CAESAR II and Rigid Elements: What is the point?Components that are part of a piping system, such as valves, flanges, and other elements, are much heavier (and stiffer) than the pipe that connects to them. How does CAESAR II handle this? By using rigid elements. Let’s take a look at what a rigid element really is and how it affects your piping stress analysis.
CAESAR II is a 3D beam element solver that computes the element stiffness matrices and load vectors and then solves for displacements, forces, moments, reactions, and stresses. The resulting information is compared against allowable stresses from the codes of construction. While code standard calculations evaluate the pressure-containing capability of all piping components in a piping system (usually through the specifications of listed components), the code standards do not attempt to evaluate mechanical load on non-pipe components (such as a gate valve).
Why not? Well, where is a system going to fail first, on the pipe, a valve, or a flange? Typically, it will be the pipe. Additionally, the geometry of non-pipe components are too varied and complex, so accounting for non-homogenous surface area calculations to determine failure on a component that has a very low chance of failing compared to pipe is just a moot point.
Enter the rigid element
This parameter can be applied manually using the Rigid selection box on the classic piping input screen.Click the selection box to turn it on and two important things occur:
The first is that you have a new parameter that the object carries, “Rigid Weight”. You can assign any number you want to the “Rigid weight” as it pertains to the object you are modeling. For example, if this element was simulating two flanges and a gate valve, you would sum those individual component weights and enter that value into this field.
The second thing that occurs is that your element is displayed differently in the modeling window. By manually entering the rigid element, I have converted element20-30 to a bright green color with some symbolic geometry so that I can quickly recognize this element during my analysis. Rigid elements can look differently so don’t worry about that too much right now.
So now that we have one, how does this affect the analysis?
Well, the job of the rigid element is to transfer stresses across the element. When CAESAR II computes the stiffness matrix during an analysis and solves for loads, the rigid will transfer the load from one side of the pipe to the other. This is a very useful property for modeling more complex types of analysis. For example, let’s take a look at modeling a vessel attached to piping in a CAESAR II analysis.
Here I have a vessel modeled as the large, vertical cylinder, and a nozzle attachment modeled as the two rigid elements connected to the horizontal piping run on the right. You’ll notice there is a
(thin, green element coming from the center of the vertical vessel extending to the flanged connection on the vessel side.
What is important to understand here is that rigid elements retain thermal expansion as a property even though their main purpose is to transfer loads during an analysis. So what I have effectively done in this case is accounted for radial expansion of the vessel by connecting my rigid element to the centerline of the vessel and extending it to the vessel’s radius.
There is more going on here but start to think about how you can apply this tool as a means to get more precise results from your analysis. Let’s recap and describe a few more properties about this tool:
- Rigid elements main function is to transfer stresses between nodes in a piping system.
- A rigid element can be weightless, or you can define it a specific weight.
- Thermal expansion can be modeled within a specific application.
- The inherent stiffness of the element for analysis is based on ten times the wall thickness of your piping system.
- CADWorx will automatically export all non-pipe sections as rigid elements with their weight that was assigned in the specification with the C2OUT command.
- Rigid elements will not “fail” during an analysis.
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