a curved wall panel made of fiberglass - polyester resin composite and having a main wall section , in which steel straps are circumferentially embedded , and an inwardly recurved section at each side thereof which terminates in a radially disposed and outwardly extending flange . the side - by - side flanges of adjacent panels are conjoined to form a joint which is outwardly spaced from the pair of flanges . the plastic portions of the bolted butt joint formed thereby are in compression only from the bolts and from the hydrostatic pressure while the steel bolts and straps are in tension only . a plurality of conjoined ring panels form a ring module . outwardly disposed ring panels at the top and bottom of each wall panel enable a ring module to be placed on the rim section of a base formed of pie - shaped elements and to be stacked to form a field - erected tank having horizontally disposed and movement - permitting ring joints above and beneath each ring module . because the thin - walled , creep - susceptible composite in the compressed butt joints and in the ring joints are not under tensile stress , there is substantially no stress concentration nor loci for accelerated corrosion in the tank , so that any size tank can be precisely designed and fabricated .