Abstract: A bulk storage vessel of generally cylindrical upstanding shape is formed with a plurality of wall segments which are assembled together in end to end relation to form subassemblies of circular shape. These subassemblies are then flange interconnected along upper and lower edges to provide a cylindrical vessel. Each of the wall segments includes a curved side wall of generally rectangular shape having roll formed flange structures along both its upper and lower edges and a vertical end flange at one end. The upper edge flange structures include an outwardly extending horizontal flange section and a downturned vertical flange section integrally rolled therewith. Along the lower edge, the flange structure includes an outwardly extending horizontal flange section, a downturned vertical flange section and an inwardly extending flange section joined to the lower edge of the downturned vertical flange and deflectable for interlocking engagement with the upper edge flange structure of an adjacent lower subassembly.