A tank structure

A double containment prismatic tank has outer and inner walls (1, 2) made by stacking H-beam sections on top of each other and joining them along their longitudinal flange edges and at their abutting end faces in joints (5). In the joint areas internal stays (tension beams) (3) are connected to the inner wall (2) to improve the structural efficiency of the tank. The stays (3) are connected to theinner wall (2) by means of brackets (6) which extend in a smooth and tapering manner to the sides of the joint (5) area. In the joint (5) the outer and inner flanges (7, 8) of the beams (4) are joined by welds (10). However, the webs (9) of the beams (4) are not welded together in the joint, but are instead recessed and terminated in a smooth curve so as to form an opening (11), thus avoiding anycontact between the outer and inner walls (7, 8) that are not base metal and thereby avoiding a risk of fatigue crack propagation from the inner wall (2) to the outer wall (1). The inner flange (8) of the beam sections (4) is