By Joann Gonchar, AIA
This past spring, the owners of the 55-story Bank of America Tower, which sits catty-corner from New York City’s Bryant Park, celebrated the building’s opening with a reception in the lobby—almost two years after the first occupants moved in. If the “opening” party seemed a bit anticlimactic, the event did mark an important milestone. It coincided with an announcement that the $1 billion, 2.2 million-square-foot tower had achieved Platinum certification under the U.S. Green Building Council’s LEED Core & Shell rating system—making it the first U.S. skyscraper to achieve this designation.
Designed by Cook+Fox Architects and jointly owned by the developer, the Durst Organization, and the bank (which is also the lead tenant), the 1,200-foot-tall, glass-clad, steel-framed building rises from a 7-story podium that conforms to Manhattan’s street grid. It then tapers and seemingly twists to achieve a sleek, crystalline form. A host of integrated strategies helped the tower earn Platinum, including rainwater and graywater recycling, an advanced air filtration system, a concrete mix that replaces about 45 percent of the Portland cement in the foundations and core with blast furnace slag, and a cogeneration plant that produces both electricity and steam for on-site use.
The building has many green bells and whistles, to be sure. But these features were chosen on the basis of operational and economic criteria, as well as sustainability goals, say project team members. “The clients weren’t interested in demonstration technologies that wouldn’t work,” says Serge Appel, AIA, Cook+Fox project architect.
The developer was willing to consider unusual strategies, but not without thorough evaluation. Before settling on cogeneration for example, consultants vetted several other on-site energy generation technologies. They monitored wind velocities with an anemometer mounted on the roof of 4 Times Square (an adjacent Durst tower completed in 1999 and widely considered the first green commercial highrise in the U.S.). But the results showed that conditions were too gusty for wind turbines. They explored incorporating photovoltaics into the skin and the podium roof, and determined that both would be in shadow too much of the time. They investigated geothermal energy, but decided the site was too tight for the number of required wells. They even discussed generating methane from tenants’ paper waste in an anaerobic digester. However, the bank was worried about the security of its discarded documents.
They eventually implemented a 4.6 MW natural gas-fired cogeneration plant which went on-line this summer. It is expected to satisfy about 65 percent of the building’s annual electricity demand. The strategy, also known as combined heat and power (CHP), derives its efficiencies from making use of the heat that is a byproduct of the generation process. At the Bank of America, the heat is used to make steam, which in turn heats the building and its domestic water supply. It also is used to operate an absorption chiller for cooling.
As with most office buildings, the tower’s demand for electricity is lower during off hours. However, “the economics of the CHP would only make sense if it could run pedal to the metal 24-7,” says Scott Frank, PE, a partner at Jaros Baum & Bolles, the project’s mechanical engineer. So, in order to even out the load profile, designers included a 44-tank thermal energy storage system. It makes ice at night with excess electricity. During the day, the melting ice supplements building cooling. The team estimates that the CHP plant, working in concert with the energy storage system, will reduce daytime peak electricity demand by 30 percent.
The CHP plant, which designers say is the first large-scale installation of its type in a New York City office tower, was the building’s most logistically challenging feature to realize. The team needed to route natural gas lines through the densely occupied structure and isolate the equipment for noise and vibration. There was also a maze of permitting hurdles, including approvals from the fire department and the local utility.
Although the CHP plant was the most effort-intensive building system, other features also involved careful coordination. For instance, the project’s construction manager, Tishman, oversaw subcontractors installing base-building components of the underfloor air system, such as core wall cladding, corridor curbs, and perimeter fin-tube enclosures. Meanwhile, the tenants’ individual fitout contractors were responsible for installation of elements within the office spaces, including the raised floor panels. In order for the system to function properly, all needed to follow strict installation guidelines and maintain the air-tightness of the floor plenum.
For Gensler, the architect that designed the bank’s LEED Gold office space and trading floors, a key challenge was the limited availability of green materials when the firm started its work seven years ago. For example, principal Ej Lee wanted all of the wood in the millwork to be certified by the Forest Stewardship Council (FSC). But Lee and her team could not find suitable veneers and decided that only the substrate would be FSC-certified.
Another difficulty was devising a layout compatible with the client’s corporate culture that would also allow access to daylight and views for a majority of occupants. Gensler pushed for private offices positioned next to the building core and surrounded by open workstations. But the bank maintained that it would need perimeter offices to attract and retain executives. The realized scheme does have perimeter offices, but with all-glass fronts facing the rest of the interior floor area in order to limit obstruction of views and allow daylight penetration.
The tower’s exterior curtain wall is made up of floor-to-ceiling, double-lite insulated units of low-iron glass. To help control heat gain and glare, the units include a low-e coating as well as a ceramic frit that covers 60 percent of the glass where the curtain wall meets the floor and ceiling. The pattern gradually decreases in density toward the vision portion of each panel. Non-metallic spacers in the aluminum mullion system and extra mineral wool insulation at the floor slabs help achieve a U value for the assembly of 0.38—a thermal resistance that is better than most glass towers built in New York City over the last decade, but still below prescriptive code requirements.
Although a more solid facade would have likely provided greater thermal resistance, the team maintains that the all-glass skin was crucial to the building’s architectural expression and its economic model: The transparency “allowed us to get market-rate rents and invest in other [high-performance] systems,” explains Don Winston, PE, director of technical services at Durst.
Even with its crystalline curtain wall, the building’s energy model shows a 20.97 percent cost savings over a building designed to meet the 2004 version of ASHRAE 90.1, Appendix G, according to the project team. If only core and shell energy are considered (i.e., if the tenant spaces are excluded), the model indicates performance that is 54 percent better than the standard.
Winston has been monitoring the tower’s performance, and preliminary results indicate that it is operating more efficiently than the energy model. But without more data, he says, “I’m not confident to say just how much better.” Winston is committed to releasing the actual performance information, but not until the CHP plant has been up and running for at least a year. We hope he will share that data with us for publication in GreenSource.
Owner A joint venture between the Durst Organization and Bank of America
Project developer The Durst Organization
Design architect Cook+Fox Architects
Executive architect Adamson Associates Architects
Interior architect Gensler
Engineers Jaros, Baum & Bolles (mechanical); Severud Associates (structural); Mueser Rutledge Consulting (geotechnical)
Construction manager Tishman Construction Corporation
Code consultant JAM Consultants
Cogeneration design Waldron Engineering & Construction
Commissioning agent The Fulcrum Group
Exterior wall consultant Israel Berger & Associates
Energy/environmental consultant Viridian Energy & Environmental
LEED consultant e4 inc.
Lighting consultant Cline Bettridge Bernstein Lighting Design
Roofing and waterproofing Darius Toraby Architects
Solar design/photovoltaic consultant Solar Design Associates
Wind consultant altPower
Base building acoustician Shen Milsom & Wilke
Exterior maintenance consultant Entek Engineering
Structural system Owen Steel Corp.
Thermal storage Calmac Manufacturing Corp. Icebank
Cogeneration plant Solar Turbines Mercury 50
Underfloor air distribution Tate Access Floors
LED lighting IO Lighting