18.104.22.168 Large Rotation Restraints
A large rotation restraint is a restraint which rotates away from its original line of action due to a pipe movement in a direction orthogonal to the original line of action. This may occur when the restraint is provided by a rod or a strut and the orthogonal displacement is large. Since the length of the rod must remain constant, the rotation from the original line of action is:
Since the length of the rod must remain constant, the pipe will be forced to travel through the arc defined by the rod rotating about its point of attachment to the structure. This is illustrated in Figure 3-104. Note that the effect of the swing is much more pronounced when using short rods.
These types of restraints are solved by initially doing the analysis with the restraints acting along their as-installed line of action. After the analysis is complete, the movement of the pipe at that location is noted. Based upon the movement of the pipe orthogonal to the line of action of the restraint, a new position on the arc of the restraint is calculated. The component of that position representing forced displacement along the line of action of the pipe is then used as an imposed displacement for the next analysis. After this analysis, the position of the pipe is checked again, the new position on the arc is recalculated, and the process is repeated, until the line of action of the restraint (and the position of the pipe) is unchanged (within a tolerance, which can be set by the user) from one analysis to the next.
Large rotation restraints are entered in CAESAR II through the use of the term XROD, YROD, or ZROD. The direction in which the rod runs from the pipe is designated by the sign (+ or -, where no sign is taken to be the same as +). This type of support can be used to model supports where load bearing is sensitive to the angle of the line of action.
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