A prismatic tank has outer and inner walls and internal horizontal stays. The walls include a plurality of horizontal beam sections. Each beam section has two parallel flanges interconnected by a web and a pair of opposing end faces. The beam sections are stacked one on top of the other and joined together along adjoining longitudinal edges of respective flanges and at end faces of respective beam sections such that a joint is formed between a first end face of a first beam section and a second, abutting end face of a second, abutting beam section. The web of the first beam section is recessed at the first end face and the web of the second beam section is recessed at the second end face so as to leave a first opening in the joint.