6 Replies Latest reply on Jan 9, 2017 2:40 PM by Bill McEachern

    Solve Free-Body Diagram Using Reactions

    Martin Taylor
    Martin Taylor Dec 31, 2016 8:23 AM

      Hello!

       

      First time posting, hope you find my project interesting

       

      I'm designing a custom playground for a local park and need to calculate the tensions in some support cables.

       

      The internal structure consists of a rigid half-torus held in position by chains and cables in tension:

      POLYSTYROZOA_RENDER_FBD-3.JPG

       

      The half-torus will be covered in netting, allowing children to climb on it. Depending on the location of the children, the center of gravity of the basket system will move, causing the tensions in the cables to change:

      Tensions.PNG

      I have been struggling to solve the free-body-diagram system of equations in Matlab.

       

      I saw this video where springs were used to model tension cables. Would it be possible to set the basket to "rigid-body mode," add the springs, and view the reaction fores in Solidworks simulation?

       

      I can't even find the option for "rigid-body" in SW15. How to I begin setting up this problem?

       

      Thanks,

        • Re: Solve Free-Body Diagram Using Reactions
          Alessandro Tornincasa
          Alessandro Tornincasa Jan 6, 2017 12:43 PM (in response to Martin Taylor)

          Martin,

          very interesting problem.

          If you modeled your frame as structural members everything will be considered as beam elements. I'm not sure you can treat beams as rigid bodies.

          It works for sure with solid mesh: all you need to do is right clicking on a body in the simulation tree and choose " make rigid".

          In your case, if you want to use beams and considerazione them ad rigid you might define a custom material with very high young modulus (2 e+6 or 2e+7 MPa).

          Spring connectors can't be user with beams but only with solids and shells, you will have to find a workaround with mixed meshing.

          Another option would be modeling the cables as beams and make them trusses. Problem is that trusses resist compression: you would need to go non linear then and define a custom material that would resist only tension.

           

          Alex

            • Re: Solve Free-Body Diagram Using Reactions
              Martin Taylor
              Martin Taylor Jan 9, 2017 6:47 AM (in response to Alessandro Tornincasa)

              Alex,

               

              Thank you very much for the help!

               

              I took your advice and re-modeled the system with structural members and found the resultant forces. I accounted for the tension/compression issue by not fixing the cables that I assumed would be in compression.

               

              Here is the simulation for the "centered load" scenario, where the basket load is centered:

              CENTERED_BAKET_FEA-1.PNG

              CENTERED-ALLOY_STEEL+AL_CABLES_FEA.PNG

              This is an easy problem to solve by hand, so I was able to compare my results. The results were very close! 520lbs vs 620lbs.

               

              The problem is that the fixed ends of the cables create moments on the structure. I tried setting the cable materials to very flexible (elastic modulus 10000) but this crashes solidworks so I had to settle for 5000 series aluminum (modulus 7e+10).

               

              Is there some way to constrain the fixed joints in a single direction? So they produce no moment?

               

              Thanks again!

            • Re: Solve Free-Body Diagram Using Reactions
              Bill McEachern
              Bill McEachern Jan 6, 2017 2:24 PM (in response to Martin Taylor)

              You could do this easy enough in GEOSTAR if you have the Premium simulation license but it is a pretty old school sort of program so figuring it out is a bit of a challenge. You would not find a lot of help but there are a few souls out there that know some of how it works. I am not one of them as the last time i used it was in say the early 2000's. But I know it could do this problem with fidelity with beams and springs.