CONSTRUCTION


Struts The visible pattern formed by the structural elements themselves doesn't necessarily show grids of equilateral triangles - for the visible grids may be equilateral triangles, equilateral diamonds,or equilateral hexagons - the diamond made up of 2 equilateral triangles, and the hexagons comprised of 6 equilateral triangles. The structural members may be aligned with all lines of the three way grid or just selected ones of those lines. If the members are arcuate (spherical) they will coincide with the grid lines; if they are straight (flat) they will be chords of the great circle grid lines. Each strut is the chord of the sphere with its ends at greater distance from the center of the sphere than the radial distance of the midpoint of the chordal strut, that difference in distance being exactly that of the arc altitude. The strut is the most efficient when cigar shaped and pin ended. The mid-girth of the chordal compression strut is proportional to its length and is always substantial. Struts with a ratio of 24 units length to 1 unit in the traverse direction (i.e. 24 : 1 slenderness ratio) are prefered. The frequency of pattern as previously defined can be chosen with a view to maintaining the optimum slenderness ratio for each size framework. Islands of compression are the only residual "solids" and their diminishing size diminishes their relative weights at a cube root progression of advantage. Halving the size of a solid spar reduces its relative weight by 8. Halving the size of a hollow spar reduces weight by a factor of 4. In the above progression, as the geodesic tensegrity frequencies increase, the size of the islands of compression diminish. Generally speaking, the larger the structure the greater will be the frequency selected in order to keep the sizes of the individual struts within practicable limits for ease of manufacture, storage, packing, shipment, handling and erection. The struts may be interconnected by sliding joints locked by gravity compression throughout the great circle pattern of the framework as a whole.

VERTICES


There are many ways to deal with the connection of the struts. In my opinion, the simpler the connection the better. If the struts impinge on a single point, you avoid creating moment arms that create instability and secondary stresses at the vertices. If you must use a HUB, smaller is better, keeping these moment arms short.

FTB


The simplest method of geodesic dome construction that I've found is the FLATTENED TUBE BOLTED (FTB) method. In this system, tubular struts are flattened at each end and drilled for bolting together. The ends are flattened parallel to each other. The flattening allows two degrees of freedom at the vertex. Angular adjustment is dictated by the plane of the flattened end, allowing an unbolted end of a strut to move in or out from the dome center. Flattening also allows rotation around the center of the vertex. These two degrees of freedom for each end of the strut enable the strut ends to float in space until adjacent struts bolted to the ends lock them in place at a fixed and uniform distance from dome center in the Synergetically triangularly stabilized spherical geodesic pattern resulting from the pattern of differing strut lengths. Electrical Metal tubing (emt) also called conduit is a common material readily available and easily fashioned into the struts using common tools.

FRAME WORK ASSEMBLY Color coding is a great aid in assembly along with layer-by-layer assembly diagrams with color reference. In erecting the framework, it is best to start by assembling the struts which are to form the very top of the dome (eg. the icosacap). This can be done on the ground. Working radially outward in all directions, the dome will begin to take shape and is gradually lifted as the work proceeds until, in the end, it rests on its lower most struts against the ground or on a suitable foundation prepared to receive it. When constructing the "next layer", the downward pointing struts of the next layer are loosely attached to the last finished layer. These pivoting struts are used to support the dome. The dome is raised by pulling the finished dome section up then the loose struts "scissor" downward and outward "Bitting" into the earth raising the dome to a new height. These struts are joined at the bottom by lateral struts completing the triangulation for that layer. All subsequent layers are formed in the same manner. This method simplifies erection by utilizing only those materials required for the dome itself and also lessens the tensile forces involved in construction of the the dome. Using this method, as the layers progress from top to bottom an increasing in the number of "scissor" struts take the total load so that the load per strut is about the same throughout construction (or at least within acceptable limits). When the struts are out of proper sequence excessive stress may cause bowing and/or buckling of strut or struts in that area. In other cases, flatness may occur . The remedy to these situations is to work from the center of the irregularity, checking strut lengths and grid pattern. On domes of up to 1/2 sphere the base will automatically tend to spread apart, however on domes greater than 1/2 sphere the tendency is toward disequilibrium collapsing in on that portion of the incomplete dome which curves inward, so that temporary support and a previously prepared base are necessary. The geodesic framework based on a single sphere with no depth has a very limited size range. Low frequency members at Large radius become very large. Large domes require a a framework comprised of a truss formed of compression and tension members, a Fully Articulated Tensegrity Structure, to keep individual members relatively small and the weight minimal.

BCF ENCLOSURE The FTB system may be enclosed with thin film or fabrics or possibly using sheet metal panels. The BCF system has been developed to enclose an FTB dome using thin plastic films. Next Table of Contents