Engineering task and calculation objective
The UG29 module sizes stiffening rings for cylindrical shells under external pressure to ASME BPVC Section VIII Division 1, paragraph UG-29. Stiffening rings divide long cylinders into shorter unstiffened sections and thus raise the allowable external pressure considerably – a stiffened shell of moderate wall thickness is often more economical than an unstiffened shell with a heavy wall. Typical applications are vacuum columns, heated jackets and long horizontal vessels subject to external pressure.
The core of the check is the moment of inertia: the code requires that the actual moment of inertia of the ring cross-section – either the ring alone (I) or the combined section of ring plus contributing shell width (I') – is not less than the required moment of inertia Iₛ or I'ₛ, respectively. The module calculates both cases and directly compares required and actual values.
This allows you to calculate a stiffening ring for external pressure in full compliance with the code and, at the same time, to document whether a chosen profile (flat bar, angle, T-section) together with the contributing shell portion is sufficient or needs to be reinforced.
Standard and calculation basis: ASME BPVC VIII-1 UG-29: 2025
Calculation workflow
- Define ring arrangement and effective lengths: The position of the rings divides the shell into unstiffened sections. Each ring is assigned its effective length Lₛ – normally half the distance to the adjacent lines of support on both sides.
- Determine factor B for the ring cross-section: From the external pressure, outside diameter, wall thickness and the cross-sectional area of the ring, the governing stress parameter is formed, from which factor B and the associated strain factor A are determined via the material-temperature chart.
- Calculate the required moment of inertia: The code provides two required values: Iₛ for the ring alone and the smaller I'ₛ for the combined section of ring plus contributing shell width. Both depend on Dₒ, Lₛ, wall thickness, ring area Aₛ and factor A.
- Determine the actual moment of inertia: For the chosen ring profile, the moment of inertia is calculated about the neutral axis parallel to the shell axis – for the combined check including the contributing shell width of 1.1·√(Dₒ·t) and the common centroid.
- Perform the check: The check is satisfied when the actual moment of inertia reaches or exceeds the required value. The module compares required and actual moments of inertia for the ring + wall case; otherwise a stiffer profile or a smaller ring spacing must be chosen.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Calulation pressure | p0 | MPa(p) |
| Calculation temperature | T0 | °C |
| (Required) thickness | s | mm |
| Outside vessel diameter (without ring) | D0 | mm |
| Vessel material | Grundkörper | – |
| Required moment of inertia stiffening ring | Is | mm^4 |
| Required moment of inertia of ring + Wall | I's | mm^4 |
| Factor A (Design Variable) | A | – |
| Factor B (Design Variable) | B | MPa |
| Actual moment of inertia of stiffening ring | I | mm^4 |
| Actual moment of inertia stiffening ring+Wall | I' | mm^4 |
| Half the distance from the center of the neigh- bouring support on one side to the center of the neighbouring support on the other side | Ls | mm |
| Cross-sectional area of stiffening ring | As | mm² |
| Ring material | Verstärkungsring | – |
| Strength | K | MPa |
| Applicable material chart | Fig | – |
| Modulus of elaticity | E | MPa |
| Strength | Kr | MPa |
| Applicable material chart | Figr | – |
| Modulus of elaticity | Er | MPa |
| Minimum yield stress | Sy | MPa |
| Minimum yield stress | Syr | MPa |
| Width of reinforcement ring (radial) | RH | mm |
| Thickness of reinforcement ring (axial) | RB | mm |
Frequently asked questions
When are stiffening rings worthwhile compared with a thicker wall?
The allowable external pressure of a cylinder rises as the unstiffened length decreases. For long, slender vessels (large L/Dₒ) every additional line of support noticeably reduces the required wall thickness. Rings are economically attractive above all for vacuum vessels and jackets, where wall thicknesses far beyond the internal pressure requirement would otherwise be needed.
May the shell be included in the moment of inertia?
Yes, UG-29 permits the check with the combined section of ring plus a contributing shell width. In that case the somewhat larger required moment of inertia I'ₛ applies (factor 10.9 instead of 14 in the denominator). Important: ring and shell must be connected so that they act as one cross-section – the attachment weld must satisfy the requirements for stiffening rings (UG-30).
What has to be considered for cut-outs or interruptions in the stiffening ring?
Stiffening rings are meant to stiffen the full circumference effectively. Interruptions, for instance at saddle supports or for drain openings, are permitted only within the arc lengths limited by the code, or they must be bridged by moment-transferring members. An interrupted ring without adequate bridging must not be counted as an effective line of support in the calculation.
Does a stiffening ring automatically count as a line of support for the shell length in UG-28?
Only if it satisfies the stiffness check to UG-29. The unstiffened length L in the shell calculation to UG-28 may only be measured between effective lines of support. A ring that is too flexible does not reduce the buckling length – a common error when rings exist on the drawings but were never verified.