Walsh construction/Brian Fritz

Walsh Construction

Tudor Van Hampton / ENR Contractor Walsh Construction hoisted derricks to place cranes on the Blue Cross-Blue Shield Tower, which is adding 24 floors.

A progression of derricks, tower cranes and 230 tons of steel are setting up shop atop a Chicago office building built more than a decade ago. This may be a glimpse of the future as cities grow denser.

A $270-million “vertical extension” of the 33-story Blue Cross-Blue Shield Tower is adding 24 floors because the owner needs more space. It is also offering an eye-catching view of unusual crane work. David Scott, chairman of the Council on Tall Buildings and Urban Habitat, says vertical extensions are “quite rare.”

The tower’s architect, Chicago-based Goettsch Partners, originally designed the tower so its owner, Health Care Service Corp., could expand if needed. That time is now because “we’ve outgrown the facility,” says Andy Pini, vice president for HCSC, the operator of Blue Cross insurance in four states. Goettsch is the extension architect, with Seattle-based Magnusson Klemencic Associates as engineer of record.

Last October, workers lugged steel beams and a 17-ton derrick manufactured by Woodstock, Ontario, Canada-based Timberland Equipment Ltd. 500 ft to the roof via a service elevator, says Lou Rossetti, senior project manager of Chicago-based Walsh Construction Co., the general contractor. The chore took about 40 loads.

On top of the building, a grillage went up and expanded eastward toward Lake Michigan. Walsh extended the steel to hold a 35-ton derrick, also by Timberland, on a platform. The smaller derrick built the larger one. The platform, composed of temporary and permanent steel, was placed on existing building columns that previously had been stubbed up.

The succession of stiff-leg derricks was necessary to set two tower cranes on the roof, Rossetti says. The 17-ton derrick was disassembled, and the 35-ton derrick constructed more grillage to hold a Potain 605 tower crane, which it helped build on the roof’s west half. Walsh repeated the process to erect a second crane. The west tower crane extended the grillage with a second platform of temporary and permanent steel so that the derrick could be repositioned on the east side of the roof. From the new position, the derrick was used to build the east tower crane, also a Potain 605. Taking the tower cranes down at the end of the project, scheduled for completion in 2010, will be a reversal of this process—except at a higher starting point.

Accommodating the tight site and ensuring the stability of the cranes against Lake Michigan winds were key design issues, says John Matuska, senior project manager of Farmington Hills, Mich.-based Ruby+Associates, the lift engineer. The steel layout featured splices due to space constraints and 3-ft-deep plate girders to provide sturdiness.

When both tower cranes were up, the vertical extension commenced. By mid-March, the cranes were functioning, the first new floors were being added and the Blue Cross-Blue Shield Tower was growing. “This is the hardest thing I’ve ever worked on,” Rossetti says.

Vertical extensions could become more common as American cities change, Scott notes. The projects usually occur as rezoning allows for taller development in dense areas that can grow only by going up, such as in Hong Kong. There, Scott’s employer, London based engineering firm Arup, recently oversaw a 30-story extension of a 15-level building.