Primary stiffening ring section modulus and secondary ring minimum angle sizes (BS EN 14015:2004, §9.3)
◉ BS EN 14015:2004 §9.3 STYLE CHECK · NOT A REGULATORY SUBMISSION TOOL
⚠ Indicative tool only, and structurally distinct from the existing API 650 and IS 803 Wind Girder calculators. EN 14015's primary ring formula directly incorporates wind gust velocity and solves for required section modulus, unlike IS 803's approach (which solves for maximum unstiffened height given a fixed thickness) — the two aren't interchangeable. Verified directly against this edition's page images, with the section modulus formula independently checked for its stated diameter cap (tanks over 60m are treated as 60m).
Identification
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When a Primary Ring Is Required (§9.3.1.1–.2, reference)
Open-top tanks: a primary stiffening ring is required to maintain roundness under wind loads, located at or near the top of the top course, preferably outside the shell. Fixed-roof tanks with a roof structure are considered adequately stiffened by that structure, so a primary ring is not normally required. Secondary stiffening rings may still be required for either type (§9.3.3) to maintain roundness over the full shell height.
Primary Stiffening Ring — Required Section Modulus (§9.3.2.1, Eq.7)
Z = 0.058 × D² × Hf × Vw²/45². Tanks over 60m diameter are treated as 60m for this calculation.
Result
Diameter used (capped at 60m)—
Required minimum section modulus Z—
Stiffener cross-section (Figure 7, Note 1): for sections a, c, d, e forming part of a stiffening ring at a stairway opening, cross-sectional area shall equal 32es² (es = shell thickness at that point). Brackets (§9.3.1.9): required when the horizontal leg/web dimension exceeds 16× its thickness; spacing shall not exceed 24× the width of the outside compression flange.
Top Corner Ring (§9.3.2.3)
Required when the primary stiffening ring is located more than 600mm below the top of the shell.
Result
Minimum top corner ring angle size—
Secondary Stiffening Ring — Minimum Angle Size (Table 17)
Sized by tank diameter alone, not by design loads (§9.3.3.1). Uses D from the Primary Ring card above.
Result
Minimum angle dimensions—
Vacuum design basis for secondary rings (§9.3.3.5): open-top tanks, 5 mbar irrespective of design wind speed; fixed-roof tanks, the design internal negative pressure per Table 3 (not reproduced here). A secondary ring shall not be located within 150mm of a main circumferential tank seam.
General Minimum Sizes (§9.3.1.6, reference)
Minimum angle for use alone, or as a component in a built-up stiffening ring: 60×60×5mm. Minimum nominal plate thickness for formed/built-up rings: 5mm if width ≤600mm, 6mm if width >600mm. Walkway sections: ≥600mm clear width, located 1m below the top corner ring, with handrailing on the otherwise unprotected side.