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By Nancy B. Solomon, AIA

As originally built, the greenhouse could not meet current code requirements for seismic and wind loads. In particular, explains structural project engineer Nancy Tennebaum, "there was no way to get loads from the top of the upper dome to the foundation." Her firm worked closely with the architects to strengthen lateral resistance without disrupting the conservatory's delicate look.

The arches in the upper and lower domes (and at the corners of both wings) were reinforced with stainless steel plates cleverly concealed between two outer layers of wood. The jambs and sills that once formed the clerestory structure between the upper and lower domes were replaced with a carefully detailed series of wood-clad stainless-steel vierendeel frames supported on eight interior columns below. The upper dome transfers the lateral load into the vierendeel, which in turn transfers the load into the lower-dome arches. These forces are then transferred into vertical, braced frames made of tube steel and 3/4-inch-diameter rods within the walls of the dome room. The tops of the braced frames are stabilized by a new horizontal truss, which was fabricated from very slender stainless-steel components--3-inch-diameter tubes and 5/8-inch-diameter rods--to blend inconspicuously within the original structural elements. The forces from the braced frames are transmitted to a new, more substantial concrete foundation.

Distribution pipes for the original hot-water heating system ran above grade along the perimeter of the rooms. The piping was covered by benches for potted plants--an arrangement commonly found in Victorian-era greenhouses. The modern-day conservators, however, wanted the option of being able to plant at grade along the edges of most of the exhibit spaces. ARG therefore designed a below-grade utility trench, covered by cast aluminum grating, that paralleled the midpoint of the interior walkways. "Everything was run through those trenches-- not only our heating lines, but also our electrical, telecommunication, and irrigation systems," explains ARG Project Manager Debbie Cooper.

High moisture content inherent in greenhouse operations meant that proper airflow was critical. The wings of the conservatory had been designed with glazed ventilation panels along the perimeter base and roof ridges. Unfortunately, the wood frames of the lower panels had rotted out long ago, so the units were replaced in their entirety by fixed, cast concrete panels. In addition, the wood frames of the upper panels had warped and twisted so much over time that they too were inoperable. To avoid such problems in the future, ARG instead specified ventilation panels of moisture-resistant fiberglass to match the original wood profiles. The architects also added fiberglass-framed ventilation panels at the very top of the central dome, where none had been before. The lower sash of the clerestory is also operable, as before. Most of the structure's upper and lower vents open automatically when ongoing monitoring devices detect a need for increased airflow. Some ventilators, however, were left with the original manual operating hardware and not connected into the automatic system to retain some of the historic hardware and to provide some airflow in the event of a power failure.

ARG was able to salvage most of the decorative woodwork but the structural wood elements, for the most part, had deteriorated beyond repair. Replacement in kind with lumber milled from freshly cut old-growth redwood was out of the question because of the city's environmental policies. The team therefore had to find an acceptable alternative. They considered pressure-treated young-growth redwood, as recommended by a wood research consultant for its durability. During a testing phase of the project, however, this approach proved less than optimal because the treatment process caused some splitting and warping of the long, narrow lumber required for the arches and would have triggered objectionable delays in the construction schedule. Fortunately, the City identified another option that was acceptable from both preservation and sustainable-design perspectives: buckskin redwood logs. These logs had been cut years ago but left behind because they either were too small or fell into ravines. The surface of such logs, which have lost their bark long ago, turns a "buckskin" color- hence the name. Lumber also came from stumps and logs that had fallen due to storms. Each piece of lumber was evaluated for strength and density to ensure that the project's performance standards would be met.