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Understanding Wooden Domes

Before you start constructing your ALBATA GEODESIC DOME SYSTEMS dome, it is useful to learn something about wooden domes in general. There are three basic types of wooden domes: Panel Domes, Hub and Strut Domes, and Stressed Skin Domes. The three types of domes, when completed look very much alike, yet they are very different in structural concept. Actually, it is hard to tell one type from another after they are finished.

Panel Domes are made of prefabricated panels using plywood, or other coverings, on either 2x4 or 2x6 wooden frames. The panels are then bolted together. A panel dome, in structural terms, is a composition of self contained elements, like building blocks. Each "building block" transfers load to the next 'building block',etc. Strength depends entirely on the structural integrity of each panel and on the quality of the bolted connections.

Hub and Strut Domes are constructed like a truss. Wooden members connect to steel hubs to form a multifaceted frame of triangles. Strength of the hub and strut dome depends on the capacity of the members connecting the hubs to carry loads. Panels in the hub and strut domes are infills, and have no structural value.

In Stressed Skin Domes plywood is the primary structural material. The strength is in the plywood. To gain an understanding of stressed skin construction, imagine a flat thin sheet of metal. It has little or no strength. When you form this sheet into a spherical shape like a mixing bowl, it provides 40 times its normal strength. This is because of stress distribution. To illustrate, try to break a pencil. The pencil breaks when you apply a small amount of force in the middle, by bending the ends down. Now, try to break a pencil by pulling on both ends, you can't break it. The reason is that in the first test very high stresses develop at the outer surfaces while stresses in the center remain at zero. In the second test stresses distribute evenly and the entire pencil participates in resisting load.

This distribution is applied in a dome. When a load is applied the stresses immediately start "radiating" away from the point of application of the stress, Stresses are low in magnitude, and that means you can use thin sheet material. This also means that domes do not hold a "live" load such as standard vertical walls holding up a roof and all of it's components.

Using a composition of thin sheet material and ribs is a total structural system called STRESSED SKIN. It is the structural system used in aircraft, the space shuttle, unit body automobiles, and in the ALBATA Dome.

Which of the three types of domes is the best? Well, each has its own merits. The Panel Dome, for example, has an advantage in that it can be field assembled in the shortest time, but it also requires the most materials to build. It also may require a crane, or the lifting of heavy parts, and extra cost of shipping. The Hub and Strut dome has an advantage in the way it allows a variety of panel materials to be used, but the patented hub systems are expensive to buy and sometimes complicated to assemble. The Stressed Skin Dome is extremely economical in the use of materials. It has light ribs and stiffeners that serve as nailers to connect the plywood, and it is the simplest to assemble with the color coded system.

Which Type of dome is stronger? This is often asked. The simple answer is: When a dome is designed to resist a given load it should be as strong as the next dome designed for the same load. In this regard, the stressed skin dome comes through with high marks. Panel Domes and Hub and Strut Domes have bolted connections. When wood shrinks, bolts have a tendency to loosen. The proper way to deal with bolted connections in wood is to tighten the nuts after a structure has been in place for a period of five to six months. That cannot usually be done in a dome, so you may end up with loosely connected panels or struts, which are hidden behind the insulation and interior finishing. This greatly reduces the strength and integrity of the dome itself.

In stressed skin domes, no bolts are used in structurally important places. Plywood is glued and nailed to ribs with nails close together like stitching. Nail joints are unaffected by shrinkage in the wood, If anything, shrinkage helps to tighten the joints. Therefore, there is no significant movement which might severely crack plaster, cause squeaking, or weaken the dome in any way.