Air-Formed Concrete Domes
Using 2D drafting software, I can define a complex 3D form by drawing a series of arc-shaped building sections. I use these sections to further develop the building's exterior and interior elevations and to build a scale model of wire for each design. I build two models at a time, keeping one and mailing the other to the client.
The sections also define the shape of the balloon. To communicate with balloon manufacturers, I show the location of these sections in plan and define the width, height, and radius of each section. These drawings give the manufacturers an accurate document by which to further describe complex curves and generate balloon gore patterns to use in bidding and fabrication.
These balloon drawings are also sent to the structural engineers who use them, along with photographs of the models and exterior elevation drawings, to define the shapes mathematically and to build digital models. Using finite element analysis, the engineers study the 3D models of the forms and shell openings to determine shell thicknesses and steel placement patterns.
From Hot Air to Warm Space
Once at the construction site, and tied down to the foundation ring, it takes the balloons about 30 to 45 minutes to inflate for a small to medium-sized building. Then foam is sprayed onto the interior surface of the fabric. The foam serves as the form for the steel rebar armature and has an insulating value about twice that of a comparable thickness of fiberglass.
The interior surface of the foam is then covered with a 3/4-inch (19-millimeter) layer of concrete, or "preshell." Next, steel reinforcing bars are pinned in place, like latitude and longitude lines, on the inside surface, leaving openings to frame windows and doors. Finally, the steel grid is sprayed with concrete to the required thickness.
This second layer of concrete performs three functions. Along with the steel, it provides the structure of the building. Second, the concrete provides fireproofing because the urethane foam is no longer exposed to the interior of the space. Third, the insulated concrete shell serves as a thermal mass capable of storing and reradiating thermal energy.
Sometimes, balloons can be peeled off and reconditioned for subsequent reuse. More often they are left in place and covered with an exterior coating. Among the many coating choices are elastomeric paints, synthetic flexible stucco, resins used for truck-bed liners, ceramic tile, and even metal shingles.
This construction system reduces the number of components necessary to build a structure and it uses the fewer materials more efficiently. The system also produces an extremely strong and energy conserving buildings
Due to the absence of corners where stresses accumulate, the domes are earthquake resistant, and they can sustain up to 300 mile- (480-kilometer-) per-hour winds. They require about half of the energy for heating and cooling when compared to conventional construction systems. This is due to the combined effect of reduced exterior surface area, high insulation values, and insulated thermal mass.
From Experiment to Credibility
In 1985, after a national conference of the leaders in the industry, the American Concrete Institute formally recognized this construction process and called it "Air Supported Forming of Thin Shell-Concrete Structures." They may have given the technology a clumsy name, but ACI recognition was a shot in the arm for those of us who work in this area.
The forming of compound and complex shapes by the inflation of balloons allows us to create building forms not feasible with other methods. Building forms can be lyrical and sinuous as well as efficient.
Moreover, the strength of the steel-reinforced concrete shells allow us to berm earth against them and to sculpt the way the building meets the land. By emulating the flowing curves and natural forms of the landscape, echoed in the shapes of the shell openings, designs can be made to appear in harmony with nature.
The future of concrete dome technology looks bright. It is scalable and has been applied to buildings from 30 feet (9 meters) up to almost 300 feet (90 meters) in diameter. Energy and materials efficiency; wind, earthquake, and fire resistance; and design flexibility and aesthetics will continue to provide compelling reasons to adopt this established design and construction technology.
Jonathan Zimmerman, NCARB, is an architect based in Marin County, California. His Web site contains more information about dome building.
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