RING STRUCTURES

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Since this particular structure was to be a walk-in dome with a 10'-diameter base, it was a simple matter for Mr. C. to lift the center of the structure and push it to overhead height. Under normal conditions, just the tension of each ring against the others is enough to maintain the dome's shape. But to demonstrate how that framework could be inexpensively reinforced, Corey made up some simple clamps and ground pegs, using scrap pieces of PVC pipe treated, as before, with hot oil.

He used the same temperature and method as he had previously, except that instead of enlarging the pipes with a swage, he flattened them in the vise and then formed both ends of each clamp into a hook around the body of the tool . . . which had a diameter approximating that of the piping used for the framework. The finished supports, which locked between adjoining rings to keep them from sliding or spreading, were 6" (more or less) plastic scraps with hooks at each end. The ground pegs were perhaps twice as long, with one end hooked and the other cut at an oblique angle to make driving easier.

In addition to fabricating the clamps and pegs, Corey introduced another feature to the structure . . . one that plays an important role when the dome must support weight above and beyond that of a plastic or fabric covering. He called it the base or tensioning ring, and its function was obvious: Under load, of course, the six frame rings experience compressional forces. But—because of the structure's dome shape—these forces translate into an outward pressure at the point of contact with the ground. To keep that force in check, Bernard merely laced a seventh ring, which was over three times greater in circumference than the individual frame hoops, between all the rings at the base. Then he coupled its ends together with glue.

SLIP ME SOME SKIN

Like the framing material, the covering used on a ring structure is pretty much a matter of choice . . . but the intended use of the completed assembly does limit the builder's options somewhat. If the end product is to be a miniature greenhouse, for example, a transparent jacket is desirable. Although clear polyethylene would be the ideal "quick and dirty" selection, that material could be expected to last through only one season of direct sunlight. On the other hand, vinyl or butyl sheeting might serve for as many as three years but would be more expensive initially. And, while UV-treated clear PVC film could endure a decade, it likewise would require a considerable investment.

When protection or heat retention is important, of course, other materials would be preferable. These could range from multi-ply, nylon-reinforced screening or an insulated UV-treated pool covering to simple polypropylene tarp or canvas sheets.

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