Why the Annular Plate Zone Is the Most Critical Part of a Tank Floor
The annular plate — the outer ring of floor plates that form the junction between the tank floor
and the tank shell — is the highest-consequence zone in any above-ground storage tank. It is
subject to:
External bottom-side corrosion — the annular plate sits directly on the foundation ring,
creating a crevice environment at the plate-to-ring interface that traps moisture and
accelerates corrosion from below
Shell-to-floor weld stress — the fillet weld connecting the shell to the annular plate is a
structural stress concentration, subject to fatigue cracking from thermal cycling, liquid
movement, and settlement loading
Differential settlement effects — uneven foundation settlement induces bending stresses
in the annular plate and its weld to the shell, leading to cracking that can propagate into the
shell
Internal product-side corrosion — the annular plate zone is often where internal bottom
corrosion concentrates, particularly under accumulated water or sludge
Failure in the annular plate zone — perforation, weld cracking, or plate collapse — represents a
critical tank integrity event. API 653 specifically requires that the annular plate zone receives
focused inspection attention, with minimum remaining wall requirements more stringent than
for the general floor area.