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By Nancy B. Solomon, AIA

Case study: structural fiberglass

Climatic conditions—bright sun, ample rain, hurricane winds, and saltwater—along Florida’s coast challenge most building materials. So, when designing an exterior staircase for a house addition near Sarasota, Florida, architect Toshiko Mori, AIA, of New York, who is chair of the Department of Architecture at the Harvard University Graduate School of Design, gave it some serious thought.

“Wood can deteriorate due to heavy rains, extreme humidity, and prolonged exposure to salt; steel can rust; and a heavy concrete stair would add unnecessary weight,” she notes. But Mori only had to look toward the water to see a structural material well suited for a marine environment: fiberglass. Used by boat builders for years, the composite material is also popular in other recreational sports—such as skiing, surfing, and fishing—where strength, lightness, and flexibility are valued. It can double as both structure and surface and is easy to maintain. But it is not typically used in architecture to integrate several different building components—such as treads, risers, landings, guardrails, and handrails—into one homogeneous assembly.

Mori turned to boat builder Eric Goetz in Bristol, Rhode Island—who makes vessels, some of which have competed in the America’s Cup, for clients worldwide—to explore the material’s architectural possibilities. Goetz’s shop, which has fabricated a handful of sculptural and nonstructural fiberglass elements for other architects, will be fabricating Mori’s stair. “This is leading edge,” says Goetz, who is not aware of any previous structural application of fiberglass in architecture.

To build any large, curvy shape out of fiberglass, explains Goetz, his shop starts with the designer’s CAD drawings. These are converted into computer aided manufacturing (CAM) software, which guides laser-cutting and routing machines in cutting plywood pieces that are subsequently assembled into the basic mold. Fiberglass fabric is impregnated with epoxy resin and draped layer by layer over this mold, which has been covered by a film of Teflon to prevent the fiberglass from sticking. The exact number of layers is determined by the structural requirements of the component.

After half the layers have been set, the fabricator covers the assembly with a vacuum bag that removes all the air inside, thereby fusing all the layers of fiberglass together. “It acts like a giant clamp,” explains Goetz. They let it harden overnight, add a closed-cell foam filler as the core material, then layer the rest of the fiberglass on top of the filler. The entire assembly is again placed in a vacuum and then allowed to harden. The fiberglass is removed from the mold and painted. Goetz explains that his shop can build any shape. However, it is more difficult to laminate a sharp point in fiberglass. He recommends a minimum radius of about 3 to 4 millimeters.