A Pioneer in Bundled-Tube Design, Iconic Willis Tower Turns 50

Fifty years after its construction, the Willis Tower stands tallest among Chicago’s 126 skyscrapers and remains an icon in its cityscape. Formerly named (and widely known as) the Sears Tower, the 1,450-ft-tall building designed by Skidmore, Owings & Merrill (SOM) revolutionized the tubular system that has continued to inform contemporary architectural design.

John Zils, a native Chicagoan and retired SOM associate partner, walked by the Willis Tower construction site every morning on his way to work in the early 1970s, when he was employed as a structural engineer with the company. On his visits, Zils saw firsthand many of the intricacies--and quirks--that went into building what would be the world’s tallest skyscraper for the next 22 years.

“Here was a massive building--76,000 tons of steel--and speed was essential,” Zils recalled. “[The erectors] would erect two floors a week, which was very quick. The column sections were two stories high, so they would erect those columns, and when they hit two levels, then they could do the floor framing.”

At 110 stories high, the structure was a feat for its time and the first to employ the bundled-tube system introduced by lead structural engineer Fazlur Khan, who was also the mastermind behind another Chicago landmark, 875 North Michigan (formerly the John Hancock Center). Khan’s tubal design innovation set a precedent for SOM, and the completed Willis Tower embodied the designers’ belief that a building’s structural makeup should permeate its form. Khan spoke extensively with Modern Steel Construction about the bundled-tube approach back in 1972.

Willis Tower’s nine steel-framed tubes allowed it to reach its envisioned height with monumental efficiency, requiring only 33 lb of steel per sq. ft (the Empire State Building, in comparison, required 66 lb per sq. ft). Each tube has 75-ft-long sides, for a total of 225 ft on each side of the building.

“It had to be more than just a tube,” Zils said. “It had to be something stronger, something stiffer, and [the solution] was to bundle the tubes.”

Each of Willis Tower’s bundled tubes is intrinsically rigid with no internal supports, according to SOM.

As Zils emphasized, speed was crucial in completing a project of this scale in a timely manner--and the contractor, Morse/Diesel (now AMEC), had to get creative to sustain that efficiency, he said. Instead of bringing all the workers down for lunch and back up again, they brought in a moveable lunch cart and hoisted it up to the level of the building at which they were working each day.

“They would stay right onsite, and it was a very efficient way to do it,” Zils said. “They could eat and not lose a lot of time.”

The contractor also positioned cranes on all four sides of the base of the building so that when American Bridge Company trucks brought in pieces of steel, they could be lifted directly into place on the building, Zils said. The steel never touched the ground.

“With most projects, they bring the steel to the site, and it’s offloaded from the truck onto the ground or some staging area, then sorted out and lifted into place,” Zils said. “Sears Tower wasn’t that way.”

Since 1974, visitors to Willis Tower have lined up for the ear-popping elevator ride to the 103rd-floor observation deck--the highest vantage point for 50 years of change.

Icons don’t remain static, of course. Willis Tower recently got a new base! Learn more about it in the August 2020 issue of Modern Steel Construction, or you can flip through these images, including a couple of animated models, to see what will greet visitors for the structure’s future.