At completion in 1973, it surpassed the World Trade Center towers in New York to become the tallest building in the world, a title it held for nearly 25 years; it remained the tallest building in the Western Hemisphere until the completion of a new building at the rebuilt World Trade Center site in 2014. The building is considered a seminal achievement for its designer Fazlur Rahman Khan. The Willis Tower is the second-tallest building in the United States and the Western hemisphere – and the 16th-tallest in the world. More than one million people visit its observation deck each year, making it one of Chicago's most-popular tourist destinations. The structure was renamed in 2009 by the Willis Group as part of its lease on a portion of the tower's space.
As of April 2018, the building's largest tenant is United Airlines, which moved its corporate headquarters from the United Building at 77 West Wacker Drive in 2012 and today occupies around 20 floors with its headquarters and operations center. Other major tenants include the building's namesake Willis Group and the law firms of Schiff Hardin and Seyfarth Shaw. Morgan Stanley plans to move to the building in 2019, and to become the building's fourth-largest tenant by 2020.
The Willis Tower observation deck, called the Skydeck, opened on June 22, 1974. Located on the tower's 103rd floor, it is 1,353 feet (412.4 m) high, making it the highest observation deck in the United States and one of Chicago's most famous tourist attractions. Tourists can experience how the building sways on a windy day. They also can see far over the plains of Illinois and across Lake Michigan to Indiana, Michigan, and Wisconsin on a clear day. Elevators take tourists to the top in about 60 seconds, and allow tourists to feel the pressure change as they rise up. The Skydeck competes with the John Hancock Center's observation floor a mile and a half away, which is 323 feet (98.5 m) lower. Some 1.7 million tourists visit the Skydeck annually. A second Skydeck on the 99th floor is also used if the 103rd floor is closed. The tourist entrance can be found on the south side of the building along Jackson Boulevard.
In January 2009, Willis Tower's owners began a major renovation of the Skydeck, including the installation of retractable glass balconies, which can be extended approximately 4 feet (1.2 m) from the facade of the 103rd floor, overlooking South Wacker Drive. The all-glass boxes, informally dubbed "The Ledge", allow visitors to look through the glass floor to the street 1,353 feet (412 m) below. The boxes, which can bear 5 short tons (4.5 metric tons) of weight, opened to the public on July 2, 2009. On May 29, 2014, the laminated glass covering the floor of one of the glass boxes shattered while visitors were sitting on it, but caused no injuries. The broken glass was replaced within days, and tourist operations resumed as before.
According to wikipedia