Abstract: Vertical boards or tank segments are compressed by outer horizontal bands that cause the vertical edges of these tank segments to press together. Each tank segment has a web and edge structures that are thicker than the web measured in a radial direction in the finished tank. The compression of the horizontal bands causes these edge sections to exert a bending movement on the web, bending the web outwardly until it contacts the outer horizontal bands. One size of tank segment, therefore, serves for a large number of different diameters of cylindrical tank. The webs are formed flat, and the thickened edges are preferably tongue-and-groove configuration.