Tower Design Banks on Potential Expansion

ABN AMRO building features a composite structural system
May 2003

by Craig Barner

Like the funds inside, the ABN AMRO bank under construction in Chicago's West Loop could multiply.

Construction of the approximately $185 million tower began in August 2001, and a second tower might be constructed next to it, said Boris Hornjak, project executive in the Chicago office of New York-based Turner Construction Co., the general contractor. "It depends on the owner and how they wish to proceed," he added.

The rising tower is on the eastern half of the block bounded by Madison, Clinton, Jefferson and Washington streets, and the potential No. 2 structure would go up on the western half. The design of the would-be, two-tower plaza affected the phase one building, said Joe Burns, principal in the Chicago office of New York-based Thornton-Tomasetti Engineers, the structural engineer.

Space is limited on the block, and strategies were implemented to fit everything, he said. The caissons and columns on the first tower's western edge, for instance, were designed to support the potential phase two tower's eastern edge.

"The gravity of the tower that would come in the future would land atop the columns and caissons we have already installed," Burns added.

Besides space, Burns said planning took into account other elements, such as accommodating Chicago's ever-present winds. The buildings' designs were tested in a laboratory to ensure the two together wouldn't cause an unacceptable acceleration of the wind speed, in addition to other tests to make certain the structures' lateral resistance and cladding could withstand the loads.

Going Dutch

The phase one tower will be the North American headquarters of Amsterdam-based ABN AMRO Bank N.V. and should be finished in September.

The structure will help consolidate the financial giant's Windy City holdings, which include LaSalle National Bank, Cragin Federal Bank for Savings, Talman Home Federal Savings and Loan Association and other businesses. The firm's existing operations are scattered across numerous Loop locations.

The 1.4 million-sq.-ft. structure will have 30 floors, Hornjak said. The bottom-six floors, made up of 60,000 sq. ft. of space, will have twice the area of the upper floors, he said. The bank's considerable data processing and check-processing operations will be located on three of the lower floors.

Besides the shell and core, Turner is also overseeing the interior construction of floors three, four and five. Schaumburg, Ill.-based Power Construction Co. is handling the build-out work on the other floors.

A cornice partly clad in stainless steel crowns the building and provides it with a decorative draw, said David Kelly, senior project engineer for Turner.

Viewed from the southeast, the building brings to mind a ship. The wide bottom six floors resemble a hull, and the narrow upper 24 floors that come to a point at the corner of Madison and Clinton streets look like a sail.

Overcoming Sitework Deficits

Workers encountered sitework obstacles.

For example, six dewatering wells were needed to drain water from the large site before excavation could start, Kelly said.

The water table was lowered more than 40 ft. so caisson shafts would not collapse during excavation, he added. The sitework took time because 85 caissons were placed about 90 ft. deep.

"They pretty much drilled continuously," Kelly said. About 60,000 cu. yds. of dirt was removed from the site.

Another issue involved integrating the foundation walls' temporary bracing with the walls of the post-tensioned concrete basement, Hornjak said. "We had to design the post-tensioning around the internal bracing and then take [the struts] out after the basement was constructed," he added.

Structural Payback

The building's composite structural system of concrete and steel is paying dividends.

The tower's 150-ft.-long, 30-ft.-wide core, which holds the elevators, stairs and some rooms, is composed of five concrete cells, Thornton-Tomasetti's Burns said. The core tapers as it rises, leaving only two cells at the top. The perimeter is held up with structural steel.

The core provides the building with its primary lateral resistance against the wind. "That frees up the rest of the steel framing to be light, open and simple," he added.

The strength of the core's structural support is efficient because it lessens the number of steel columns to hold up the building, Hornjak said. Only a few columns are located north and south of the core, and there are zero columns east and west of the core.

A bank requirement was to have wide spans for broad views, he said. "We have a 45-ft. perimeter-to-core span," he added.

About 6,000 tons of steel make up the structure, and the erection was intricate, Kelly said. Every level, for instance, has a 1-ft. raised floor to provide space for air delivery.

"We had two different top-of-steel elevations [on every floor]," he added. "We had a top of steel in the core and a top of steel in the bays."

The building features a floor-to-floor height of 13 ft., 6 in.

Y-shaped columns made up mostly of steel tubes will support the cornice - sometimes referred to as the trellis - at 30 ft. intervals, Kelly said. Because of their large size, the Ys were shipped in pieces, welded together on the ground and lifted into the building.

"The trellis hangs out over the building," he added.

Dressing for Success

A unitized curtain-wall system dresses the building exterior.

The green and blue panels were fabricated off-site, delivered on pallets and lifted into the building with a crane, Hornjak said. Typically, the panels were installed floor by floor going up and in a counterclockwise fashion.

The exception to the pattern is the hoist bay, which will be clad last.

Erecting and cladding the point on the corner of Madison and Clinton was dicey, Kelly said. The point is made up of stacked horizontal triangles the full height of the building, and the horizontal beams meet at a vertical member rising every three floors.

"Ironworkers had to shimmy up that column in winter at the edge of the building to receive the beam," Hornjak said.

A triangle-shaped platform was designed as a staging system to clad the point, and rigging was secured and dropped from the roof inside the point to support the platform, Kelly said. The point will have no floor.

Tolerances were strict throughout the building because play of only 2 1/2 in. between the edge of the flooring deck and the inside of the curtain wall was available. "The curtain wall is not very forgiving of errors," Hornjak said.

No rework has been done because of dimensional errors.

Making MEP Sense

The building's under-floor air-plenum system eliminates ductwork and allows tenants the flexibility to adapt air conditioning to their needs.

Chillers on the seventh and 29th floors cool the air, and fan columns in the core deliver air to the underside of each floor where there are tap-offs, said Kevin Luoma, associate with Chicago-based Environmental Systems Design Inc., the MEP engineer. Diffusers permit the air into the occupied spaces.

The return via natural draft is accomplished through the ceiling plenum, Hornjak said. "There's no fan sucking up the air: It's just recirculating," he added.

The advantage is that if tenant moves or a floor layout is redesigned, only minimal adjustments are needed.

"You don't have to worry about tearing down ductwork and laying out a whole new system," Luoma said. "You just have to take the diffusers in the floor and the tile, move it and, boom, you're done."

Because the pressure drops slightly on the building perimeter, dedicated fan-coil units ensure that space remains comfortable, Hornjak said.

The Dearborn Center building is believed to be the first structure in Chicago with an under-floor air plenum, and ABN AMRO is likely the No. 2 building.

Redundant electrical generators and computer chillers help guard against shutdowns, he said.
"The mechanical contracts are probably three times the value compared with a regular core-and-shell building," Hornjak said. "This is a bank with high-tech stuff in it."