If someone can help me, I'm modelling a L shape (Cross Bracing) on a pipe rack structure which is vibrating significantly.
When I'm going to run the dynamic analysis to determine the natural frequency, The system requires to set the control paramethers, specially the one that says if it is a lumped mass or consistent mass.
Would someone explain me what is the difference between these 2 options and which is the one I need to use.........????? Is the system automatically giving me the lumped masses......??
Using consistent mass I got 9 natural frequencies starting with 2.5 Hz, which is the one I could merely visualize in the field.
I appreciate your support on this matter.
Regards.........

Historically, piping flexibility analysis has approached dynamics using the "lumped mass" forumulation. Typically this has been more than adequate.

However:
1) Theoretical (academic) type problems (such as a cantilever) have noticeable error in the eigen solution - especially with minimal nodes.

2) Some "real life" systems if not modeled with at least one node between supports or bends can also be in error.

Rather than try to perform some type of "node insertion" into these systems, we opted instead to go with the exact approach - "the consistent mass matrix". Read the "help text" on this option for details.


Usually the response of a piping system is dominated by the 1st mode. So that is usually what you observe.