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Long barrel shells permit intriguing architectural designs and long spans. Shapes range from a low-rise arc to a high-rise cylindrical arch. The economy of long-barrel shells is increased with the usage of multiple shells. Folded-plate roofs are adaptable to very long spans and their cantilevers can be applied advantageously in the exterior design. Folded-plate roofs have a distinct architectural design and permit great areas of column-free space. Ridges and valleys can be sharp or blunted. A flat deck enclosing the channels formed by the folds may be used to integrate mechanical and electrical systems. Hyperbolic paraboloid umbrella shell roofs can be designed economically to meet a variety of building needs. Multiple re-use of forms makes them practical. The low stresses in a hyperbolic paraboloid shell require only a minimum thickness of concrete. Generally the shell thickness depends upon the concrete cover required for the reinforcement. Hyperbolic paraboloid inverted umbrella shell roofs are becoming increasingly popular due to their economical use of construction materials, simplicity of their structural action, and inherent beauty. They may be sloped, square, or rectangular and are particularly economical where repetition makes form re-use possible. Dome shells may be free-standing structures or designed in clusters. Adjustable segmental forms are especially economical where clusters are used. Domes may be supported uniformly or may touch the earth at as few as three points. They may be pierced as desired for natural light, or appropriate light fixtures may be used.