Geodesic domes are made up of a complex system of triangular members and their connections, which work together transmitting loads laterally to the foundation. Since the triangle is the strongest geometrical configuration known to man, the geodesic dome performs under massive loads, unparalleled by many other structural systems. There are several different material combinations that can comprise the superstructure of a geodesic dome, along with various substructures. The discussion below focuses primarily on a small dome with wooden force members pin connected and supported by a concrete pier foundation.
Subsystems and Interactions:
I. Struts: Struts are the force members in a geodesic dome that act in compressive and tensile forces to resist loading. What exactly is geodesic? Geodesic is a Latin term meaning “earth dividing.” Imagine the earth as a perfect sphere, with its longitudinal lines dividing it into equal halves. These longitudinal lines are called great circles. The geodesic dome has members which follow three sets of principal sets of great circles intersecting at 60 degree angles, subdividing the dome surface into a series of equilateral spherical triangles. (Ching, Francis D.K.: A Visual Dictionary of Architecture). The more complex this system of triangles, the more spherical the dome becomes. The structure as a whole is subjected to bending moments, but the individual struts are rigid and only subjected to tension and compression forces. Applied loads are distributed through one strut to the pins. The pins transfer the loads to the next strut, and this process continues until the loadings reach the foundation. A diagram of the load distribution is shown below:
Geodesic Load Distributions
Photo Courtesy of: Switala, Szurek, Duroseau
II. Pins: The pins are used at vertices to hold the struts together. The pins must be able to resist the compressive forces transferred through the struts. All of the vertices must have a pin connection, which allows forces to be transmitted through to the foundation. The pins should be weather treated to resist damage due to environmental conditions.
III. Substructure: The foundation transfers loads from the superstructure down into the earth. The applied loads consist of live, dead, wind, seismic, and gravitational. All of these must be withheld by the foundation. Although the geodesic dome is primarily a self-supporting structure, its foundation must carry the applied loads and anchor it into the earth. Typically, a circular concrete slab is poured onto the earth. The struts and piers are bolted and welded to the slab as shown in the diagrams below:
Pier Foundation Details
Photos Courtesy of Good Karma Domes
The size and depth of the concrete piers depend on local building codes and the soil conditions. The flared bottom of the concrete pier helps to distribute the loads equally throughout the soil.
The geodesic dome has many properties that make it an ideal building structure for different uses. The floor plan variations within a geodesic dome are limitless. No load-bearing interior walls are required to support the roofs, resulting in an open spaced plan. Partition walls can be directly framed into the dome shell, or they may be free standing space divisions. Up to 50% of the lowest ring of triangles can be removed, and these openings can be replaced with traditional doors and windows. The choice of a geodesic system yields less material costs. Since the sphere is a mathematical maximum, it encloses the most area for the least amount of material. An example of a dome home is shown below:
Photo Courtesy of Alpine Domes
Along with the open space plan, the geodesic dome’s structural stability makes it a valuable resource to resist against excessive loading, such as winds or seismic vibrations. They have been used as radar towers in Antarctica under up to 200 mph winds for over 25 years. Since spherical shapes amplify light, as opposed to rectangular which absorb, superior lighting distribution makes a spherical shape perfect for a greenhouse. Other common uses of geodesic domes include:
- Churches - Bulk Storage
- Garages - Office Complexes
- Gymnasiums - Cabins
- Ice Rinks - Aircraft Hangers
Limitations: The main limitation of a geodesic dome is the somewhat odd shape. The diameter and height of the sphere are completely dependent upon each other. In order to reach an attainable height, the diameter must sufficient. Interior heights near the edges of the dome are rather short and awkward. This can be eliminated by adding riser walls to the dome, increasing the height and volume of the dome.
Typical Materials Used: Geodesic domes may be built with various construction materials. Many manufacturers offer complete kits for small dome building, such as greenhouse and residential. Wood is typically used in these smaller structures. For massive domes, steel and concrete are used to increase the structural stability.
Construction Issues: The construction of small geodesic domes rather easy in comparison to other structural systems. Many manufacturers selling geodesic kits state that they can be assembled within a few days by the competent builder. The difficulty of assembling the struts varies directly with the frequency of the dome. The frequency is simply the number of smaller triangles within a large one. Examples of one and two frequency spheres are shown below:
Photos Courtesy of Good Karma Domes
The higher the frequency of the dome, the more spherical it becomes, and the more complex the construction is due to the addition of smaller members. It can be seen above that the individual struts decrease in size as the frequency increases. The shorter struts are stronger against buckling forces, which makes them applicable in larger dome structures. In general, the construction is dependent upon its required use and its environmental setting.
Numeric Parameters: The triangular spherical shape of geodesic domes give it structural capablilities unmatched by many other structural systems. Steel geodesic domes have been wind tunnel tested to withstand up to 200 mph winds. A geodesic dome can be any size, as long as the height and span correspond. One of the design concerns is the weight of the struts compared to their spans. Below is a preliminary design chart for steel and aluminum geodesic domes.
Photo Courtesy of: Cowan, Henry J.: Architectural Structures. NY, NY: Van Nostrand Reinhold, 1991.