| Applies To | |
| Product(s): | AutoPIPE |
| Version(s): | ALL; |
| Environment: | N/A |
| Area: | Results |
| Original Author: | Bentley Technical Support Group |
| Date Logged & Current Version | Feb. 2017 10.01.00.09 |
Problem:
Why are there different Participation Factors and Captured Modal Mass results on different analysis runs where nothing has been changed in the Frequencies sub-report in AutoPIPE?
Solution:
Although a model may appear symmetric. l, see the frequencies are repeated. This happens for example for a vertical pipe supported on the ground. The first mode vibration can be in any horizontal direction, with same frequency. It will have one X and one Z mode with same frequency. But since X and Z are arbitrary for the structure, any two modes that are orthogonal will be fine and that is the reason the participation factors are changing and that should be fine.
If so concerned, AutoPIPE can force the modes to be in X and Z instead of skewed in XZ plane. That can be done if the model is updated with a slightly different support or anchor stiffness in X or Z. e.g instead of RIGID. Recommend using a value close to rigid value. The rigid value is in the Tools>Model Options> edit>.
Notice the participation factors are flipped from X to Z to make it orthogonal.