6. 14 l which the "aclive" systems (plumbing, electricity, fire protection, heating, ventilating, air conditioning, communications, and vertical transportation) are placed. Materials available for _passive systems of the prototype colony are essentially limited to those refinable from lunar soil. (See Section VIII.2.1 for lunar soil composition.) Steel and silicon glasses are primary construction materials for the total colony structure with oxygen, silicon, calcium, sodium, and aluminum as by-products. The availability of these materials signals a different approach to construction than is currently in practice in the United States. On Earth, "light construction" usually means wood, clearly unavailable to the prototype. Section VII.4.6 outlines basic structural concepts for habitable facilities. A steel frame structure with columns typically on Sm centers is bolted together and rested on the inner hull by a series of rollers. The rollers enable the buildings and hull to expand differentially during spin-up and also guarantee independence and flexibility in the case of thermal expansion and contraction. Materials for "exterior" walls and other partitions allow a wider degree of choice. Glass and metal panels are obvious possibilities. A compressed baked-finish calcium wall board would be possible and could offer the textural feeling of plaster or "drywall". Slag is available as a cheap insulator and could be pressed into bricks for a masonry feel. Silicon meshes might be woven to approximate canvas for screens and awnings. The major failing of these materials is their uncompromising "hardness". The "soft" side of the materials spectrum should be filled from Earth, by importing much of the furnishings, bedding, and cushioning. This imported softness is essential to the humanization of an otherwise overly metallic environment. Components of active systems can be thought of a conduits which carry fluids, electrical energy, or people-sized containers to points of application. Electrical wiring is commonly copper enclosed in a petroleum derivative shield. Plumbing supply and drainage pipes are commonly copper, plastic, or cast iron. Mechanical systems piping is usually steel with air ducts fabricated from galvanized steel
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