PURPOSE:To obtain sound welding by piercing spot faced holes to at least one of the mutually butt parts of special shape materials of a projecting shape in section for butt welding of the bottom plate and side plate of a flat bottommed cylindrical tank thereby producing penetration beads within the spot faced holes in the stage of welding, and facilitating penetration. CONSTITUTION:The bottom plate parts 6'' of special shape materials 6 are butt welded 7 to the outer side edge of a bottom plate 3, and the faces 9, 9 between the units of the materials 6, 6 are butt welded 8. At this time, spot faced holes 10, 10 are provided oppositely to the end faces 9 to produce penetration beads in the holes 10 in the stage of welding, thereby facilitating penetration.
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