Willis Tower (formerly known as Sears Tower) is an iconic building in Chicago and the third tallest building in the United States, as well as the fourteenth tallest building in the world (as of 2024 data).
Building Overview
Height and Structure
Total height: 442.3 meters (excluding antenna), including antenna total height 527.3 meters, with 108 floors above ground and 3 floors below ground, covering a total area of 416,000 square meters.
Design Features: Designed by SOM, it employs an innovative “Bundled Tube Structure” that forms a wind-resistant system through nine progressively tapering square steel tubes, reducing wind pressure while enhancing stability. The maximum displacement under measured wind speed at the top is only 46 centimeters, far below the design allowance of 90 centimeters.
Historical Significance
World Record: Upon completion in 1974, it surpassed the World Trade Center in New York with a height of 442 meters, becoming the tallest building in the world, a record it held for 24 years until surpassed by the Petronas Towers in 1998.
U.S. Ranking: Currently, it is the third tallest building in the U.S., following One World Trade Center in New York (541 meters) and the Chicago Spire (472 meters).
Design and Technological Innovations
Structural Breakthroughs
Bundled Tube Structure: Proposed by structural engineer Fazlur Khan, it consists of nine steel tubes layered to form a cross or double-tube structure, reducing steel usage to 1.62 kilograms per square meter, saving 50% compared to traditional frame structures.
Materials and Craftsmanship: The exterior features black aluminum and coated glass curtain walls, with a total steel weight of 76,000 tons, designed to withstand wind pressure of up to 3 kilopascals.
Functional Layout
Office and Observation: Once the headquarters for Sears, it now serves as a comprehensive office space, with the 103rd floor featuring the “Skydeck” at a height of 412 meters, offering panoramic views of four states, equipped with four glass observation decks (The Ledge) extending 1.5 meters for an exhilarating experience.
An enduring Chicago landmark
Chicago’s Eternal Landmark
For decades, the tallest building in the world, the 110-story Willis Tower (formerly Sears Tower) marked a major step in exemplifying and defining SOM’s belief that a building’s structure should inform its exterior profile.
The 110-story Willis Tower (formerly Sears Tower) was the tallest building in the world for decades, marking an important step in SOM’s architectural philosophy that “a building’s structure should reflect its appearance.”
The 4.5-million-square-foot tower was the headquarters for Sears, Roebuck & Co. from the time of its completion in 1973 until 1995, when the merchandiser relocated to suburban Chicago. The building’s design, flexible floor plan, and iconic stature have made it a premier address for a host of companies. Today, the tower is well-maintained and remains a center of activity in Chicago’s Loop.
This 4.5 million square foot building has served as the headquarters for Sears, Roebuck & Co. since its completion in 1973 until 1995 when the company moved to suburban Chicago. Its unique design, flexible floor plan, and iconic stature have made it a preferred office location for many companies. Today, the building is well-maintained and continues to be a hub of activity in Chicago’s Loop.
Pragmatic Engineering
Pragmatic Engineering Design
The iconic setback design of the structure was conceived in response to the original client’s space requirements: Sears wanted a building that incorporated not only large office floors, necessary for the company’s operations, but also a variety of smaller floors for tenants requiring less floor area.
The iconic setback design was intended to meet the original client’s spatial needs: Sears wanted a building that could accommodate not only large office floors necessary for the company’s operations but also a variety of smaller office floors for tenants with lower space requirements.
Pioneering the use of bundled tube construction, the basic structure consists of nine 75-foot-by-75-foot column-free square tubes at the base, forming a cellular-tube frame. The nine bundled structural tubes, each rigid within itself without internal supports, rest on reinforced concrete caissons that go down to bedrock. The tubes are bundled together as a closed square up to the first 50 stories, and then terminate at varying heights to create the tower’s distinctive, multi-tiered form.
The building pioneered the use of bundled tube construction, with its basic structure consisting of nine 75-foot by 75-foot column-free square tubes at the base, forming a cellular-tube frame. The nine bundled structural tubes, each rigid without internal supports, rest on reinforced concrete caissons that extend down to bedrock. The tubes are bundled together as a closed square up to the first 50 stories, then terminate at varying heights to create the tower’s distinctive multi-tiered form.
The structural steel frame was pre-assembled in sections and then bolted into place on site. The lightweight building skin, a black aluminum and bronze-tinted glare-reducing glass, serves as an insulator between the interior and exterior structure in order to maintain a relatively constant temperature, which minimizes the expansion and contraction of the frame.
The structural steel frame was pre-assembled in sections and then bolted into place on site. The lightweight building skin, made of black aluminum and bronze-tinted glare-reducing glass, acts as an insulator between the interior and exterior structures to maintain a relatively constant temperature, minimizing the expansion and contraction of the frame.
Above the Clouds
Above the Clouds
In 2009, the Ledge at Willis Tower was created to enhance the building with an innovative new observation deck on the 103rd floor. In 2021, SOM continued its 50-year stewardship of Willis Tower with a transformative renovation of the Ledge. Interior enhancements, improved lighting, building technology upgrades, and a new exhibition space further immerse visitors in Chicago’s historic skyline.
In 2009, the Ledge at Willis Tower was established to enhance the building with an innovative new observation deck on the 103rd floor. In 2021, SOM continued its 50-year stewardship of Willis Tower with a transformative renovation of the Ledge. Interior enhancements, improved lighting, building technology upgrades, and a new exhibition space further immerse visitors in Chicago’s historic skyline.
History and Renaming
Construction Background: Groundbreaking in 1972, completed in 1974, originally the headquarters for Sears, Roebuck & Co., converted to commercial use after the company moved out in 1992.
Renaming Event: In 2009, due to the Willis Group acquiring naming rights, it was renamed “Willis Tower,” which sparked local controversy but was ultimately retained.
Safety and Facilities
Fire Safety System: Equipped with smoke detectors, automatic sprinklers, and rooftop evacuation platforms, elevators operate in sections (high-speed elevators directly to the 33rd and 66th floors).
Anti-Terror Measures: Once listed as a target by Al-Qaeda, security has since been enhanced.
Cultural Significance
Chicago Landmark: Reflecting Lake Michigan, it represents the pinnacle of 20th-century American economy and engineering technology, now a core tourist attraction in the city.
Architectural Heritage: The bundled tube structure has been widely applied in super-tall designs, such as Shanghai Tower and Taipei 101.
Client: Sears Roebuck & Co.
Site Area: 3 acres
Building Height: 1,450 feet
Number of Stories: 110
Building Gross Area: 4,455,844 square feet
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