14 Replies Latest reply on Mar 18, 2017 1:57 PM by Alessandro Tornincasa

    How to get a positive BLF when analizing Tanks filled with liquids

    Alessandro Tornincasa

      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:

      Loads1.PNG

      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:

      Cattura.PNG

      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

        • Re: How to get a positive BLF when analizing Tanks filled with liquids
          Bill McEachern

          you could try reversing the load and then hope for a negative one.

          • Re: How to get a positive BLF when analizing Tanks filled with liquids
            Ryan Dark

            Hi Alessandro,

            Looking at your setup, if you are not interested in buckling of the tank itself then why are you keeping the tank in the analysis?

            • Re: How to get a positive BLF when analizing Tanks filled with liquids
              Jaja Jojo

              if your interested in the LEGS why is the THIGH is right there? you could remove to shorten analysis

              • Re: How to get a positive BLF when analizing Tanks filled with liquids
                Duarte Vasconcelos

                If you're interested on the legs, remove the vessel walls and concentrate on the legs.

                 

                Linear buckling is similar to frequency analysis. You can almost relate natural frequencies to buckling modes.

                 

                The stiffness of the walls is much smaller than the stiffness of the legs, and you'll get to many buckling modes before you can see any modes on the legs. As it will happen with frequency.

                So the easiest way is to simply isolate the legs. That's one of the main assumptions with FEA. Leave only what you want to analyze.

                 

                Now, regarding the legs, the vessel is symmetrical, as are the loads. Without making any study, I would say that the first modes will be: tangential rotation through the center of the vessel, axial up and down displacement. And because the legs are tubes, perhaps some bloating and collapsing of the tubes (like the vessel walls) can also be some of the early modes.

                 

                This type of model will collapse if there is an imbalance in the force, or on the support under the legs.

                I would advise that you apply the force/pressure in a way that would cause that imbalance, and see the resulting buckling modes.

                 

                These analysis should be done with the complete base of the vessel, but without the vessel walls and upper part.

                 

                But I also agree with you, and I would rather perform a non linear buckling analysis (arc length method). It would be much more accurate, and you'll probably observe a buckling mode not shown by linear buckling analysis.

                And as Ryan said, I would analyze a leg individually with NL analysis, taking advantage of the vessel symmetry.

                Look at the take a look at exercise 1 of lesson 3, of the SW Simulation Premium Nonlinear training manual for reference.

                  • Re: How to get a positive BLF when analizing Tanks filled with liquids
                    Bill McEachern

                    Reducing the model to just the legs if the tank is much less stiff than the legs seems like a sensible idea. I would do a linear buckling estimate of the complete leg assembly with a vertical load to see what sort of modes are likely and then do a NL arc length or displacement control analysis - I doubt there is any snap back that would be of interest and the arc length control has been a bit busted since like 2012 or so - seems to take inordinately long time to solve. They may have addressed it in later releases. Either way, you typically need to add some out of plane displacements/loads in an NL analysis of this type if they will not develop on their own which may not be the case here as the legs look tapered. For other modes to emerge (like an axial rotation) it might be required. It doesn't hurt in any case as they are insignificant in the big scheme of things.  I would add some trivial load to any surface involved in a low order linear buckling mode. This will allow the out of plane forces to develop. I would strongly discourage using any sort of symmetry model as it will suppress any non symmetrical modes, though in this case, if it is leg crippling that initiates the failure it wouldn't matter much. Then again I can't really tell from the image exactly what you are dealing with here in terms of leg construction like whether its an  enclosed tube or 3 plates with a side open.

                    In other codes there is often a capability to add very small displacements from a frequency analysis or buckling analysis to impose non perfection into a model so that out of plane forces develop naturally. Though, I don't think the jury if fully in on how well it mimics actual imperfection.

                  • Re: How to get a positive BLF when analizing Tanks filled with liquids
                    Chris Clouser

                    I think everybody needs to back the heck up a step or two.

                     

                    Simulation DOES NOT WORK for non-Euler buckling in static analysis!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

                     

                    Ok?

                     

                    I can't speak to it's non-linear capabilities because I use Nastran, but I would probably start there.

                     

                    You've been warned.

                      • Re: How to get a positive BLF when analizing Tanks filled with liquids
                        Alessandro Tornincasa

                        Agree, you can find this solution in the KB:

                         

                        What is the theory used by the linearized buckling analysis of SolidWorks Simulation?

                         

                        The theory used by the linearized buckling analysis of SolidWorks Simulation is Euler buckling.

                         

                        Non Linear Buckling

                         

                        One major difference between nonlinear buckling, and the linear (Eigenvalue) buckling, is that nonlinear buckling phenomenon includes a region of instability in the post-buckling region whereas linear buckling only involves linear, pre-buckling behavior up to the bifurcation (critical loading) point.

                      • Re: How to get a positive BLF when analizing Tanks filled with liquids
                        Alessandro Tornincasa

                        Thank you all for your toughts and comments, they all come from sound reasonement and make a lot of sense. I will follow your suggestions.

                         

                        It was a nice, practical, and constructive conversation on a real design issue that could occur on that type of tanks.

                         

                        Alex