SPHERE SEGMENT - COLUMN SUPPORTS
If a dome is built as less than a half sphere, a tension ring of steel bars, plates, or wires is required at the base to carry the thrusts of the shell. In this case, the ring has been made big enough so that it assists in distributing the reaction of the columns into the dome. The direct stresses in the shell are mostly compressive in this structure and are so small that the stress calculations are hardly necessary. There are bending stresses in the shell wall due to restraint of the thrust ring and to change in temperature. Therefore, the thickness of the shell is increased in the vicinity of the thrust ring. Otherwise, the shell thickness is a minimum and may be 2 1/2 to 3 inches for spans up to 150 ft.
Due to the double curvature of domes, buckling is seldom a factor in the design. Domes have been built with a thickness of 6 inches for a span of about 300 feet. With long spans, however, walking on the roof is like walking on a giant balloon because of the spring action of the shell.