Getting Zero's Natural Frequency for the first 4 modes ; any idea why this happen

[rharbi]

Hello all,

I'm working on piping modal analysis through ANSYS but the surprise is that I got 4 zeros natural frequencies and very low 5th and 6th values. Although I put all supports boundary conditions.

I would highly appreciate your guidance and comments

Regards,

 

[Tmoose]

more info needed. zeros means very soft supports. rigid body motion. Did you use some kind of "soft spring " option to stabilize a cranky model?

 

[rharbi]

Thanks Tmoose

Indeed it was like free free model. DO you know how could I estimate piping elastic support stiffness correctly;

Second question shall I consider adding point mass in modal analysis; I'll not perform any static or harmonic analysis jsut modal analysis