The 238-ft-tall Auditorium Building, completed in 1890, was once the tallest building in Chicago. It bears on so-called “wedding cake” stone footings that progressively widen to spread out the massive load. They rest on a mat of steel beams and rails embedded in concrete. Timbers permanently submerged in groundwater provide final support.

Draining the neighboring site would be disastrous. “You can't dewater your spot to bring the water table down,” explains Julie Allman, Power's senior project manager. The builder brought in Roselle, Ill.-based Case Foundation Co. to drill rock and belled caissons, 24 in total, and Thatcher Foundation Inc., Gary, Ind., then installed steel sheeting, roughly 40 ft deep around the site's perimeter, to protect the Auditorium's underpinnings. Excavators could then dig out the vertical campus' 23-ft-deep basement.

With the foundation secure, contractors then turned to the problem of delivering concrete for the floor slabs and eccentrically-located core—which sits close to the north wall. After the first floor was in place, crews shored it to support ready-mix trucks so they could back into the building, rather than deliver concrete from the street. Another problem was the tower crane, which would foul up schedules if placed inside the core. Crews couldn't stick the crane outside the building either, as the owner of an adjacent parking lot would not allow any penetrations in the lot's asphalt.

Chicago-based structural consultant Chris Kohout, principal of ACK Engineering Services Ltd., devised an innovative solution to hang the crane about 10 ft above grade using steel beams tied back into the core.

“It's not something that's done very often,” says Kohout, who designed the so-called “diving board” for steel erector Chicago Decking Inc., a subcontractor to fabricator Zalk Josephs, Stoughton, Wis.

The cantilevered foundation consisted of two steel plate girders, roughly 6 ft deep and 35 ft long. According to David Eckmann, principal of Seattle-based Magnusson Klemencic Associates and the building's structural consultant, the solution worked well for the project team. “They had no problems with it,” he says.

One last structural sleight of hand is an six-story bustle of the building that cantilevers over the south side, close to the Auditorium, without disturbing the footings below. The historic building “has settled a fair amount over the years,” Eckmann says. “We didn't want it to move any more as a result of our building.”