Dear Ray

Thanks for your reply.

Yes, I was calculating natural frequency using the numeric method from the published paper by C.E. Freese and PV-Elite was calculating using eigen solver. Now I could confirm my calculation.

As you have correctly mentioned, vibration is actually less in vessels supported on legs, problem is during calculation of seismic force as per ASCE 2010, where natural frequency is the critical parameter for calculating seismic co-efficient.

I understand that, it is generally client's or designer's responsibility to select the method. But, I would like to know whether there is any standard practice followed by industries about selecting method.

Regards
Prudhvi Teja