Wind Stability of the Empty Tank Shell During Construction: API 650 §5.9.7 and Interim Stiffening Rings

Why the Empty Shell Is the Most Vulnerable Configuration

During tank erection, shell courses are assembled from the bottom up — or in full-wrap sequences for larger tanks. At any point before the roof is installed and before any product is inside, the structure is an open-top, thin-walled cylinder. This is the weakest configuration the tank will ever occupy during its service life.

Wind acting on a cylindrical shell creates external pressure on the windward side and aerodynamic suction on the leeward side. The combined effect is an ovalling (out-of-round) deformation mode: the cross-section tries to distort from circular to elliptical. For a thin shell with a large unsupported height, the elastic buckling resistance against this mode is low — often far lower than the design wind pressure.

The completed tank resists this deformation through three mechanisms: the roof structure, the hydrostatic stiffness of the product, and the permanent wind girder. During construction, none of these are present. The bare shell, particularly in the upper courses where plate thickness is at its minimum, is exposed.

This is not a theoretical concern. Tank shells have buckled during erection in moderate wind events — events that would have been entirely non-governing for the completed tank. The root cause in nearly every documented case is the same: the §5.9.7 check was either not performed for the erection stage, or it was performed too late for the interim ring to be included in the fabrication package.

API 650 §5.9.7: The Stability Check

API 650 §5.9.7 requires that the empty, open-top shell be checked for stability against wind loading during the construction period. The check evaluates whether the shell can resist the design wind pressure without elastic buckling, taking as the critical parameter the unsupported shell height at each stage of erection.

The critical wind pressure the shell can resist is a function of three geometric quantities: the unsupported height L between stiffening elements, the nominal shell diameter D, and the shell plate thickness t of each course within that unsupported span. The relationship is strongly sensitive to L — halving the unsupported height increases the critical pressure by a factor of roughly four.

The design wind pressure p_wind is derived from the construction-period wind speed. API 650 does not mandate a specific construction wind speed; the common practice is to use the same ASCE 7 (or applicable local code) design wind speed used for the completed tank, recognising that exposure during erection is typically short. Some owners and contractors specify a reduced construction wind speed — 25 m/s or 35 m/s are common values — but this must be agreed with the engineer of record and stated explicitly on the construction drawing.

The stability check compares p_wind to p_crit for each unsupported span. If p_wind exceeds p_crit at any stage of erection, the shell is unstable and action is required before that stage is reached on site.

When an Interim Stiffening Ring Is Required

When the §5.9.7 check identifies a failing unsupported span — that is, p_crit < p_wind — the solution is to add a circumferential stiffening ring at an intermediate height within that span. The ring divides the unsupported height into two shorter segments, each with a higher critical pressure than the original full span.

The ring location is chosen so that both resulting spans satisfy p_crit ≥ p_wind. For a shell with uniform thickness over the failing span, the optimal ring placement is at mid-height. For a shell with varying course thicknesses (thicker at the bottom, thinner at the top), the ring is typically placed higher — at or just above the transition from the thicker to the thinner courses — since this is where the combination of low t and long unsupported length is most critical.

The interim ring may be temporary or permanent:

• Temporary: Installed for the erection period only, removed after the permanent wind girder and roof structure are in place. The removal note must appear on the construction drawing.
• Permanent: Retained as an intermediate wind girder in the completed tank. This eliminates the risk of the ring being left in place accidentally, and may improve the completed tank's wind resistance — but must be included in the design analysis for the finished structure.

The ring cross-section must provide sufficient stiffness to act as a nodal line for the buckling mode. A flat bar or angle welded continuously around the shell circumference is typical. The size is generally governed by practical handling and welding considerations rather than by the stiffness requirement, provided the ring is continuous and properly welded.

How Construction Sequence Affects the Check

The §5.9.7 check must be performed for each distinct stage of erection — not just for the final shell configuration before roof installation. This is where the most common errors occur.

A typical erection sequence for a large-diameter tank proceeds as follows: the bottom plate is laid and the first two or three courses are welded in place, forming a short, thick-walled cylinder. Additional courses are then added. The upper courses are thinner because hydrostatic pressure is lower near the top. By the time the full shell height is erected — but before the roof ring and roof structure are in place — the configuration is at its most vulnerable: maximum unsupported height, thinnest plates in the upper zone.

