January 2019 update: stiffening rings definition is vastly improved and simplified with NextGen version 2019. The following article applies to versions up to 2018.3.
Stiffening rings (or stiffener ring, stiffeners) are components positioned externally or internally to prevent collapse under external pressure load. Rings are considered "effective reinforcements" and contribute reducing "maximum unsupported length" (or "buckling length") of a shell or item, which is the maximum length on said shell without any element considered effective against buckling. These elements are usually body flanges, heads, and rings.
With NextGen, a stiffening ring can be modeled starting from its section shape. This section can be composed by one, two or three rectangles combined. As said, the ring can be placed internally or externally. Central rectangular section can be placed right, left or centered and this helps to cover all the common configurations.
User should keep in mind that ring rectangular sections are enumerated counting from the closest (welded) to the vessel's surface, moving towards the farthest. Welded section is always the one named "bottom plate". If a ring has a single section, values for "bottom plate width" and "bottom plate height" must be filled. If a ring has two rectangles in its section also "mid plate" height and width must be filled.
Calculation codes validate rings comparing its second moment of area with a required value. To maximize this value it is necessary to displace more surface as possible far from the reference axis (vessel surface). When small values of the second moment of area must be balanced, a single vertical plate can suffice. For larger values, a horizontal flange should be added on top of the vertical plate.
Here are some common configurations and how to obtain them with NextGen.
To obtain a ring having a single vertical rectangular section, "bottom plate base" should be filled with plate's thickness while "bottom plate height" with its height.\ Keep in mind that even if in the component image a three sections ring (I-beam) is displayed, a single rectangular section can have its base smaller than its height.
A "T" shaped ring is made using two rectangular sections, one for the vertical part and one for the topmost horizontal one. Top flange helps to maximize the second moment of area of ring section. It can be designed as follows:
This ring has the same values of the "T" shaped one, except that its mid plate is placed to the left with respect to top flange.