The Geodesic dome reference article from the English Wikipedia on 24-Jul-2004
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Geodesic dome

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The American Pavilion of Expo '67, by R. Buckminster Fuller, now the Biosphère, on Île Sainte-Hélène, MontrealEnlarge

The American Pavilion of Expo '67, by R. Buckminster Fuller, now the Biosphère, on Île Sainte-Hélène, Montreal

A geodesic dome is an almost spherical structure based on a network of struts arranged on great circles (geodesics) lying on the surface of a sphere. The geodesics intersect to form triangular elements that create local triangular rigidity and distribute the stress.

Of all known structures, a geodesic dome has the highest ratio of enclosed area to weight. Geodesic domes are far stronger as units than the individual struts would suggest. It is common for a new dome to reach a "critical mass" during construction, shift slightly, and lift any attached scaffolding from the ground.

Geodesic domes are designed by taking a Platonic solid, such as an icosahedron, and then filling each face with a regular pattern of triangles bulged out so that their vertices lie in the surface of a sphere. The trick is that the subpattern of triangles should create "geodesics", great circles to distribute stress across the structure.

There is good reason to believe that geodesic construction can be effectively extended to any shape, although it works best in shapes that lack corners to concentrate stress.

An early example of a geodesic dome was the 1913 planetarium dome at the Carl Zeiss plant in Jena, Germany. However, the most famous individual to be associated with geodesic domes was Buckminster Fuller, who popularised them in the 1940s in line with his "synergetic" thinking.

Fuller had hoped the dome would address the emerging housing crisis - he also had hopes for his dymaxion house. The geodesic dome has also been used to provide a stable structure for industrial buildings and stadiums.

The dome was introduced to a wider audience at Expo '67 the Montreal, Canada World's Fair as part of the American Pavilion. The structure's covering later burned down, but the structure itself still stands and, under the name Biosphère, currently houses an interpretive museum about the Saint Lawrence River.

A number of people have built homes in the shape of a geodesic dome. Domes have a number of advantages.

They are very strong. The basic structure erects very quickly with a small crew, and light-weight pieces. Domes as large as fifty meters have been constructed in the wilderness from rough materials without a crane. The dome is also aerodynamic, so it loses relatively little heat to wind chill. Solar heating is possible by placing an arc of windows across the dome: the more heating needed the wider the arc should be, to encompass more of the year.

However, as a housing system the dome has several problems.

On the mundane side the entirety of the funishing and fitting world is designed with flat surfaces in mind, and installing something as simple as a sofa results in a half-moon behind the sofa being wasted.

The shape leaves the vast majority of the interior surface unusable because of the sharply sloping roof lines. For example, in a 20 foot tall dome, only the bottom 8 feet or so are really usable. This leaves a large volume that must be heated, yet cannot be lived in.

Dome builders find it hard to seal domes against rain. The most effective method with a wooden dome is to shingle the dome. Another method is to use a one-piece reinforced concrete or plastic dome. Some domes have been constructed from plastic or waxed cardboard triangles that overlapped in such a way as to shed water.

Methods of construction

Wooden domes drill a hole in the width of a strut. A stainless steel band locks the strut's hole to a circle of steel pipe. This method lets the struts be simply cut to the exact needed length. Triangles of exterior plywood are then nailed to the struts. The dome is wrapped with several stapled layers of tar paper, from the bottom to the top in order to shed water, and finished with shingles.

Temporary greenhouse domes have been constructed by stapling plastic sheeting onto a dome constructed from 1x1s. The result is warm, movable by hand in sizes less than 20 feet, and cheap. It should be staked to the ground, because it will fly away in strong wind.

Steel-framework domes can be easily constructed of electrical conduit. One flattens the end of a strut, and drills bolt holes at the needed length. A single bolt secures a vertex of structs. The nuts are usually set with removable locking compound, or if the dome is portable, have a castle nut with a cotter pin. This is the standard way to construct domes for jungle-gyms.

Concrete and foam plastic domes generally start with a steel framework dome, and then wrap it with chicken-wire and wire screen for reinforcement. The chicken wire and screen is tied to the framework with wire ties. The material is sprayed or molded onto the frame. Tests should be performed with small squares to achieve the correct consistency of concrete or plastic. Generally, several coats are necessary on the inside and outside. The last step is to saturate concrete or polyester domes with a thin layer of epoxy compound to shed water.

A CGI geodesic sphere rendered using freeware DOME Software and POV-Ray softwareEnlarge

A CGI geodesic sphere rendered using freeware DOME Software and POV-Ray software

Some concrete domes have been constructed from prefabricated prestressed steel-reinforced concrete panels that can be bolted into place. The bolts are within raised receptacles covered with little concrete caps to shed water. The triangles overlap to shed water. The triangles in this method can be molded in forms patterned in sand with wooden patterns, but the concrete triangles are usually so heavy they must be placed with a crane. This construction is well-suited to domes because there is no place for water to pool on the concrete and leak through. The metal fasteners, joints and internal steel frames remain dry, preventing frost and corrosion damage. The concrete resists sun and weathering. Some form of internal flashing or caulking must be placed over the joints to prevent drafts. The 1963 Cinerama Dome was built from precast concrete hexagons and pentagons.

See also

Fullerene

External links