Keeping work moving typically ranks alongside safety and quality atop every construction project’s priority list. But when it came to building the reinforced concrete core of the Western Hemisphere’s tallest elevator test tower at ThyssenKrupp’s new $200-million North American corporate headquarters in Atlanta, that concept was applied literally as well as figuratively through the use of slipform construction.
Topped out in mid-March by Brasfield & Gorrie after approximately eight weeks of continuous slipform pours from a moving formwork platform, the 74-ft-long by 56-ft-wide tower core rises 398.5 ft from a 16-in.-thick mat slab founded with bedrock anchors approximately 55 ft below grade. The process used nine different concrete mixes totaling just over 7,130 cu yd to consistently form the structure’s 18-in.-thick exterior walls and 10-in.-thick interior walls separating nine test shafts.
Taking advantage of its close proximity to The Battery Atlanta sports and entertainment venue, the test tower will double as a technology demonstration showcase when it opens in 2021, just shortly after its final completion.
“Unlike most elevator test towers that are hidden in rural locations, we wanted this to be in a highly visible location and ultimately be a point of pride for the community,” explains Steve Allen, ThyssenKrupp Elevator’s vice president of engineering.
A 52,000-sq-ft, full-height standard unitized curtain wall will extend approximately 27 ft off the east face of the core to house examples of ThyssenKrupp’s advanced elevator system, including the ropeless MULTI system. The other three sides will be clad with approximately 40,000 sq ft of unitized perforated metal panels. Topping the tower, a 20-ft-high steel-framed structure will contain mechanical rooms and observation decks.
Jump-form construction was originally considered for the tower core, according to John Haba, principal-in-charge for Gensler, which designed the tower as part of the three-building headquarters complex. However, ThyssenKrupp’s concerns about the speed and safety of that process led the project team to look to slipform, which the manufacturer used for a similar 800-ft test tower in Rottweil, Germany.
In addition to saving approximately three months of construction time and providing a safer environment for workers via the enclosed platform, “slipform also offered the ability to create the super-flat concrete surface necessary for the MULTI elevator technology,” Haba says.
Partnering with Austrian slipforming specialist Gleitbau enabled Brasfield & Gorrie’s team to quickly familiarize itself with a method rarely used in North America while also providing the design team with constructibility input.
“We already self-perform concrete work, so this was an opportunity to explore a way to expand that expertise,” says operational manager Blake Riedling. “We did a lot of due diligence to identify the best system and expertise that complemented our own.”
One of the biggest challenges to designing the slipform system was preparing for installing nearly 190 openings and more than 3,300 embedment plates for interior and exterior structural steel framing.
“Up to 200 pieces on a single elevation had to be erected within a very short time frame,” says Gleitbau project manager Nikolaus Weise-Schmidbauer.
Getting ready to rise
With details of the slipform system’s elements finalized, Gleitbau shipped five containers of equipment across the Atlantic to the project site. Assembled over two weeks, the work platform was fitted with enclosure panels around the perimeter and engineered lumber decks sourced from Oregon to ensure flat surfaces for workers.
The team also needed to secure an around-the-clock supply of concrete—at least 5.5 cu yd an hour for the duration of the placement, which would be bucketed up to the work level via a tower crane. “That helped us maintain the consistency of the mix,” Riedling says. “We were concerned that a pump carried the risk of becoming jammed and not having enough volume.”
As the crane would also be used to deliver rebar, embedment plates and other supplies as the tower rose, the scheduling of deliveries had to be as precise as the quantity and types of the materials themselves.
“It was very much a just-in-time approach with an organized plan for every elevation,” says project manager B.J. Haisten.
Crews started slipforming the mat slab on Jan. 22. With the jacks lifting the platform approximately 1 inch every 15 minutes, the carefully choreographed sequence of rebar installation and concrete pouring into the 4-ft-high forms worked its way clockwise around the platform, gradually building up exterior and interior walls.
With each 10-in. lift, the process began again. Embedment plates and frames for openings were integrated into the process. The platform’s pace ensured that the concrete would be sufficiently cured to allow finishers on lower platforms to do their work, keeping vertical surface deviations to no more than plus or minus ¼ inch, well within the specified design tolerance.
“Our approach was ‘slow and steady wins the race,’” Haisten says, noting that it took about two weeks for the team to fully adapt to the process. “We went from a new concept and a bit of chaos to everyone being used to their roles.”
The tower eventually began rising by an average of 7.5 ft per day, with approximately 30 workers on the platform at any one time. “Fortunately, we were working before the coronavirus hit, so there were no concerns at the time about social distancing,” Haisten says.
Staggering rod ties among the project’s ironworkers eliminated the risk of bottlenecks, while the pace of curing provided ample time for finishers on the platform’s lower levels to achieve the needed smoothness requirements. Access was provided by a buck hoist fitted with rolling tie that was extended as the work platform rose higher.
Riedling notes that the platform’s protective sound-dampening blankets helped minimize high-rise distractions for workers, while heat generated by the concrete’s hydration provided a welcome respite from the cold temperatures and persistent rain. The only pauses in the 24/7 slipforming operation occurred during three scheduled day-long tower jumps. Setting a cold joint in the wall, the Brasfield & Gorrie team used the brief downtime periods to clean the forms and perform any necessary maintenance before resuming work.
“Cleaning is so important for slipform because it’s your home for the duration of the project,” Riedling says. “Otherwise, you risk having building up layers that could affect the wall. It’s a constant effort.”
The last of the tower’s slipformed concrete was poured on March 18, ending a process that encountered few hiccups over 54 days and that had no recordable safety incidents.
“That’s a credit to the more than 100 employees who, at the time, could have easily gone to any other project in the area and worked normal hours,” Riedling says. “They were willing to pick up a new skill set quickly, and then excel at it.”