Shell Course Thickness Design Under API 650: One-Foot and Variable Design Methods

The Governing Principle

The API 650 shell design is fundamentally a hoop stress check. Hydrostatic pressure from the stored liquid creates a circumferential (hoop) tensile stress in the shell plate. This stress increases linearly from zero at the liquid surface to a maximum at the shell-to-bottom junction. Each shell course must have a thickness sufficient to keep hoop stress below the material allowable.

The basic hoop stress relationship is: σ = p × r / t, where p is the hydrostatic pressure at the design point, r is the tank radius, and t is the shell thickness. Rearranging for the required thickness gives the familiar API 650 §5.6 formula.

The One-Foot Method

The 1-foot (0.3 m) method is the simplest and most widely used approach. The design point for each course is taken at 0.3 m above the bottom of that course. This is a conservative simplification that slightly overstates the pressure at the governing cross-section, resulting in slightly thicker plates.

The required thickness for each course by the 1-foot method is:

The ordered (actual) thickness is the greater of the design thickness t_d and the hydrostatic test thickness t_t (which uses water SG = 1.0 and a higher allowable stress St). It must also meet the minimum thickness requirements from Table 5-1 (e.g., 6 mm minimum for D ≥ 15 m).

The Variable Design Point Method

The variable design point (VDP) method, described in API 650 §5.6.4, locates the actual point of maximum stress in the shell more precisely. Because shell bending effects near course junctions reduce the effective hoop stress below the membrane value, the true governing cross-section is not at 0.3 m but at a variable distance from the bottom of each course that depends on the thicknesses of adjacent courses.

This makes the method iterative — you need an initial assumed thickness to calculate the design point location, then recalculate the thickness at that point. The VDP method typically yields plate thicknesses 5–15% lower than the 1-foot method on the lower courses of tall tanks, which translates directly into steel tonnage and cost savings.

When to use VDP: For tanks taller than 12 m with tight cost constraints, or where material procurement is limited to specific gauge thicknesses, the VDP method pays for the extra calculation effort. For standard-height tanks, the 1-foot method is usually preferred for simplicity and clarity in design documentation.

Corrosion Allowance — Common Mistakes

The corrosion allowance (CA) is added to the minimum required thickness, not to the ordered thickness. The ordering sequence is:

1. Calculate t_required (design or test, whichever governs).
2. Add CA to get t_min_ordered.
3. Round up t_min_ordered to the next available standard gauge thickness.

A common error is adding CA to an already-rounded thickness, which results in ordering 1–2 mm extra on every course. On a 20-course, 40 m diameter tank, this adds several tonnes of unnecessary steel. TankCode 650 applies CA correctly and shows you exactly which requirement governs each course. See our shell module walkthrough for a step-by-step guide to reading these outputs.

Gauge Rounding and the Gauge Database

API 650 requires that ordered plate thicknesses conform to commercially available gauges. The standard gauge series used in most projects is defined in the workbook reference data. TankCode 650 rounds to the nearest available gauge automatically, and shows you the over-thickness margin on each course — useful for corrosion life calculations.

For more on how shell design connects to the overall weight and foundation load calculations, see our getting started guide.