Dear Dave and Richard,

With your above discussion on modal analysis, please solve the following queries,

I am doing a vibration check for a slug flow line using static equivalent approach (DLF=2) and modal analysis. I applied the slug force on every elbow of the piping after the source of slug force (Say after PCV). My vibration frequency limit is 5Hz and above. For static case, all is fine. When i check for modal analysis, the starting frequecny is less than 1Hz for friction stiffness is 0 and 2Hz for friction stiffness is 1000 (as per Mr. Dave instruction), in both the cases i could not reach the frequency atleast 4Hz. In my model there are many places where the rigidty is less, to increase the rigidity i have to increase the number of supports and restraints.

My query are

1.In the first modal analysis, my starting frequency is 2 Hz (friction stiffness = 1000), so to increase the frequency to 5 Hz, can i able to find a point in the Dynamic output result where the frequency of the paticular node is less, so that i can increase the rigidity of that paticular portion of piping. (To be clear in my query, there are many portion of piping system where the rigidity is less due to non-availability of support location, so using the dynamic analysis - Frequency result, can i able to point out, in which point or part of the piping portion the natural frequency is less).

2. In my case, the strum check failure occurs, can i ignore it or not? If not how to rectify it?

3. Under the control parameters i selected the load case for non-linear restraint as Max. Operating condition with slug forces. Is the frequency shown in the dynamic output consider those slug forces or not?

Please clarify.....