9 Replies Latest reply on Feb 14, 2014 1:30 AM by Jared Conway

    Real CG & SolidWorks CG

    Lakhvinder Singh

      Hi All,

       

      Please see attached file. I am trying to model a container which has components inside it (not shown/modeled here as we do not need to analyse those components). I need to apply accelerations to the ACTUAL REAL LIFE CG of the container which is shown on the upper side. But when I apply accelerations, SolidWorks apply accelerations to the CG location provided by SolidWorks (shown on the lower side).

      Is there any way I can make a new ACTUAL REAL LIFE CG and apply accelerations to it and conduct a simulation? Any ideas?

       

      Thanks in advance.

       

      Regards,

       

      LS

        • Re: Real CG & SolidWorks CG
          Jared Conway

          how are you applying accelerations?

          what type of study are you running?

          • Re: Real CG & SolidWorks CG
            Shaun Densberger

            There are two ways you can approach this:

             

            1. You can add in remote masses at the CG locations for your components and connect them to the respective local areas on your frame.
            2. You can add in distributed masses over the respective local areas of your frame.
            3. You can calculate out what the equivalent loads would be for a give acceleration and apply those over the respective areas (make sure to still add in gravity to load the frame's mass).

             

            Which one of these you will use depends on how the components are connected to the structure. For the items above, you need to take the following into account:

             

            1. Unfortunately, it looks like SW only allows you to use a rigid connect between a remote mass point and another point/edge/surface, so you'll only want to use the first method if your connection points between the frame and masses are over a small area. Any surface or edge you tie your remote mass to becomes infinitely rigid. This can cause a large amount (or even an infinite amount) of unreal stiffness.
            2. If you use the distributed mass feature, then you're not going to capture the moment of inertia associated with the masses.
            3. The big downside to applying equivalent loads is that you have to manually calculate them out. This becomes a bigger pain if you need to check several acceleration directions.
            • Re: Real CG & SolidWorks CG
              Lakhvinder Singh

              Shaun/Jared,

               

              This is a static study. Most of the times we need to analyse a structure provided by the client. The client will only give the ACTUAL CG of the structure, and would not give details of the components inside. So what we do is we apply accelerations of 1 g to the mass of the structure (at the CG location).

               

              1. Adding remote masses at CG locations of each components is not possible, as the client does not have that data. They would only give the actual CG location, with the maximum gross weight of the structure/container/equipment.

               

              2. In a beam analysis/stick figure program like STAAD.Pro we normally make the structure and additionally make a small heavy beam at the location of the CG to simulate the mass of the structure. We then apply accelerations to this CG location. Is there something similar we can do in SW Simulation?

               

              Regards,

               

              LS

                • Re: Real CG & SolidWorks CG
                  Shaun Densberger

                  OK, given the limited information that you have, let's take a step back. What type of results are you looking for, and how accurate do they need to be? If all you know is the location of a CG and the design of the structure (but not how the any of the components connect to the structure), then you're results are going to have a lot of uncertainty (regardless of the numerical convergence of an analysis).

                   

                  The methodology you gave when using STAAD can be used in SW as well. However, based of what you said, it's hard to say if that method is even applicable to what would physically happen. In SW, you could use with a beam element or a small 3D element to capture the mass of your CG, but now you're left with the issue of "how and where" do you connect that CG mass to the structure. This is partly a FEM issue, and partly a design issue. The "where" part is tied to the mechanical design. Are the components (i.e. the net CG) connected near the corners of the frame, or are they in the middle of the beam? The "how" part is tied to the mechanical design as well, and the concern is "how stiff are the components making up the CG?" Are the components making up the CG very stiff, like a car's engine block would be with respect to the car's body, or are the components relatively flimsy? The answers to these questions effect how you model the system to get applicable results.

                  • Re: Real CG & SolidWorks CG
                    Jared Conway

                    Let's get more general, what is he application? What are the accelerations coming from? What do you want to learn?

                  • Re: Real CG & SolidWorks CG
                    Lakhvinder Singh

                    Jared,

                     

                    The container has to be lifted in an offshore enviroment in rough sea conditions, by a crane located on an another vessel. In these conditions, the container can see high acceleration values. DNV or ABS classification society codes specify that accelerations values of 1g are applied, including factored load to do an analysis.

                     

                    Regards,

                     

                    LS

                    • Re: Real CG & SolidWorks CG
                      Lakhvinder Singh

                      Attached is the spec. as requested.