Combining Models in CAESAR II

There are instances where a single CAESARII input file contains several, separate pipe models (runs). Possibly, these runs are from different lines, or possibly they are part of the same line and need to be connected. This article discusses the latter condition, how to properly connect separate models in a single input file.

The situation of separate models can occur in several ways:

- The model consists of piping and structural steel elements.

- The model consists of piping and vessels, where the vessels are actually modeled.

- Several pipe runs are being combined into a single model, possibly from a CAD interface.

To begin, assume all elements (pipe, structure, or vessel) are available in the current input file. At this point, there is no connection between these distinct models. Attempting to run the analysis will result in the solution for distinct (i.e. not connected) models. Attempting to plot the input shows all models emanating from the plot origin (the start node of the first model). These results are correct and predictable, because CAESARII has not been informed as to how the models connect to each other. For example, consider an cantilever composed of two halves, anchored at 10. In this model, nodes 40 and 50 are supposed to connect to each other, i.e. they are the same point in space.

1st half: 10--------20--------30----------40

2nd half: 50--------60--------70----------80

If the user plots this model, as it is, the result is:

Result: 50--------60--------70----------80

because both sections plot on top of each other, starting at node 10. If the user specifies the coordinates of 50, such that they are the same as 40, the resulting plot is:

Result: 10--------20--------30----------50--------60--------70----------80

Specifying the starting coordinates for each distinct model will produce correct plots - however, the analysis still sees distinct unconnected models. Positioning models in space relative to each other has no effect on the global stiffness matrix! The models must be tied to each other from a "stiffness" point of view. The above model of the cantilever is still incorrect.

There are two ways to properly tie models together, from both a geometric (plotting) and a stiffness (computation) point of view. The first and easiest method is to change one of the node numbers. For the example cantilever, changing node 40 (on the 1st half) to node 50 will produce the proper connection. Similarly, (on the 2nd half) changing node 50 to node 40 will also produce the proper connection. The connection formed in this manner is as if the two halves are welded together, all six degrees of freedom are connected.

The second, and more powerful way of connecting two models is via the "Restraint with CNODE" option. This option will connect the geometry for plotting purposes, but the stiffness connection can be specified individually for each of the six degrees of freedom. For the above cantilever, restraining 40 with a CNODE of 50 in [A]ll directions produces the same, proper connection as changing the node numbers did. A hinge could be defined by restraining 40 with a CNODE to 50 three times, once in the X direction, once in the Y direction, and once in the Z direction. (Note that a hinge in a cantilever is an unstable model!) The "Restraint with CNODE" option is most useful when connecting pipe to steel using "resting" (+Y) supports.

The "Restraint with CNODE" option is a very powerful, flexible feature. This subject is discussed in detail in the CAESAR II manuals.

The important thing to remember about connecting models is that a correct geometry plot is a necessary, but not a sufficient condition for a correct stress analysis.


[Go Back]