Bob,

Thank you very much for all your suggestion. It helps me a lot.

I agree that the best solution for this case is to restrict the line avoiding high forces and moments during operation (neutral points). The problem is that not always you have stell structure to support in the point you need.

This system is one of severals that I need to meet the frequency requerement (mainly lines of flare and those connect to reciprocal machines). Imagine how many support I will save if instead of using 0 I uses 1 (for example) for the stiffiness friction for factor. The frequencies are very sensitive to this factor.

Anyway, considering the lack of information on this friction effect modal analysis issue I think I will follow your instructions.

Regards,
Roberto