Hi,

I'm performing a (linear) buckling analysis on a tank filled with liquid.

I've fixed its legs and put variable pressure to simulate liquid load:

I'm using shell elements.

In this kind of tanks, the main concern is buckling of the legs, therefore it's important to perform FEA to determine optimal geoemtry and sheet metal thickness.

When I run the analysis I get all negative buckling load factors caused by pressure. Like this:

This tank is going to be emptied very slowly, therefore negative pressure is not expected, and so it won't buckle that way.

I need to find positive buckling load factors, but I can't because the max number of modes you can set is 200. The 200th mode is still negative.

My questions are:

- is there a way to "filter out" negative buckling modes ?

  In frequency for example there's an option to find frequencies above a certain value in order to exclude low frequencies (like rigid body motions). I was wondering if buckling had a similar option.

- Given the limit of 200 modes, how can you calculate positive BLF's

I was thinking about replacing pressure with a remote load with rigid connection: this way you can find some positive BLF's.

My fear is that this would make my buckling load factor results anyway invalid.

Any experiences with this situation or suggestions ?

Alex