Large Rotation Rods - Basic Model - CAESAR II - Reference Data

CAESAR II Applications Guide

Reference Data

Large rotation rods are used to model relatively short rods where large orthogonal movement of the pipe causes shortening of the restraint along the original line of action.

Large rotation rods can be entered in any direction. Use the Type list on the Classic Piping Input dialog box, to select the rotation rod: XROD, YROD, or ZROD. After the rotation rod is selected, the restraint options change as follows:

  • Gap changes to Len, which is the length of large rotation swing.

  • Mu changes to Fi, which is the initial load on the restraint if used to model variable support spring hanger. (Imagine the large rotation rod as providing a bowl in which the pipe node is free to move.)

Enter large rotation rods only where they are needed. Repeatedly using large rotation rods when they are not necessary can cause the system to become unstable during the nonlinear iteration. First, analyze the system without the large rotation rods, and then add large rotation rods where horizontal movement at support points is greatest. Usually, you should add only one rod in an area at a time.

The rod angle tolerance is currently set at 1º.

Large rotation is generally considered to become significant when the angle of swing becomes greater than 5º degrees.

As for any other support, connecting nodes can be used for large rotation rods. Graphically, the connecting nodes and the restraint node do not have to be at the same point in space. There is no plot connectivity forced between large rotation rod nodes and connecting nodes.

The plus or minus signs (+/-) on the large rotation rod determine the orientation of the swing axis. A positive YROD (+YROD) is equivalent to an YROD and indicates that the concave side of the curvature is in the positive Y-direction.


The rod pivots about the structural steel support. There is a very short swing arm, so even a small amount of horizontal movement produces a relatively large swing. The output report for this restraint shows X- and Y-direction loads.