Page E2.2 . 06 June 2007                     
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    HOK Straw Bale

    continued

    The new facility was to include a 31,000-square-foot (2,900-square-meter) administration building, a 25,000-square-foot (2,300-square-meter) maintenance building, a bus wash facility, a fueling island for buses, and a publicly accessible fueling station. By making a compressed natural gas fueling station accessible to the public, the city could reduce hydrocarbon and ozone emissions from other vehicles in the community as well as from its own buses.

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    Before beginning design, HOK held a charette with the owner, engineering consultants, and landscape architect to develop consensus on some ambitious sustainability goals.

    Straw Bale Aesthetic

    Straw-bale buildings are characterized by thick walls, a larger building base, and elevated wall foundations. HOK embraced these characteristics to define an aesthetic for the Santa Clarita facility: pronounced plinth, deeply recessed glazing, and exposed wood structural frame. The basic elements function as finished structures, further reducing unnecessary material use.

    Rather than exploit the structural qualities of straw bale, as is often done in residential construction, the architects designed a long-span, heavy-timber frame of high-recycled-content engineered wood.

    Leaving the frame exposed above the straw bale reduces the need for toxic finishes, and a continuous line of horizontal glazing made possible by this structural system promotes interior daylighting. Lime plaster covers the straw-bale walls inside and out, and metal shingles made from high-recycled-content copper cover the exterior wall surfaces that are not straw. Exterior materials are light-colored to maximize reflectivity, recalling traditional adobe construction of the southwest.

    Venturing into a "new" technology carried risks for the architects and builders, who came away with a few "lessons learned." For instance, they initially attached the bales to the footing using wires instead of strapping, creating a certain instability during construction. When the temporary wood bracing was removed, untrained contractors climbed on bales to reach their work, causing misalignment and requiring rebuilding.

    After a rainstorm during construction, a mistake by the roof contractor allowed the bales to get wet, and 60 percent of them had to be replaced. In hindsight, the team understands that greater job-site quality control and better communication is needed, especially when workers are engaging in unfamiliar practices.

    Energy Advantages

    "Straw-bale construction may be a rediscovered technology," says HOK's senior project manager Charles Smith, "but it is appropriate and sustainable by today's standards. When combined with more recent technologies such as under-floor air distribution, high-performance glazing, and daylighting as it is in this project it can be part of a powerful strategy for creating an energy- and resource-efficient building. We were able to exceed California Energy Efficiency Standards by over 40 percent."

    The HVAC system is an efficient water-source heat pump. Chilled water is generated by an on-site cooling tower. Under-floor air delivery eliminates the need for overhead ducts, leaving the ceiling unobstructed for better daylight reflection. The raised-floor system uses concrete-filled metal pans, which are left exposed to eliminate the need for carpet or other floor coverings in most spaces.

    The desert climate, with large diurnal temperature swings, is ideal for cooling by nighttime ventilation. Cool night air is brought into the administration building where it chills the interior surfaces' thermal mass, preconditioning the space for the following day. The HVAC system is designed to condition space only 7 feet (2.1 meters) above the floor. Each occupant can control the airflow and temperature of his or her work area.

    The building also incorporates high-performance glazing and a well-insulated roof. Combined with the thick straw-bale walls, these create a super-insulated envelope that moderates temperature fluctuations and protects the indoor environment from the hot, dry daytime conditions.

    The relatively narrow floor plates no more than 60 feet (18 meters) deep mean that all spaces have access to daylight and views. Deep roof overhangs shade the glazing while protecting the perimeter of the straw bale walls from direct water infiltration. The daylighting strategy reduces reliance on electric lighting and the associated heat loads. Skylights over the interior corridors and lobby limit the amount of electric light required in those areas.

    A solar canopy bus shade structure, consisting of a 129.6 KW-DC photovoltaic system, provides on-site renewable energy that meets 45 percent of the facility's annual energy demand.

    Material Choices

    Because straw bale contains no volatile organic compounds (VOCs) or other toxic compounds, it contributes to excellent indoor air quality. Other materials were also selected based on renewable and/or recycled content, local availability, low toxicity, durability, and ease of maintenance.

    There is very little carpeting, and suspended ceilings are used in only a few rooms. All casework and millwork is constructed of wheat straw board instead of the conventional medium-density fiberboard (MDF) or plywood. Wheat straw board is rapidly renewable and contains no VOCs.

    Building systems are electronically monitored 24 hours a day for comfort and efficiency by a digital control system. The straw-bale walls are continuously monitored for moisture content. The City of Santa Clarita expresses a commitment to ensuring occupant satisfaction and reports that all systems are functional and providing the most cost-effective return.

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    Project Credits

    Architect: HOK (Culver City, California)
    Structural Engineers: NYA (Los Angeles)
    MEP: Syska Hennessy Group (Los Angeles)
    Civil Engineering: Development Resource Consultants (Anaheim, California)
    Geotechnical: Neblett Associates, Inc. (Huntington Beach, California)
    Alternate Fuels: Marathon Technical Services (Heidelberg, Ontario, Canada)
    Maintenance Equipment: Maintenance Facility Consultants (Houston, Texas)
    Code/Life Safety: Schirmer Engineering Corporation (Torrance, California)
    Cost Estimating: Davis Langdon Adamson (Santa Monica, California)
    Acoustics: VSA Associates (Whittier, California)
    Landscaping: Katherine Spitz Associates (Marina del Rey, California)
    Telecom/ITS: National Engineering Technology Group (La Mirada, California)
    Graphics: Selbert Perkins Design (Playa del Rey, California)
    Specifications: Robert K. Cloud (Santa Monica, California)
    Hardware: Security Hardware, Inc. (Anaheim, California)

    AW

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    SUBSCRIPTION SAMPLE

    Santa Clarita (California) Transit Maintenance Facility designed by HOK, with walls of straw bale.
    Photo: John Edward Linden

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    Straw-bale walls make deep window openings.
    Photo: John Edward Linden

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    Straw-bale wall installed on a high foundation.
    Photo: John Edward Linden

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    Section through straw-bale wall.
    Image: HOK

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    Santa Clarita Transit Maintenance Facility site plan.
    Image: HOK

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    East and west elevations.
    Image: HOK

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    Central courtyard.
    Photo: John Edward Linden

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    Copper covers the walls that are not straw.
    Photo: John Edward Linden

     

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