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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Sidebar 2 Fresh Air and Clean Energy

By D.Y., S.B., and S.C.

As the HGA Design team rethought every aspect of the Minnesota State Capitol, it kept in mind the need to make the 100-plus-year-old structure as energy efficient as possible while still maintaining comfort levels for employees and visitors.

One way the team achieved this was by replacing the building's ventilation systems with 15 air-handling units (AHUs). The AHUs are sized to meet the minimum outside air requirements of ANSI/ASHRAE Standard 62.1-2010 and are provided with economizer controls. Energy efficiency was further enhanced by using custom air handlers with fan-wall technology.

In addition to the economizer control, the units are optimized by demand-controlled ventilation. Duct-mounted carbon dioxide (CO 2 ) sensors in the main return ductwork for each AHU drive open the units' exterior air dampers to introduce more fresh air. Minimal ventilation is used when the building is not highly occupied. Such measures are particularly important in a building with the unique occupant load that a state capitol experiences; periods of high occupancy are separated by long stretches of very low occupancy.

Just as important, the team relocated all the building's fresh-air intakes to the roof. Before the remodel, most of the building's outside air was brought in at ground level, and the original intakes were near parking areas. Not only did this result in poor indoor air quality, but it jeopardized security. The engineering teams, the architects, and the Minnesota Historical Society (MHS) collaborated to find appropriate locations for rooftop intakes and the ductwork to carry the fresh air throughout the building. The new intakes are copper-roofed "doghouses" with louvered walls, and they drastically improve indoor air quality and thermal comfort while reducing the security threat.

The building is also now connected to the extremely efficient District Energy St. Paul for both chilled water and high-temperature hotwater heating. Very little square footage is needed for this system, so little had to be taken away from other building functions. But chilled water from District Energy St. Paul is not available on a year-round basis. So for those spaces requiring constant cooling-such as the telecommunications, electrical, and data rooms-energy efficiency was gained by using a variable refrigerant volume (VRV) system.

Additionally, the building's lighting controls were completely overhauled to meet the demands of a state-of-the-art office building. The capitol was the first public building in Minnesota to be electrified. Electric lighting was a relatively new technology when Cass Gilbert designed the building, and the lighting levels at the time were considerably lower than what is demanded in buildings today.

The challenge to the restoration's lighting designers was to provide flexibility to satisfy the competing demands of the MHS, which wanted the lighting to be close to the historic levels for some tours, and legislators, who needed lighting levels in their chambers to be adjustable. A lighting control system was installed and programmed to provide that flexibility. At the push of a button, one can select from four preset scenarios: historical tour, working level, broadcast (for television shows broadcasting from the building), and janitorial.

Additionally, long-lasting light-emitting diodes (LEDs) were used in 99 percent of the light fixtures. The mechanical systems work in concert with the retrofitted lighting and improved electrical infrastructure to meet the energy-efficiency goals of the project.

-DY, SB, and SC

Civil Engineering, December 2019, © American Society Of Civil Engineers. All Rights Reserved

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