The critical failure mode that the check frequently misses is this: an intermediate check stage passes because the unsupported height is still short and the exposed courses are thick. As erection progresses, the upper thin courses are added, extending the unsupported height into the zone where p_crit falls below p_wind. If the interim ring was not planned from the start, it cannot be added in the field without a design change and re-fabrication.

Best practice is to check stability at intervals corresponding to each course addition, identify the critical stage, and specify the interim ring elevation on the construction drawing before any plates are cut.

A Practical Calculation Walk-Through

Consider a tank with the following parameters:

• Nominal diameter D = 30 m
• Shell height H = 12 m (seven courses)
• Construction wind speed V = 35 m/s
• Course thicknesses: courses 1–2 at t = 10 mm, courses 3–5 at t = 8 mm, courses 6–7 at t = 6 mm
• Each course height approximately 1.8 m (courses 1–6) and 2.0 m (course 7)

The design wind pressure from V = 35 m/s is approximately p_wind = 0.75 kPa (using a pressure coefficient of 1.1 and air density at standard conditions).

The approach is to transform the multi-thickness shell into an equivalent shell of uniform thickness equal to the thinnest course (6 mm), then compute the critical pressure for the full transformed height. Thicker courses contribute a shorter equivalent length proportional to (t_min / t_i)³, compressing the effective span. In a formal project calculation, this transformed height should be shown explicitly course by course so the check is traceable back to the API 650 construction-stability procedure.

For this example, checking the upper three courses (courses 5–7) as a span above a hypothetical ring at 7.2 m elevation: the unsupported height is 4.8 m, the thinnest thickness in that zone is 6 mm (courses 6–7). Using that transformed-shell procedure gives approximately p_crit = 0.82 kPa — which exceeds p_wind. This zone is stable.

Now check the full shell height with no interim ring: L = 12 m, using the transformed equivalent height for the full seven-course shell. With the thinnest course governing, the transformed height is longer than 12 m because the thin upper courses dominate. The resulting p_crit falls to approximately 0.41 kPa — well below p_wind = 0.75 kPa. The full height without a ring fails the check.

Adding an interim ring at 7.2 m elevation (top of course 4) splits the shell into a lower span (7.2 m, governed by 8 mm plate) and an upper span (4.8 m, governed by 6 mm plate). Both spans, when evaluated individually, yield p_crit values exceeding 0.75 kPa. The check passes with the interim ring in place at this elevation.

Coordination With the Fabricator

The interim stiffening ring must appear on the construction drawing — specifically, the shell erection drawing that the fabricator uses to cut and roll the shell plates. The drawing must show:

• The ring elevation above the bottom plate (measured to the ring centreline)
• The ring cross-section: flat bar, angle, or built-up section, with dimensions
• The weld detail: typically a continuous fillet weld on both sides of the ring flange
• If temporary: a clear note — for example, "INTERIM ERECTION RING — REMOVE AFTER PERMANENT WIND GIRDER AND ROOF RING ARE INSTALLED AND ACCEPTED"
• If permanent: designation as an intermediate wind girder on the tank GA drawing, included in the design analysis

Many construction incidents trace back to one of two coordination failures. First, the ring appears in the design calculation file but does not make it onto the approved construction drawing — the erection foreman never sees it. Second, the ring is on the drawing but without a clear removal note; the erection crew installs it and leaves it in place, and the completed tank is delivered with an unchecked structural element that was not part of the final design basis.

The engineer of record should confirm that the interim ring detail has been transferred to the issued-for-construction drawing set and that the erection procedure references the ring explicitly — including the stage at which it must be in place before the next course is erected.

For fixed-roof tanks, it is worth verifying whether the roof structure, once erected, provides sufficient stiffness to substitute for the ring. API 650 §5.9.7 allows the roof to serve this function if it meets the stiffness criteria — but the roof must be fully installed and the connection to the top shell course completed before the ring can be removed or its installation deferred.

Coordination between the structural engineer, the fabricator's detailer, and the erection contractor is the point at which §5.9.7 compliance most often breaks down. The calculation is straightforward; ensuring the result reaches the field is not.

Related reading: Continue with Wind Girder Design Guide, Fixed Cone Roof Design, and External Pressure Design to keep the full API 650 design workflow connected.