DOME MAGAZINE: Winter 1992-93, Vol. 5 | No. 2
This article will attempt to define some of the basic details among residential dome designs that are available, and also attempt to explain the more basic geometry that is used by many dome manufacturers today.
Once again, this is not intended to place any particular emphasis on one type of system or another, but to try to explain the basic differences, and hopefully, to make it easier for interested prospective dome owners to understand the systems that are available.
Two categories that evoke interesting comparisons are panelized systems and hub and strut systems. The main feature of the panelized system is that the triangular panel sections are assembled in the factory. The framework is assembled and the skin (plywood) is attached much the same as with the systems used in conventional, square panelized housing.
The panels are set in place on site and bolted together. Assembly time is quite rapid and only requires one or two days to completely set the whole framework. The framework is usually 2 x 4 or 2 x 6 lumber (Figure 1).
Hub and strut systems use one 2 x 4 or 2 x 6 at each intersection. These struts are held together with metal connectors and hubs (Figure 2). Using single wall systems of 2 x 4 or 2 x 6 for the struts limit the type and the amount of insulation that can be used.
Some dome manufacturers also use a double walled system that allows for more insulation, if needed (Figures 3 & 4).
For the super-insulated dome, manufacturers generally use a 2 x 6 load-bearing strut on the exterior and suspend a 2 x 4 strut to the interior. This provides a space to install additional insulation, and also provides support for the interior panels. The cavity is usually 14.5” deep, to hold 12” of fiberglass, and provides a 2 1/2” air space for cavity ventilation. A plywood strut will accomplish the same space for 12” of insulation.
General Dome Geometry
Most dome manufacturers today base their design on the five-sided geometric figure called the pentagon. Each dome structure has a pentagon at the top, and one pentagon emanating from each point of the top pentagon, making a total of six pentagons in each dome.
A common question that most lay people ask is, “What do you mean by ‘frequency’?” This word describes how the main triangle is subsequently divided to make up the whole dome. For beginners, let’s look at the two-frequency design.
Two-frequency Domes
This dome has ten sides at the base and represents 1/2 of a sphere at the base point. It also means that it has only two different length struts and only two different sized triangles (Figure 5).
The two-frequency structure lends itself to smaller domes, 32 feet in diameter or less (garages, cabins, greenhouses and smaller residences), because the struts generally get too long for the larger-sized domes.
Three-frequency Domes
This design has fifteen sides at the base and is truncated (cut off) at the 3/8 or the 5/8 points of the sphere line (Figure 6).
In other words, the 3/8 design cuts off 2/8 for the sphere before the midpoint, and the 5/8 design cuts off 1/8 of the sphere after the midpoint. This particular system is the most widely used by manufacturers of contemporary domes. Each design has its place in the fundamentals of residential housing. Some manufacturers use 4/9 and 5/9 truncations as the basis for their designs, but these differences off no real significance.
Four-frequency Domes
This dome has twenty sides at the base, and the spheres are cut off at the 1/2 and 5/12 point (Figure 7). The purpose of the four-frequency design, generally, is to keep the strut length down on the larger domes.
The above categories represent a large portion of the domes that are manufactured today. There are many interesting versions of these systems, both in larger residences and commercial buildings.
As true dome proponents, we are alone in the housing market. We will not get any help from our competition (the square house industry). It is nice to believe that they are going to give us something besides grief, but don’t bet on it. They may toss some of us a few crumbs, but in general we must make our own way amid all of the propaganda that is strewn in our path to keep the dome from becoming the housing of choice.
Relative Merits
Arguments prevail among dome manufacturers relative to the merits of each system, but the fact remains that any dome that is built, using normal consideration to construction details, far exceeds any square house in terms of cost per square foot, use of materials, energy efficiency and safety. Were we to aim our arguments at the square house industry—-where they should be aimed—-instead of at each other, the biases most certainly would be diminished.
I have been in the dome business exclusively since 1975. during that time we have seen several large dome companies go out of business. I am of the belief that the dedication to the dome concept was not there.
Lack of dedication, trying to be all things to all people, such as building square housing and dome housing both, means simply that true dedication is not on board. So, once again, I call on the dome industry to accept the challenge, as true believers in the dome concept, to make room for our differences and attack the common enemy, the square house industry. We hold all of the aces: most efficient use of material, most energy efficient, and the strongest of all residential structures. We should be shouting this from the rooftops.
Which reminds me of the old saw that says, “We have met the enemy, and they are us!”