Large diameter carbon steel storage tanks are constructed from strip metal wound in a helical path in which the tank is built upwardly. The strip is fed to a support assembly arranged circularly on a base, and the upper edge of the strip that is fed to the support assembly is aligned with the lower edge of the helical turn immediately there above so that the edges are spaced apart in a vertical plane from each other and then butt welded together. The strip is vertically adjusted to maintain a predetermined spacing of adjacent helical turns of the strip in the area of the welding thereof. The support structure in the area of the welding includes parts positioned on opposite sides of the strips, the inside one of which may be collapsed to permit removal of the assembly following completion of the tank. Top structures accommodating the helically wound strip are utilized.