Abstract: A pressure resistant square panel made of steel or cast iron comprises flanges extending along peripheral edges of a pressure receiving surface of the panel, a bottom portion along bottom edges of the flanges of the pressure receiving surface, flat portions inside the bottom portion and higher therethan, and reinforcing ribs substantially X-shaped as viewed in a plan view and extending along diagonals of the panel and progressively increasing their heights from corners to a center of the panel.

Abstract: A high pressure resistant and water tight panel assembled tank consisting of unit panels, each unit panel is formed with a protrusion portion in the form of a pyramid or truncated pyramid progressively increasing its height toward a center of the square wall surface of the panel and has securing flanges at the peripheries of the panel. The unit panels consist of (a) bottom wall panels of which securing flanges are folded in a direction opposite to the extending direction of the protrusion portion and (b) side wall panels of which securing flanges are folded in a direction the same as that of the protrusion portion and having partially thicker portions, and the securing flanges of the wall panels embracing sealing seats therebetween are clamped together outside of the tank to form the assembled tank.