A bottom-line building

The success of a multi-tenant, speculative office building is dependent on the developer's ability to convince tenants that by moving to its building they can expect to experience one or more of the following benefits: a reduction in operating costs, an improvement in organizational efficiency, an enhanced quality of the work environment, or the flexibility to accommodate growth and change.

These objectives are hallmarks of a new Chicago office building that is 95% leased at a time when the city's vacancy rate for Class A space is about 10%. The 50-story tower, initially known as One North Wacker, has been renamed UBS Tower for its signature tenant.

UBS Tower is an investment-driven building, notes James Goettsch, design principal with Lohan Caprile Goettsch (LCG), the building's Chicago-based architect. He says The John Buck Co. (JBC), the building's Chicago-based developer, is able to attract tenants — particularly, more sophisticated ones — that are seeking to make their operations more efficient. "Existing buildings just can't efficiently accommodate the needs of these companies," Goettsch says. "Despite a relatively high office vacancy rate, a few large tenants need the kind of space the UBS Tower provides."

"I think tenants are looking for something different than they were in the 1980s, when they wanted to be in a snazzy building and surrounded with luxury," he adds. "Everything now is really business-driven. Tenants are moving into this building because it's reducing their real estate costs."

Higher-level communications

A major attraction is the building's technical platform for voice/data services. Utilities typically bring their services to the basement or first floor. But at UBS Tower, services are brought to a "network point of presence room" on the 18^th floor, the building's primary mechanical level. This makes connections easier for tenants, and also accommodates multiple vendors. The mid-building location saves cabling costs for high-rise tenants.

Services are routed from the "net pop" room through secure voice/data closets, some with fire-rated doors. Daniel Murphy, principal-in-charge for project mechanical/electrical engineer Environmental Systems Design, says this arrangement is a new concept in the delivery of services. It is particularly suited for a more security-conscious environment.

The UBS Tower has two telecommunications risers that extend the full height of the building. Tenants also have rooftop direct access to risers for voice/data transmission by satellite.

Built-in redundancy is another lure for tenants. Electricity is supplied by three separate feeds from different utility substations, in conjunction with an automatic throwover switch. The electrical system provides a standard 6W/sq. ft, expandable to 10W/sq. ft.

The building's cooling needs are supplied by a district cooling system that provides chilled water and makes it unnecessary for UBS Tower to have its own cooling towers or chillers. The system's multiple cooling plant locations in the downtown area provide redundancy if any single plant goes down. In addition to this existing capacity, roof mechanical wells and pathways to the roof are provided for tenants who want to install supplemental cooling equipment.

Gilding a basic plan

It is difficult and costly to make the structure of a building like UBS Tower the primary device for architectural expression, says Steve Nilles, technical principal with LCG. However, the structure did offer an opportunity to be the source of architectural definition and detail. At 675 ft., UBS Tower is not exceedingly tall. In Chicago, experience has shown that the quickest and most economical structural system for a building like UBS is a concrete core to house fixed elements. The core is surrounded by a structural steel frame with columns 30 ft. on center and a 45-ft. span from the core to the perimeter.

"We didn't start with the idea of creating a form that emphasizes exposed structure," Nilles says. The goal was to refine and delineate the basic building plan in order to create architecture without imposing structural premiums. For example, 5-ft.-long moment arms shop-welded to columns were used to create gentle curves on the east and west facades at virtually no cost premium. The building's exterior infuses interest into an essentially rectangular plan "We knew we had to be very efficient structurally to make the pro forma work," Nilles says.

Columns clad with stainless steel extend the full height of the building. Their embossed linen finish provides a light-diffusing surface that creates soft reflections. Nilles says the linen-finished panels present a good appearance under virtually all lighting conditions.

Extensive value engineering was performed on the curtain wall, working closely with wall supplier Antamex. For example, it was determined that reducing a 9-in. bull nose profile by 1 inch would save thousands of dollars. "These are the kinds of things only a fabricator would know," Nilles observes.

UBS Tower’s straightforward expression of structure is emphasized by stainless steel-clad columns that extend the full height of the building.

"One of the unique things about tall buildings is the importance of the structure," Goettsch says. The stainless steel-clad columns extend above the roofline at the corners of the building, providing a straightforward expression of the structure. The top of the building also clearly expresses the core. It extends 36 ft. above the surrounding frame, enclosing elevator machines and other mechanical equipment. UBS Tower steps back three times on its east elevation as elevator banks top out.

Glass yields to stone

The incorporation of stone at the base of the building to convey a sense of quality was discussed early in the design, but the designers did not want to use stone in the typical manner. Instead, they proposed bringing the curtain wall's high-tech aesthetic down to the lobby level, covering the building core with white Carrara marble, and glazing the lobby with glass so transparent that the core walls would be visible even from outside the building.

The 180-ft.-long, 45-ft.-wide lobby faces south. Its 40-ft.-high, cable-supported wall of ultra-clear glass allows natural light to flood the space, even on cloudy days. In contrast to the remainder of the building core, the elevator vestibules are highlighted with light from luminous ceilings. Vestibule walls are clad with white marble.

The lobby glass wall, a dramatic example of the dematerialization of the membrane that separates interior and exterior space, is the result of combining extremely clear glass with a cable net system used for the first time in the U.S. (see "Glass and Steel Unite," BD&C, April 2002). The system utilizes 3/4-in.-diameter stainless steel cable to support 5x5-ft. glass panels. The glass also has a nonreflective coating.

"It is ironic that one of the most interesting and unique aspects of the lobby, the net wall, is the least visible," Nilles says. "No matter what the lighting level, day or night, you won't see the glass." JBC principal Andrew Nieman describes it as "the building's number one talked about feature."

Offering raised access floors as a tenant option poses a quandary during construction because it is usually not known which tenants will install these floors until leases are signed. Nilles worked with other Building Team members to develop a "staggered stair" approach to this situation. When a raised floor is installed, a 6-in. concrete topping is applied to the stair tread immediately below the floor landing, adding one step between floor levels. Conversely, the raised floor eliminates one step between the landing and the floor above.

Tight fit for new caissons

A site riddled with caissons remaining from earlier buildings led to the use of the highest capacity pilings ever utilized in Chicago, according to Thomas Poulos, project manager for structural engineer Thornton-Tomasetti Group. Existing caissons were tightly spaced only 22 ft. apart. The concrete core accounts for about three-fourths of the total building load. To support the weight of the core, it was necessary to reach a bearing strata with greater capacity than that provided by the hardpan layer, and also to be able to fit new caissons between existing ones. Caissons with a bearing capacity of 500,000 pounds/sq. ft. were authorized by the city, which normally limits capacity to 400,000 pounds/sq. ft. These caissons were constructed down to rock.

Working in close proximity to the existing caissons was nevertheless a dicey experience. Timber lagging used to hold back the soil when they were constructed had rotted, allowing the resulting empty space to fill with water. "It was a logistical nightmare to be able to get a [new] caisson in without nicking an old caisson and flooding the new one," Poulos says.

The building used 15 pounds of structural steel per square foot, about two pounds per square foot less than what has been required for comparable buildings, according to Poulos.

Construction challenges for the UBS Tower were typical of those encountered on high-rise projects, according to Paul Laskowske, Chicago operations manager for general contractor AMEC. Most material deliveries were made at street level, while other operations, including the concrete plant, occurred one level below, on the lower level of Wacker Drive. Laskowske described the project as "a well-coordinated team operation."

Construction costs