Banking on steel

August 11, 2010

When Chevy Chase Bank decided to build its new $175 million headquarters on the last available site at the highest profile intersection in Bethesda, Md., the owner had two primary building requirements: an open floor plan and high floor-to-ceiling heights.

The project's structural engineer, Tadjer-Cohen-Edelson Associates Inc. (TCE), Silver Spring, Md., evaluated 25 different computer-modeling schemes for the building structure, incorporating post-tensioned concrete and structural steel to determine the optimum framing plan. Although the steel framing system chosen cost about $1 million more than a post-tensioned concrete system, the owner selected a composite steel structure as the most economical way to provide 50-ft. clear spans and 8-ft.-10-in. floor-to-ceiling heights on most of the building's 15 floors.

'Steel offered lower pricing compared with a post-tensioned concrete structural system,' says Xinzhang Li, senior project engineer for TCE. 'Standard concrete could not make the 50-ft. span at a reasonable cost.'

The project benefited from an owner who addressed construction issues at the outset of the design process, says the architect, the Washington, D.C., office of New York City-based Brennan Beer Gorman and Monk/Architects & Interiors (BBGM). 'We wanted the floors to be as column-free as possible,' says Page Lansdale, group vice president for corporate real estate for the bank.

Use of the lightweight structural steel system facilitated column transfers at different floors, which BBGM required to provide more open office spaces and parking, says Li.

The bank favors a modular office plan with few partitioned work spaces, which are more easily arranged in an open-span environment. 'We're a dynamic company with a lot of growth and change,' explains Lansdale. 'We're constantly reorganizing our modular offices.'

Modular office calls for raised floors

A raised floor system was installed throughout the headquarters' two 15-story towers to negate the need for power poles extending into work spaces from the ceiling.

To achieve the 8-ft.-10-in. floor-to-ceiling heights, the steel fabricator, Lynchburg Steel, Monroe, Va., cut 68-by-15-in. holes in the 24-in.-wide steel I-beams through which mechanical ductwork passed (see sidebar, page 44).

Steel framing and the 8-in. raised floor added to the building height as well as construction cost. 'A steel building is going to be a bit taller than a flat-plate or post-tensioned concrete building, and adding 8 inches to every floor throughout the towers made them even taller,' says Lansdale. 'This increased the amount of façade area. But we worked through all the design issues and we still found it affordable.'

Why not a single 32-story tower instead of two 15-story towers? Building restrictions in Bethesda and around the Washington, D.C., area prohibit the construction of buildings taller than 200 feet from the roofline to the top of the highest occupied floor. Penthouses and appurtenances, however, can be built higher.

In addition, Domenic Giordano, BBGM's partner in charge of the project, says that a taller single tower would have resulted in floors with considerable windowless space. 'Ideally, most clients want 40 feet from the elevator core to the face of the windows,' he says. 'Beyond 40 feet you end up with a lot of space no one wants.'

'We're at about 45 feet from the core to the perimeter wall,' says Lansdale, contentedly. 'It's a very good dimensional fit for us.'

Floors 2, 3 and 4 connect the towers of the 750,000-sq.-ft. building. The base of the building is limestone clad. Precast concrete panels clad the fifth floor through the penthouse.

David Beer, BBGM's design partner, says the building's height and mass reinforce its urban character and distinguish it from its immediate neighbors. Strong vertical elements and a crenellated top express the building's identity as a corporate banking complex.

Giordano says that the style evokes the 1930s-era Art Deco office buildings, but with a more understated, modern tone.

Because the building is located on a tight site, the steel erector used two tower cranes to erect the project's 6,900 tons of steel, says Glen McEwen, senior project manager for general contractor Clark Construction Group, Bethesda, Md.

With the project surrounded by office buildings and located on a major commuter route, contractors used the building's plaza as a staging area for the structural steel and precast concrete. Two ornate lobbies, replete with onyx stone, woodwork and bronze paneling, required substantial coordination of materials, says McEwen.

Electrical system maintains power

The bank headquarters began as a speculative office project, but it contains mechanical and electrical systems not commonly found in this type of building.

An inadequate power grid in the area is prone to outages, according to Lansdale. With technology being the 'nerve center' of the bank, crews installed an emergency power system that enables the building to remain 80 percent operational in a power outage. The system is backed up by two redundant systems.

The building's heating, ventilating and air-conditioning system employs an energy-saving cooling system that makes ice at night while energy rates are low. Resulting chilled water is circulated through the building to cool the air, limiting the need to operate chillers during costlier peak daytime hours. Lansdale claims the system will pay for itself in three years.

A free-cooling heat exchanger provides chilled water to condition the building by evaporative cooling. Two 800-ton chillers in the below-ground parking garage use a glycol system to make the ice. The glycol solution is pumped into 52 ice tanks containing water, says Vasudevan Niar, vice president of mechanical and electrical engineer Joseph R. Loring & Associates, Washington, D.C. Water is chilled and ice is formed on the tank coils. When the tanks are filled the chillers turn off.

Theater adds drama to project

The new bank also includes a 705,000-sq.-ft. below-grade parking garage that contains five levels and accommodates 2,000 vehicles.

A condition of the master plan obligated the bank to build a 400-seat performing arts theater on the grounds. Located on the eastern side of the site, the Roundhouse Theater is leased to a local theater group that will present its first performance in May. BBGM also designed the theater. Another community amenity is a 40,000-sq.-ft. public plaza with water fountain.

The bank completed its move into the building in December, following its September completion. Previously, its operations and branch services had been spread across three locations in Bethesda and Chevy Chase. 'We're very pleased,' says Lansdale.

         
 

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