for continuous insulation and weatherproofing layers.

From an aesthetic point of view, the facade features a curtain wall made of black anodized aluminum with vertically running bronze-tinted glass panels, accentuated by black lines emphasizing the tubes. Large windows maximize natural light and views, while the tinted glass reduces reflections and serves, to some degree, as an insulator to maintain a relatively constant temperature, minimizing the expansion and contraction of the framework.

The vertical evolution of the building is also defined by the structural tubes we talked about previously. As some of these tubes are lower than others, the tower draws an asymmetric stepped-back silhouette that has become recognizable not only within the Chicago skyline, but as one of the greatest skyscrapers of all times.

Another material found at the Willis Tower is metal tiles , used in the lobby floor with a stylezed design based on a bundled tube structure.