The Willis Tower is a 110-story, 1,450-foot (442.1 m) skyscraper in Chicago, Illinois. At completion in 1973, it surpassed the World Trade Center in New York to become the tallest building in the world, a title it held for nearly 25 years; it was the tallest building in the Western Hemisphere for 41 years, until the new One World Trade Center surpassed it in 2014.

Willis Tower is considered a seminal achievement for architect Fazlur Rahman Khan. It is currently the second-tallest building in the United States and the Western hemisphere – and the 16th-tallest in the world. Each year, more than one million people visit its observation deck, the highest in the United States, making it one of Chicago’s most popular tourist destinations.

ARCHITECTURAL MARVEL: WILLIS TOWER

The Willis Tower, formerly known as the Sears Tower, showcases a revolutionary structural system called the bundled tube design. This concept, pioneered by Fazlur Rahman Khan, consists of nine square tubes clustered together to form a larger square. As the building rises, the tubes terminate at different heights, creating the iconic stepped appearance of the tower.

WIND RESISTANCE ENGINEERING

Chicago’s nickname, „The Windy City,” posed significant challenges for the tower’s engineers. To combat wind sway, the building incorporates a series of innovative features. The structure’s tapered design and its corner indentations help reduce wind forces, while two 300,000-pound dampers on the 76th floor counteract building movement, ensuring occupant comfort.

ELEVATOR TECHNOLOGY

The Willis Tower boasts an impressive elevator system, featuring 104 high-speed elevators, including double-decker elevators that serve two floors simultaneously. The fastest elevators can reach speeds of up to 1,600 feet per minute, making the journey from ground level to the 103rd floor in just 60 seconds.

SUSTAINABLE RETROFITTING

In recent years, the Willis Tower has undergone extensive renovations to improve its energy efficiency. The building now features a state-of-the-art window system that significantly reduces heat gain and loss. Additionally, the implementation of smart building technology and energy management systems has resulted in substantial reductions in energy consumption.

SKYDECK ENGINEERING

The Skydeck on the 103rd floor features „The Ledge,” a series of glass boxes extending 4.3 feet from the building’s facade. These boxes are designed to support up to five tons and can be retracted into the building for maintenance. The glass floor consists of three layers of half-inch thick glass laminated into a seamless unit.

CONSTRUCTION MILESTONES

During its construction, the Willis Tower set several records. The foundation required the largest single concrete pour ever attempted: 2,000 trucks delivered 19,000 cubic yards of concrete in a single day. The building also utilized a then-novel construction technique called „kangaroo crane” jumping, where cranes „climbed” the structure as it rose, significantly speeding up the construction process.