Unanchored tank modal analysis

[alishst]

Hi

Is the modal analysis of unanchored tanks (simply placed on the ground) with abaqus any reliable given the nonlinear nature of the problem? I've obtained some results for convective and impulsive modes but don't know if they are any valid or not.

 

[FEA way]

It should be possible to obtain valuable results for this case in Abaqus. This software is made for nonlinearities and it can handle fluid-structure interaction (or acoustic-structural coupling actually) too. How did you model the problem ?

There are some papers about it. For example these two:
- "Nonlinear Finite Element Analysis of Unanchored Steel Liquid Storage Tanks Subjected to Seismic Loadings" by H.N. Phan, F. Paolacii and P. Mongabure
- "Fluid-Structure Interaction Problems: An Application to Anchored and Unanchored Steel Storage Tanks Subjected to Seismic Loadings" by H.N. Phan, F. Paolacii

 

[Erik Panos Kostson]

we have discussed this and shown that acoustic elements can be used (it can be hard to identify modes, but they work).

So if it is not anchored you can let it sit on top of the ground and have frictional contact between the tank and the ground, and then run it with nonlinear transient dynamics say typically with seismic loads (you cannot use modal since it is a linear analysis and does not account for the complex contact between the two structures).


Now I have never done anything like that (measurements or FEA), so I cannot comment if the modes you calculate (probably free suspended tank), are close to the real modes when the structure rests on the ground - obviously the BC are quite different.


In any way seismic transient nonlinear transient dynamic analysis uses implicit time integration since it needs to account for all nonlinearities (contact, material geometry), and thus a mode superposition is not used, but an implicit scheme, so modes are not used.