Back in the 15th and 16th centuries, mariners traversed the Mediterranean Sea and also sailed to the New World on small but functional vessels called caravels, ships with broad bows, high narrow stern castles and three masts.

In keeping with the ocean-going themes of Magellan Development, the Caravel is also the name of the company's new 29-story condominium building at the northeast corner of Dearborn and Ontario streets in Chicago's booming River North neighborhood.

And while this Caravel primarily enjoyed smooth sailing on its journey to completion, there were moments when the construction team had to do some navigating.
For example, the tight site was close to two other large construction projects being completed simultaneously.

"The major design challenge is it's on a very small, 14,000-sq.-ft. lot," said Jim Loewenberg, president of Loewenberg + Associates, the Chicago-based architectural and planning firm specializing in multifamily housing. "This is a tremendously congested area, and there were four major structures being built within a block of the site.

"And of course, being on the feeder lane into the expressway, special planning had to be given to traffic. We couldn't work during certain periods of the day, couldn't take deliveries because these are major arterial streets."

Caravel's Log

Work on the Caravel began in August 2001 and was slated for completion in June. With a budget of $34 million, the tower will feature 125 condominium units above six floors of parking and approximately 5,000 sq. ft. feet of ground-level retail on the Ontario Street side of the building.

The structure is built of cast-in-place concrete, with windows and sliding glass doors serving as the cladding. The Caravel's top two, or 28th and 29th floors, are penthouse duplex units.

Spandrel glass panels line the north and south sides of the 29th floor for aesthetic purposes, and "nice spires of steel," crown the building's roof for additional appeal, said Ken Chura, project manager with Chicago-based Walsh Construction Co.

Navigating a Tight Site

Chura said the compact site necessitated a great deal of coordination between contractors working on the Caravel and two other nearby projects: the Grand Plaza apartment towers and the Millennium Centre condominium.

The contractors got together to coordinate material deliveries so streets wouldn't be blocked, a situation closely monitored by the city, Chura said. In addition, Walsh had to work with contractors on the Millennium to coordinate their tower cranes because booms on the cranes at both projects swung over the same area of sky above crowded Ontario Street.

Chura added that because of the constrained site, Walsh had just 20 ft. on the south and 25 ft. on the west to stage the entire job. That meant only one delivery could be made at a time. "We had to have a coordination schedule for deliveries in half-hour increments throughout the day," he said.

"That became extremely critical because we were on a four-day concrete cycle. Every fourth day we would pour a floor, and the concrete trucks would take up the staging area. So every fourth day, there could be no deliveries."

Concrete was poured using the tower crane, as well as concrete pumps that pumped the concrete from the street level to the floors above.

Though the delivery of exterior material was somewhat delayed by space constraints, the team adhered to Walsh's nearly two-year construction timetable, Loewenberg said.

Getting Chilled Water

While most buildings feature chillers designed to cool the water for the air-conditioning system, the Caravel is to receive chilled water from Unicom, a utility, eliminating the need for a chiller. The result is reduced construction and maintenance costs, Chura said.

If a cooling tower someday must be added, space is included in the design, said Martin Finerty, president of Structural Associates, a structural engineering firm based on Chicago's Northwest Side.

"We did make provisions for a cooling tower," Finerty said. "That's kind of a city requirement, in case Unicom can't provide that service anymore."

Another distinctive feature of the building is that it features a solid concrete core with 12-in.-thick concrete walls enclosing the elevators and stairwells, Finerty said. That's a departure from some buildings, where stairwells are enclosed with drywall partitions or with a mix of concrete walls and drywall partitions.

Though more costly to construct, a concrete core offers three chief benefits. "It provided the vertical support for the floor slabs, the lateral resistance against the wind, and an additional measure of fire safety for the residents," Finerty added.

Transferring Tower Weight

Like most buildings featuring parking below residential units, the Caravel includes a transfer girder level allowing the weight above the garage to be transferred to different columns in the parking area. Columns that carry the weight of the higher floors and the exterior walls are spaced differently than the columns in the garage. While columns are uniformly spaced in the garage, the aesthetics of the living units require non-uniform spacing, Finerty said.

The construction team used a standard flat-plate concrete design, with no post-tensioning required.

The strength of the concrete used in the building was chosen to help increase aesthetics and functionality in the condominiums. A 10,000-psi concrete was employed for the columns, enabling column size to be reduced and living space therefore maximized.

But Chura acknowledged the 10,000 psi concrete is pretty standard. "It's on every building now," he said. "It's evolved over the last 10 years."

Finerty added that contractors are now using 12,000 and higher psi.

Calling the Caravel, "a fairly high-end project," Chura said condominium buyers will enjoy a luxurious lobby with mahogany walls and marble floors. Wood floors, granite countertops and marble bathrooms are standard in each unit.

Caravel units boast larger balconies than featured in most residential high-rises. Chura said that's one of the developer's requirements.

Finerty said that architecturally, he thinks it's a great-looking building.
"That's because of the curved facade at the lower level, the double cantilevered balconies, the multiple setbacks and the floor-to-ceiling windows," he added.