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Starting from the top

By Bret McCormick
mortenson

There was no greater symbol of the trust and collaboration required to build Climate Pledge Arena than the preservation of the building’s 44-million-pound historic roof.

  

A protected national and local historic landmark built in 1962 for the World’s Fair, the original roof had to be propped up for 13 months while the new arena was built beneath it. With nearly 90% of its load support cut off at any one time, the 400-foot-by-400-foot structure loomed over thousands of workers that filled the site daily. The shoring team, led by structural engineers Thornton Tomasetti and contractor Mortenson, had to believe in its work.

“Our temporary roof support team had an attitude of ‘we have to be right.’ There was a lot of trust,” said Greg Huber, Mortenson’s Climate Pledge Arena project executive.    

The arena’s entire design stemmed singularly from the requirement to keep the Paul Thiry-designed roof. Climate Pledge is double the square footage of the former KeyArena. Designers achieved that increase by digging down about 15 feet in the middle of the building, and more than 50 feet at the perimeter. But the foundations were undermined, which required the roof to first be supported before any work could commence.

“I don’t think there has ever been [a project] where they supported the roof, demolished everything, excavated down, then came back up again,” said Thornton Tomasetti principal Shawn Leary, who managed the roof shoring project and was one of more than 100 people from the firm working on the project. “So basically, every day was a new condition.”    

The process commenced in June 2019 with the installation of a center shoring tower surrounded by roughly 30 vertical supports, each consisting of two 36-inch piles drilled 50 feet into the earth and supporting the roof with a steel beam. Four thousand tons of steel formed the temporary roof support.

Almost as soon as the last of the 30 temporary supports were installed, the first ones started coming out, part of a rhythmic year-long transition from permanent support to temporary, back to permanent, all while 680,000 cubic yards of dirt were excavated and a new seating bowl built.

Four anchored concrete buttresses were key supports in the original design but all four, plus 20 chevron columns, were rendered useless during the temporary shoring. A 230-foot steel kickstand supported the freestanding south buttress, crucial for the roof in case an earthquake struck. The buttresses and chevrons were integral to the new permanent support system, especially the challenging south buttress, which, because of tunneling and other work nearby, needed its own anchoring system.

“So much of it was the first time kind of setup, even for some of the most brilliant structural engineers around, really solving a problem that was completely different,” said CAA Icon Senior Vice President Rob Stephens, the firm’s Climate Pledge Arena project executive.

Once permanently supported, the roof was strengthened to meet modern demands, including the latest seismic codes, a rigging grid with 100-ton capacity and hospitable to any concert setup, and a pair of three-sided, roughly 50,000-pound Daktronics video boards.

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