Hi all,

I´m doing a dynamic analysis (earthquake spectrum) for buried pipelines. In the dynamic output show an error message: "Model is dynamically unstable. (Rigid body modes) Need some extra restraint to prevent infinite sliding and/or rotatation." What does it mean? Is it related to non-linear restrain?

Many thanks and regards!
Cesar Moreno

You can't have a non-linear restraint in dynamics. CAESAR II uses the static load case you reference (on the Control Parameters dialog) to linearize the restraints. This means "if the pipe isn't against the restraint in the referenced load case, the restraint is removed". This can lead to rigid body motion.

Somehow you need to restrain all six degrees of freedom (not necessarily at the same point though).

Ok Mr. Richard, I already restrain all six degrees of freedom in the system and doesn´t show the error message, but when the dynamic run begins for Earthquake (Spectrum), the Sturm sequence is failure because the first frequency calculated exceeds the cutoff frequency. There are too high!! This is the spectrum data points:

N Range (s) [Linear] Ordinate (mm/s2) [Linear]
1 0.01 16314.03
2 0.10 33010.65
3 0.20 39298.86
4 0.30 34668.54
5 0.40 29753.73
6 0.50 25015.50
7 0.75 17295.03
8 1.00 12870.72
9 2.00 5395.50
10 4.00 1912.95

What does it mean?
I appreciate your help...

Cesar Moreno

A "Sturm Sequence" failure simply means that there is another frequency withing 5% of the last frequency extracted. This may or may not be a problem - that is for you to decide.

When it is a problem? Which are the criterion?

Many thanks and regards!
Cesar Moreno

Read this earlier post.