The downtown Chicago condominium market is among the hottest in the nation, with abundant choices for buyers.

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Gail Lissner, vice president of Chicago-based Appraisal Research Counselors, said about 4,700 units are expected to be delivered in the Loop in 2006, up more than 50 percent from 2005's 3,100 units.

The structural design of the $250 million 340 on the Park condominium in Chicago's East Loop might help give the building sales advantages.

The building with curtain-wall cladding will have striking views. Overlooking Randolph Street, 340 on the Park is immediately north of Daley Bicentennial Plaza, northeast of the nationally famous Millennium Park and west of Lake Shore Drive. In addition to those, the 69-story high-rise will have vistas of Grant Park, Lake Michigan and the landmarked Michigan Avenue streetwall.

The building had reached the 39th level in early May. Top-out is expected in October, and the project should be finished the following October.

The concrete structure features post-tensioning, said Ola Johansson, project manager for Seattle-based Magnusson Klemencic Associates, the structural engineer.

A key benefit of post-tensioning is improved floor-to-ceiling height due to thinner but equally strong slabs over a conventional structure with "mild-steel" slabs - not a small consideration for a developer in the Loop's superhot condominium market. The floor-to-ceiling height in 340 is 9 ft. but would have been between 8 ft., 8 in. and 8 ft., 10 in. with a conventional system.

"It's something a potential buyer would compare with another building," Johansson added.

Moreover, the column-to-column span in 340 on the Park is 27 ft., Johansson said. If the building had been constructed with an 8-in.-thick mild steel slab, the column-to-column span would have ranged between 22 and 25 ft.

The wide span is mimicked in the building's first six levels where the parking is located, though those levels have mild steel slabs. The 10-in.-thick floor slab is strong enough to accommodate the wide span, whereas an 8-in.-thick floor slab in the tower has sufficient strength because of the post-tensioning.

As a result, the building's deeded 471 parking spaces are greater than would have otherwise been the case if post-tensioning had not been used above.

"So the additional parking stalls we were able to eke out with the post-tensioning system offset any construction premium to install the post-tensioning," said Ron
Klemencic, president of Magnusson Klemencic.

Klemencic, who is the immediate past chairman and still a board member of the Council on Tall Buildings and Urban Habitat, an international association of architects and engineers headquartered in Chicago, added that the building is possibly the world's tallest post-tensioned concrete building and definitely Chicago's tallest such structure.

"I believe this is going to be the tallest post-tensioned concrete building in North America," he said. "If there is a taller one, I don't know what it is."

Tensioning a Building

The post-tensioning process involves hydraulically tensioning each concrete floor slab.

Steel tendons that are anchored to posts and sheathed in green plastic are laid in the metal deck, and the concrete is poured, Brandt said. After the concrete has hardened, the cables are stressed hydraulically.

More than one cable band is involved per floor, and the number of tendons per floor ranges between 100 and 1,000.

A key issue is coordinating the layout of the cables and the lines for the electrical, plumbing and other utility systems. Sometimes, a conflict can arise between the two.

"The post-tensioning cabling has to go where it has to go to allow the building to act as it needs to for structural reasons," Brandt said. "At the same time, plumbing piping has to go where it has to go to service a bathroom or kitchen."

When a conflict arises, every party involved is assembled to find a solution, and conflicts between post-tensioning and utility lines arise "quite a bit," Brandt said. A cable could be swept in a new direction to accommodate a utility line or a utility line redirected.

Typically, the post-tensioning cable gets priority over utility lines "to allow the building to stand up at the end of the day," Brandt said. In some cases, however, the post-tensioning cables are moved and extra reinforcing steel is added in that area for stiffness. Cost-effectiveness and owner's requirements are considered in making the decision.

Buildings have been post-tensioned in Midwest before, but Klemencic said the process is more common on the West Coast and Southeast partly because of building tradition.

As a result, crews on the 340 project were a little apprehensive about the process because they wanted to maintain a three-day pour cycle per floor.

"In the beginning, they were talking about maybe four or even five days per floor, but they've been executing it three days per floor," Klemencic said. That means the project is ahead of schedule.

An Unused Site

The project is on a previously unused lot between the Blue Cross Blue Shield office on the west and The Buckingham condominium on the east.

Only a half-story is below grade in landfilled area, and the first six levels will hold the parking and face Lower Lower and Lower Randolph Street. The first condominium level is two floors above Upper Randolph Street - or the eighth level overall.

The 975,000-sq.-ft. building will hold 344 condominium units that range between about 1,000 and 5,000 sq. ft. in one-, two- and three-bedroom layouts, said Don Biernacki, senior vice president of Chicago-based LR Development Co., the developer. Customizable penthouse units start on level 57.

Neither the percentage of units sold nor unit costs was released.

About 4,000 sq. ft. of space on the seventh floor, which is flush with Upper Randolph Street, will hold retail. The building will feature an amenity floor on the 25th level, including a pool and fitness facility.

A "signature" building was sought, so a quality aesthetic design and finishes were needed, Biernacki said.

Rather than just only curtain wall, the exterior features variation with protruding and inset balconies and horizontal and vertical lines. Inside, bamboo - a sustainable material - is used for the typical unit flooring, and there will be high-end cabinets, faucets and other items.

SIDEBAR

Locating Lower, Lower Randolph

The maze of streets beneath Chicago's normal level can be confusing for those drives not used to navigating the city's lower depths.

An issue on the $250 million 340 on the Park condominium project was the access point on Lower Lower Randolph Street.

Materials are coming from points all over the country, and "the lower roads are difficult to explain to someone on how to get there," said Bert Brandt, senior project in Chicago with Bovis Lend Lease Inc., the general contractor.

MapQuest, the popular Internet site often used for navigation, would prove useless if trying to map a route to the lower depths of Randolph because it does not include the below normal grade streets.

"More than a couple times" a delivery was expected but went missing, and someone was dispatched to find the driver on the streets of downtown Chicago, Brandt said.

Other times, deliveries meant for other projects, such as those in the nearby Lakeshore East development, came into 340's lot, he added.

As a result, drivers are instructed to call the field office before making a delivery, and specific directions are given.

But the site below the city's normal grade proved an advantage when it came to constructing formwork.

The large amount of space on grade has allowed large flying forms, rather than stick-built forms, to be used on the project. A spillover benefit is that flying forms allow the teams to maintain their three-day pour cycle.

"In Chicago, you don't see flying forms often because you're in boxed-in sites," Brandt added.

Other issues arose on the project: