Dear Singh,

I apologise for the word i used as "load case" in modal analysis. Actually i mean for static load case for non-linear restraint status. Till date there is no accurate solution to calculate the dynamic effect of non-linear restraint status, so we change it to linear and we select one of the static load case to carry on.

For a complex piping system with a chance of more non-linear restraints. In selecting the static load case for it, when i select the sustained case the 1st natural freq is say 4.5 Hz, when i selct the operating case without slug force the 1st natural freq is say 5.3 Hz, with slug force the freq is 3.1 Hz. So for non-linear conditions the natural freq is varying to great extent. My client requirement for 1st natural freq is more than 5Hz.

Any how we managed to keep the natural freq more than 5Hz in all static load cases by adding additonal restraints.

Regarding lumped mass, as per your reply, it is not generally recommended to exclude the concentrated mass in the static case.

Can you please explain with an expample for lumped mass usage.

Thank you.