Bob Halvorson, principal with Chicago-based Halvorson and Partners, the project's structural engineer, says that while the approach isn't altogether unique, it is unusual nonetheless. "The most obvious solution would have been to incorporate large beams or transfer girders to mediate among podium and tower column grids, but those members tend to be very deep," he says. "In this instance, depths would have measured about 10 to 15 feet."
"Beams also would have required a grid assembly in order to transfer loads in two directions," adds Greg Lakota, principal at Halvorson. "A transfer mat achieved that while minimizing thicknesses, so we were able to kill two birds with one stone."
Plans originally called for a 72-in.-thick mildly reinforced slab, a scheme that would have required installation of 15 levels of reshoring in the existing 28-story structure," says Eric Lindquist, project manager with Lombard, Ill.-based Adjustable Forms, OneEleven's concrete contractor. "By redesigning the slab into a 5-foot post-tensioned element, we were able to significantly reduce the amount of concrete, rebar and reshoring required," he says. "We poured an initial 20 inches and stressed the post tensioning, so that the slab had capacity to support the remaining 40 inches."
As built, the 12,195-sq-ft slab accommodates 9,200 kips of force in both directions, and incorporates some 350 1/2-in.-dia tendons in both directions. "Tendon dimensions aren't unique—there are just many more of them than you'd typically find in a floor slab," says Lakota.
Parabolic tendons are atypical, with low points located 3 in. from the bottom of the slab and high points 2 in. from the top. Resulting drape—or distance between the two—is 55 in., as compared with the 6 in. typical of an 8-in.-thick floor slab.
"The biggest issue was the number of penetrations running through the mat—there are hundreds of them —that and the fact we had to hit the floor running," says Brandt. "We were starting with a structure that was half built, so trades didn't have the coordination time they would have had for a project starting in the ground. And we had only one shot to position all those paths correctly."
Prior to the pour, engineers and trades spent a month on site, coordinating the positions of mechanical shafts and electrical and plumbing conduit as crews placed 110 tons of mild reinforcing and 240,000 linear ft of encapsulated mono-strand tendons on the 140-ft by 90-ft deck, in accordance with design drawings supplied by Halvorson. "There was just so much reinforcement between mild steel and the post-tensioning," says Lakota. "Tremendous amounts of coordination were required."
To avoid clashes, Halvorson supplied crews with extensive guidelines for circumventing penetrations. Firm engineers also were on hand as Adjustable Forms performed its pours. "In many cases, we found ourselves making changes in real time," says Brandt, "so we had members of our team coordinating with Halvorson to determine the best paths forward."
Mixer trucks positioned themselves on Lower Wacker Drive, a lower deck of the thoroughfare typically reserved for through traffic and vehicles serving the street's buildings. To expedite pours, Adjustable Forms supplemented its Schwing 8800 Stationary Pump with crane and bucket placement, utilizing a Potain MD485 tower crane from Central Crane Service.
Crews closely monitored pump performance throughout pours and evaluated product for cold joints, heat of hydration and other conditions. Pours occurred a few days apart to provide the initial pour with time to cure and strengthen.