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 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.
Buckminster Fuller developed and named the geodesic dome from field experiments with Kenneth Snelson 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 of it was extremely strong for its weight, because 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 construction techniques, which are far less efficient, far heavier, inherently instable, and rely on gravity to stand up. But there were drawbacks.
While strong, domes react to external stresses in ways that confound traditional engineering. Some tensegrities will retain their shape and contract evenly when stressed on the outside, and some don't. One dome built at Princeton was hit by a snowplow, which popped the struts on the opposing side. To this day, the behavior of tension and compression forces in the different varieties of geodesic structures is not well understood. Traditionally trained structural engineers cannot guarantee their performance and safety.
The dome was successfully adopted for specialized industrial use, like the 1958 Union Tank Car Company dome 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.
Residential domes were less successful. Fuller himself lived in a geodesic dome in Carbondale, Illinois, at the corner of Forest and Cherry, but they never caught on to the extent that Fuller hoped.
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 contex, is symbolized by v.
Advantages of Domes
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.
One residential design called a dome home that employs the dome's aerodynamic properties to be resistant to high winds, such as those created by hurricanes.
Disadvantages of Dome Homes
However, as a housing system the dome has several problems.
On the mundane side the entirety of the furnishing 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. Domes tend to develop leaks because the sun heats the dome during the daily cycle and the stresses are conveyed through the structure as the sun moves through the sky much as one might break the shell of a hard-boiled egg by similaneously pressing and rolling.
In Bucky Works : Buckminster Fuller's Ideas for Today, Fuller's former student J. Baldwin states that there is no reason for a dome that is properly designed and constructed and uses proper materials to leak and that some designs cannot leak.
Sounds, smells and even reflected light tend to be conveyed through the entire structure. Lloyd Kahn, author of Domebook One, Domebook 2 and later, Shelter, became disillusioned with domes by 1973, and details many more problems with dome homes on his website calling them smart but not wise.
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.
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.
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
Last updated: 06-02-2005 04:19:06