GEODESIC DOMES
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
selfsupporting 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.
Uses:
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
loadbearing 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.