Wind Girder Design for Large Diameter Tanks: API 650 §5.9 Explained

Why Thin Shells Need Wind Girders

A cylindrical shell under uniform external pressure fails by elastic buckling — not by material yielding. For large-diameter tanks with thin shells (D/t ratios of 500–1500 are common), the critical buckling pressure can be well below the wind design pressure. API 650 §5.9 addresses this by requiring circumferential stiffening rings — wind girders — to reduce the effective unsupported shell height.

The collapse mode is oval flattening of the cross-section, progressing until the shell lobes inward. Historical tank failures in hurricanes have been attributed to inadequate wind girder design on the upper courses where the shell is thinnest.

Top Wind Girder

Every open-top tank (and most fixed-roof tanks without structural roofs) requires a top wind girder — a stiffening ring at or near the top of the shell that maintains shell circularity under wind and prevents the top course from ovalling. For fixed-roof tanks, the roof structure itself frequently provides equivalent stiffness, and API 650 §5.9.7 sets criteria for when it qualifies.

The top angle (typically a 75×75×10 or 90×90×10 angle for small-to-medium tanks) must provide a minimum section modulus Zw. When the top angle alone does not meet Zw, a dedicated built-up ring must be added. This is more common than many engineers expect on larger-diameter tanks — the required Zw scales with D², so doubling the diameter quadruples the ring requirement.

Intermediate Wind Girder — Calculating H₁(max)

When the shell height between the top wind girder and the next structural support (the tank base) exceeds a maximum unstiffened height, an intermediate wind girder is required. API 650 §5.9.6 gives the maximum height as:

Importantly, the calculation uses a transformed height based on the equivalent shell with uniform thickness equal to the thinnest course. Thicker lower courses are converted to equivalent lengths of the thinnest course, which always reduces the transformed height below the actual shell height. This is why intermediate girders are more often needed on tall tanks in high-wind regions rather than on standard-height tanks in moderate-wind zones.

Section Modulus Zw Calculation

Once the intermediate girder location is set, the required section modulus for each span is:

Zw = D² × H × 0.0001 (m³, for V = 190 km/h wind speed)

For different design wind speeds, API 650 provides a correction factor. The section modulus of common steel sections (angles, T-sections, built-up rings) must be calculated taking into account the effective width of the shell plate that acts compositely with the girder — typically 16t on each side of the ring attachment.

Practical Tips for Large-Diameter Tanks

• D > 30 m tanks: Almost always need at least one intermediate wind girder on the upper shell. Check early in the design — the girder location affects scaffolding and erection planning.
• Floating-roof tanks: The floating roof pontoon provides stiffening only when the roof is at grade or above. During initial fill and emptying, the shell is effectively unstiffened at the top — this governing case is easy to miss.
• High-wind zones (V > 190 km/h): Use the correction factor from API 650 Table 5-1. The required Zw scales with V², so a 240 km/h design wind speed increases Zw by approximately 60% versus the base case.
• Location of intermediate girder: Place it at or near the junction of the thinner upper courses and thicker lower courses — this is usually at the 1/3 to 1/2 height point on the transformed shell.

For a full worked example running wind girder design in TankCode 650 alongside shell thickness design, request a demo from our team.