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Definitions for Geodesic Dome

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geodesic dome noun. A domed or vaulted structure of straight elements that form interlocking polygons.

Encyclopedia

geodesic dome (jē’ədĕsĭk, –dēsĭk) , structure that roughly approximates a hemisphere. Popular in recent years as economical, easily erected buildings, geodesic domes are geometrically determined from a model and may be constructed from limited materials. The architect Buckminster Fuller was an early proponent of geodesics for housing and other functions. Among the best-known examples of geodesic domes have been the United States Pavilion at Montreal’s Expo 67 and Biosphere II, an experimental recreation of the ecosystem in Arizona.

The noun geodesic dome has one meaning:

Meaning #1: a lightweight dome constructed of interlocking polygons; invented by R. Buckminster Fuller

Wikipedia definition: geodesic dome

The Montreal Biosphère, formerly the American Pavilion of Expo 67, by R. Buckminster Fuller, 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. It is the only man made structure that gets proportionally stronger as it increases in size.

Of all known structures made from linear elements, a geodesic dome has the highest ratio of enclosed volume 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 sub-pattern of triangles should create “geodesics”, great circles to distribute stress across the structure.

There is 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.

History

R. Buckminster Fuller (aka Buckminster Fuller) developed and named the geodesic dome from field experiments with Kenneth Snelson and others at Black Mountain College in the late 1940’s. Researchers have found antecedent experiments like the 1913 geodesic planetarium dome at the Carl Zeiss plant in Jena, Germany, but it was Fuller that exploited, patented, and developed the idea.

The geodesic dome appealed to Fuller because it was extremely strong for its weight, its “omnitriangulated” surface provided an inherently stable structure, and because a sphere encloses the greatest volume for the least surface area. Fuller had hopes that the geodesic dome would help address the postwar housing crisis. This was in line with his prior hopes for both versions of the Dymaxion House.

From an engineering perspective geodesic domes are far superior to traditional, right-angle post-and-beam constructions. Traditional constructions are a far less efficient use of materials, are far heavier, are less stable, and rely on gravity to stand up.

However, there are also some notable drawbacks to geodesic constructions as well. Although extremely strong, domes react to external stresses in ways that confound traditional engineering. Some tensegrity structures will retain their shape and contract evenly when stressed on the outside, and some don’t. For example, a dome built at Princeton, New Jersey was hit by a snowplow. The stress was transmitted through the structure, and popped out struts on the opposite side. To this day, the behavior of tension and compression forces in the different varieties of geodesic structures is not well understood. So, traditionally trained structural engineers may not be able to adequately predict their performance and safety.

The dome was successfully adopted for specialized industrial use, such as the 1958 Union Tank Car Companydome near Baton Rouge, Louisiana and specialty buildings like the Henry Kaiser dome, auditoriums, weather observatories, and storage facilities. The dome was soon breaking records for covered surface, enclosed volume, and construction speed. Leveraging the geodesic dome’s stability, the US Air Force experimented with helicopter-deliverable units. 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, but the structure itself still stands and, under the name Biosphère, currently houses an interpretive museum about the Saint Lawrence River. A dome was constructed at the South Pole in 1975 where its resistance to snow and wind loads is important.

In the “Climatron”, built in 1960 at Missouri Botanical Gardens, the original plexiglass panels discolored and were replaced with glass.

In the 1970s the Cinesphere dome was built at the Ontario Place amusement park in Canada.

Residential domes have been less successful, due largely to their complexity and consequent higher construction costs. Fuller himself lived in a geodesic dome in Carbondale, Illinois, at the corner of Forest and Cherry. Residential domes have so far not caught on to the extent that Fuller hoped. He envisioned residential domes as air-deliverable products manufactured by an aerospace-like industry. Fuller’s dome home still exists, and a group called RBF Dome NFP is attempting to restore the dome and have it registered as a National Historic Landmark.

Chord factors

Of great importance is the chord factor, the factor by which the radius of a dome must be multiplied to yield the length of a particular strut. The chord factor is twice the sine of half the central angle of the chord, but determining the central angle requires some non-trivial spherical geometry. In Geodesic Math and How to Use It Hugh Kenner writes, “Tables of chord factors, containing as they do the essential design information for spherical systems, were for many years guarded like military secrets. As late as 1966, some 3v icosa figures from Popular Science Monthly were all anyone outside the circle of Fuller licensees had to go on.” (page 57, 1976 edition) Other tables became available with publication of Lloyd Kahn’s Domebook 1 (1970) and Domebook 2 (1971). With advent of personal computers, the mathematics became more accessible. Rick Bono’s Dome software, outputs a script that can be used with the POV-ray raytracer to produce 3D pictures of domes. Domes of differing frequencies, or amount of subdivision of a polyhedral face, require differing results. Frequency, in this context, is symbolized by v.

Advantages of domes

Domes are very strong, and get stronger the larger they get. The basic structure can be erected very quickly from lightweight pieces by a small crew. 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 withstands considerable wind loads, such as those created by hurricanes. 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.

Today there are many companies that sell both dome plans and frame material with instructions designed simply enough for owners to build themselves, and many do to make the net cost lower than standard construction homes. Construction techniques have improved based on real world feedback over sixty years and many newer dome homes can resolve nearly all of the disadvantages below that were more true of the early dome homes.

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 struts. 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.

(When this article was written the author did not know about American Ingenuity’s steel reinforced concrete dome.)

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 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.

Largest geodesic dome structures

Many geodesic domes have been built and are in use. According to the Buckminster Fuller Institute Web site, the largest geodesic-dome structures (listed in descending order from largest diameter) are:

See also

References

  • Geodesic Math and How to Use It by Hugh Kenner, University of California Press (October 1, 2003) ISBN 0520239318
  • Bucky Works : Buckminster Fuller’s Ideas for Today by J. Baldwin, John Wiley & Sons (March, 1996) ISBN 0471129534

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