Here's some background information about Willis Tower, formerly the Sears Tower, located in Chicago.
The Sears Tower was the world's tallest building until 1996, when it was surpassed by the Petronas Twin Towers in Kuala Lumpur, Malaysia.
Buildings and structures
Building planning and construction
Business and industry sectors
Business, economy and trade
Midwestern United States
Continents and regions
Points of interest
Some 25,000 people enter the skyscraper every day.
About 1.5 million people go up to the Skydeck visitors' observation deck on the 103rd floor each year.
Security cameras are located in all public areas of the building.
Security personnel monitor the building 24 hours a day.
One of the first major buildings to have a complete sprinkler system.
Structural height - 1,450 feet or 110 floors
Height with the two antennas - 1,730 feet
Height of Skydeck/103rd floor - 1,354 feet
Lowest level - 43 feet below Franklin St.
Building size - 4.5 million gross square feet
Weight - 222,500 tons or 445 million pounds
Cost to build - $150 million
Windows - 16,100
Building exterior - black aluminum and bronze-tinted glass
Architectural firm - Skidmore, Owings & Merrill
Architect - Bruce Graham
Structural engineer - Fazlur Khan
1970 - Construction begins.
1973 - Construction is completed and the building opens to the public.
1974 - The Skydeck opens for visitors.
1994 - Sears, Roebuck & Company sells the building to reduce its debt.
1996 - The Petronas Twin Towers surpass the Sears Tower in height to become the world's tallest buildings at 1,483 feet each. The Sears Tower becomes the world's third-tallest building.
December 1997 - The building is sold to a Canadian real estate firm, TrizecHahn Corp.
July 2009 - The Sears Tower is officially renamed Willis Tower. The same month, an attraction called the Ledge opens on the 103rd floor, with glass balconies and a view down 1,353 feet to the street below.
May 28, 2014 - Cracks develop in the protective coating on a glass sightseeing ledge. Officials say no one was ever in danger.
2016 - Paul Kurzawa, a film executive from the DreamWorks Animation studio, is hired to oversee redevelopment of the tower in an effort to boost tourism.
February 1, 2017 - Blackstone Group unveils its plans for $500 million in tower renovations, including revamping the lobby, adding retail and dining spaces and a rooftop garden, among other things.