A large tank for holding a fluid substance. Sidewalls of the tank are outwardly convex, or arcuate. The upper edges of the sidewalls are unrestrained so that a relatively high degree of distortion or freedom of the walls is permitted under a fluid head in the tank. The shear flow which is developed in the tank walls acts in summation as a horizontal beam which, anchored at its opposite ends to vertical beam-columns, supports the walls and contained fluid.