General contractor Bovis Lend Lease, structural engineer Magnusson Klemencic Associates and owner and developer The John Buck Co. got lucky before work began on the 52-story building called 111 South Wacker Drive in Chicago.

They realized a perfectly solid foundation for the new skyscraper already existed more than two stories below ground.

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Bell caissons dug for the foundation of the U.S. Gypsum Building, the previous structure on the site, were sound and reusable, as were the earlier building's four foundation walls, said Ron Klemencic, president of Seattle-based Magnusson Klemencic Associates.

The problem was those caissons supported a building that had stood on a 45-degree angle on the site and was only about 20 stories tall.

The 111 South Wacker building would be orthogonal to the property lines and much taller. The only way to reconcile the new geometry to that of the old building was to add new caissons to the existing ones.

Old and new caissons were tied together through the use of a giant mat slab that was laid down in a "monolithic pour," said Bill Moody, principal of The John Buck Co. of Chicago. "We believe it's the largest single pour of concrete for buildings in downtown Chicago," he added.

Jim Dushek, Chicago-based senior vice president and director of field operations with Bovis Lend Lease, said the existing foundation walls were bermed with a high-density CA 6 and rubble debris mix, and a soldier pile and lagging system was installed around the foundation mat. That stabilized the earth and helped reduce the cost
of a retention system.

Since the old caissons were used, and used effectively, the cost of the new foundation construction was a fraction of what it would have been otherwise, Klemencic said. But before the caissons were used, considerable testing was undertaken to ensure their viability, he added.

"We not only substantiated the concrete strengths of the existing caissons but also verified the bearing capacity of the existing caissons," Klemencic said. "We did that through a core drilling testing program. We go out essentially with a coring machine and drill through the tops of the caissons to sample the concrete.

"But in a couple of instances, we drilled all the way through the caissons to sample the soil beneath the caissons. All of that was done to verify the strength of the existing structure."

When complete in the first quarter of 2005, 111 South Wacker Drive will feature 1,025,000 sq. ft. of office space, with about 4,000 sq. ft. of retail and 485 parking spaces.

The high-rise building will have three occupied sections. The 12th through 28th floors will be leased by accounting firm Deloitte & Touche. No major tenant yet exists for the mid-rise floors. And the high-rise floors from 41 through 52 will be occupied by the law firm Lord Bissell & Brooks.

Cable Wall System

Even more distinctive than the foundation of the building is its use of cable wall from street level to the third floor, providing a more transparent look to the ground-level curtain wall.

Calling it "one of the most unique aspects of the building," Klemencic said this vertical cable wall features about 70 cables that span about 50 ft. from the plaza level to the third floor. Each cable is stressed to a different design tension, depending on location, span and wind loads. The tensioning stiffens the cables, permitting them to handle positive and negative wind loads.

At about 1-in. thick, the cable is a smaller diameter member than aluminum or steel mullion, so it doesn't interrupt the glass facade, Dushek said. "And the glass is very clear glass, which doesn't reflect light back," he added. "So when you're facing the building, it's more open."

Klemencic said building the cable wall was like tuning a piano. As each cable was tensioned and force was applied to the cable, the building at both the plaza and third-floor levels deflected slightly due to the applied loads.

"And as you sequentially stress each cable, the building continues to respond to these applied loads," he added. "The net result is we had to tune each cable to its optimal tension level."

A nearby building developed by The John Buck Co. 1 North Wacker Drive, features cable running both vertically and horizontally in what's called a "net cable wall."

Such a system was not possible at 111 South Wacker Drive because the wall at the new building is not straight but elliptical, which would make horizontal cable useless.

Careful attention was given to the timing of the cable wall installation, Dushek said.
The third-floor steel framing is being pulled in a downward direction and the plaza-level steel framing in an upward direction by the cables, he added.

For that reason, structural components had to be in place before tensioning could begin. Final work to bring the system up to 100 percent tension was completed in June.

The Megatrusses

111 S. Wacker Drive is a composite structure, but it is steel that plays an impressive role on the job through the use of what Klemencic termed megatrusses.

Floors 3-12 are in reality, a megatruss that supports the upper floors of the building.
The truss sits on massive 5-ft-9-in in dia. pipe columns. Two of the pipe columns start at the second basement level, and are joined at street level by four more pipe columns which rise up to the 3rd floor, said Andrew Vogl, project manager for the steel fabricator, Merrill Iron & Steel, Schofield, Wis. The pipe columns are encased in hi-strength concrete.

Floors 3 through 9 are for parking, and the columns there slope and spiral up to the 10th floor, providing the diagonal members of the truss. On the 10th floor, the sloping columns level off for the mechanical areas. On the 12th floor, the sloping columns come together with the vertical members. Then the typical office floors start.

"The building is not precisely symmetrical, and because of that the construction between the third and 12th floor is one large megatruss that helps to resist the unbalanced loading in the building," Klemencic said.

Lohan Caprile Goettsch Architects of Chicago used the underlying structure as part of the inspiration for the building's appearance, and this added to its aesthetic, Klemencic said.

Tight Site

The tight site created another issue for the construction team. In terms of staging, the team's one advantage was that Lower Wacker Drive could be used for concrete trucks making daily deliveries. Structural steel, metal deck and steel stairs were received on the north - Monroe Street - side of the building.

The steel team of Merrill Iron & Steel, working with Cordeck Sales as erector and Federated Crane, used an innovative crane technique because of the tight site. "There was not enough tail swing space for the tower crane," Vogl said. "So instead, we placed the crane inside the concrete core and created a modified hydraulic lifting system." The concrete crews installed "pockets" as they formed the core. They jumped the crane from pocket to pocket, Vogl said.

Moody said 111 South Wacker Drive should be 85 percent leased by the time it opens next spring. He added that one factor in tenant satisfaction will be the redundancies engineered into the building's forward thinking systems, which include backup electric, increased telecommunications requirements, satellite provisions for rooftop communications and provisions for backup HVAC.

"We have a dedicated location for tenants to install backup generators if they need to," Moody added. "We have redundant isolated telecommunications shafts, so a tenant could have 100 percent redundancy in telecommunications if needed."