15 Replies Latest reply on Apr 24, 2013 11:33 AM by Jared Conway

    Analyzing tube frame in SimulationXpress

    Adam Johnson

      I'm fairly new to using SimulationXpress so I need some help. I want to analyze the tube frame shown in the picture. The loads will be applied to the top tube and the lower tubes will be sitting on the ground. How do I model this part so that it will allow the lower tubes to spread apart creating tension in the lower members? I am running Solidworks 2010 profesional with SimulationXpress. Any help or suggestions would be appreciated.

        • Re: Analyzing tube frame in SimulationXpress
          Ryan Werner

          Hi Adam,

           

          You can create a Split Line on the bottom faces of the bottom tubes to restrain to.  Then only restrain it in the vertical direction and that should allow them to separate.  Hopefully it will run with just that restraint but the model may "move" since there would be no restraint keeping it from doing so.  If that happens you may have to decide on a way to restrain it in the other directions as well.  You can do something similar on the top tube to apply your loading to if you want.

           

          Ryan W.

          • Re: Analyzing tube frame in SimulationXpress
            Derek Bishop

            Does Simulation Express allow you to define constraints using reference geometry? If not, I think you will be struggling.

              • Re: Analyzing tube frame in SimulationXpress
                Jan Van Leeuwen

                SimulationXpress can still be used to do the job if you look at it a bit more specific I would say. First, it looks like the model is symmetric. So that can be used. Since you want the load acting on the top tube it will create two equal reaction forces half the size from the floor to the two bottom tubes. Then as you say indeed these two tubes like to spread and put tension as well as bending moment on the bottom connecting tubes. The outside tubes will rotate just a little bit namely. Therefore the bottom tubes cannot be restraint. Also since the connecting tubes will bend they tend to move down towards the floor till they also hit the floor. It is hard to see at the picture but it seems that the connecting tubes are smaller in diameter so there is play between the floor and these tubes. Therefore we cannot put a narrow cut-extrude in the centre of the connecting tubes to restraint the 4 tube-faces then exposed. Because such a restraint would not allow the connecting tubes move down towards the floor during simulation. (it is to be expected that the center cross section of the connecting tubes will stay perpendicalar to the floor though).

                So finally here is what I would do:

                1. use a split line to split a top section of the top tube and put the restraint in Xpress on that surface.

                2. use a split line to split off bottom sections of the two bottom tubes right where they are expected to hit the floor.

                3. put the load upwards on the two bottom faces created under item 2 and hit the combined button as well as use normal to horizontal face.

                  • Re: Analyzing tube frame in SimulationXpress
                    Derek Bishop

                    I suspect that would be a reasonable approximation but the deformation of the top member wuld be a bit out.

                    • Re: Analyzing tube frame in SimulationXpress
                      Adam Johnson

                      You are correct. There are 2 different sizes of tubes being used. I've attached a pdf showing the model I think you were describing. Let me know if this is what you meant. I've also attached the stress graphic. Does this make sense? This is the best case where the entire load is spread over the top tube. As we know the impact load from a concentrated load will cuase the most problems.

                        • Re: Analyzing tube frame in SimulationXpress
                          Jan Van Leeuwen

                          Hi Adam,

                           

                          Yes this is what I meant. Now there is one thing. Since we used a fixed restraint on the split surface of the top tube, we actually made that tube portion completely rigid. You have to consider whether that is true in practice. I do not know the details of the construction or load application on top.

                          For instance if you have some kind of halfpipe sitting on the top tube without being welded it might ask for another approach. The top tube tends to get into an elliptical shape as you can see in the stress plot therefore the radius of the splitted top surface likes to get bigger and bigger. In practice that might happen if there is no weld or such and if the halfpipe is much more stiff than the top tube. You can assume then that the load will get split in two halves between the 2 halfpipe edges and the top tube. In that case we cannot use the rigid restraint.

                          Depending on the above, the stress values near the fixed restraint can be considered as realistic, otherwise they are partly a result of our restraint method.

                    • Re: Analyzing tube frame in SimulationXpress
                      richard klassen

                      Hi Adam

                      One way to get around the fixed degrees of freedom, is to add shear release tab.   This requires knowledge of how you expect the model to deform.  The shear release tab is thin in the direction you expect deflection.  For biaxial release,for instance, use a long slender rod as a support. Be carefull that the tab does not add strength to the model.  RMKTestPart-SimulationXpress Study.analysis.jpg