23 Replies Latest reply on Sep 23, 2014 7:49 PM by Jared Conway

    Reaction Force (Again)

    Mike Armstrong

      Hi all,

       

      I have modelled an offshore support frame and applied an external force and gravity to include the self weight of the frame. (Please note the default mass properties have been overridden to account for missed components). However, when I assess the reaction forces underneath the 15mm doubler plates, the reaction in the Y-axis is greater than the applied load + self weight of the frame.

       

      Below is an image of the doubler plate and reaction forces. These doubler plates will be secured onto the deck of a barge by means of a fillet perimeter weld, hence the fixed restraint in all directions in the static simulation.

       

      Clipboard01.jpg

       

      Does anyone have an explanation why these forces are greater than the applied loads?

       

      Thanks.

       

      Mike

        • Re: Reaction Force (Again)
          Gregory Halleck

          HI Mike.  I believe you are seeing a mechanical advantage due to an offset force from your anchor (or pivot) point.  If you decouple your moment at the anchor point you would see higher forces than the applied.  You can double check your analysis by creating a simple FBD (free body diagram) on paper and comparing your results with the simulation.

           

          ..or I could be completely wrong and need another cup of coffee :-)

           

          2014-09-08 09_38_09-SolidWorks Premium 2014 x64 Edition - [Offshore Support Frame  _].jpg

          1 person found this helpful
            • Re: Reaction Force (Again)
              Gregory Halleck

              Mike - after looking at the FEA results a little more, I found an odd discrepancy between the FB force and Reaction force.  The FB force appears relatively symmetric..while the reaction force is not.  It could be the frame supports are producing an asymmetric loading condition.  Just something to consider.

               

              free-body-force.jpgreaction-force.jpg

              -Greg

              1 person found this helpful
                • Re: Reaction Force (Again)
                  James Riddell

                  There has to be a shear force on the surface in the Y(?) direction to counter-act the moment generated by the overhanging mass.

                  • Re: Reaction Force (Again)
                    Mike Armstrong

                    Gregory Halleck wrote:

                     

                    Mike - after looking at the FEA results a little more, I found an odd discrepancy between the FB force and Reaction force.  The FB force appears relatively symmetric..while the reaction force is not.  It could be the frame supports are producing an asymmetric loading condition.  Just something to consider.

                     

                    free-body-force.jpgreaction-force.jpg

                    -Greg

                    Greg, the restraints are not symmetrical so I would have expected the reaction forces in the plates to be different. However, I still would have thought that the reations would have equalled the applied load, but as you said above, it may be a result of mechanical advantage.

                     

                    Mike

                  • Re: Reaction Force (Again)
                    Mike Armstrong

                    Gregory Halleck wrote:

                     

                    HI Mike.  I believe you are seeing a mechanical advantage due to an offset force from your anchor (or pivot) point.  If you decouple your moment at the anchor point you would see higher forces than the applied.  You can double check your analysis by creating a simple FBD (free body diagram) on paper and comparing your results with the simulation.

                     

                    ..or I could be completely wrong and need another cup of coffee :-)

                    Cheers Gregory, I will have a look in the morning.

                     

                    I am sure you are correct.

                  • Re: Reaction Force (Again)
                    Mark Ankrom

                    Mike,

                     

                    The summation of reaction forces has to equal the summation of applied loads.  Common problems are

                    1. Solver is not seeing the same mass as the design model - check the .out file.

                    2. Solver is seeing loads that are not apparent in the user interface - again look at .out

                    3. There is a reaction that you are not accounting for.

                    4. A badly conditioned FEM model or a solver error.

                     

                    I tried to look at you model.  I downloaded the zip file and was able to look at the design model but there was no study included in the file.  What version of SW are you using ?  I only have SW2014.  I will take a look at your model if I can open the study.

                     

                    Mark

                    1 person found this helpful
                      • Re: Reaction Force (Again)
                        Mike Armstrong

                        Mark Ankrom wrote:

                         

                        Mike,

                         

                        The summation of reaction forces has to equal the summation of applied loads.  Common problems are

                        1. Solver is not seeing the same mass as the design model - check the .out file.

                        2. Solver is seeing loads that are not apparent in the user interface - again look at .out

                        3. There is a reaction that you are not accounting for.

                        4. A badly conditioned FEM model or a solver error.

                         

                        I tried to look at you model.  I downloaded the zip file and was able to look at the design model but there was no study included in the file.  What version of SW are you using ?  I only have SW2014.  I will take a look at your model if I can open the study.

                         

                        Mark

                        Hi Mark,

                         

                        Thanks for the pointers. I did not upload the simulation results due to size. I will upload tomorrow morning when back in work. I take it the .out file is outputted when a simulation has been undertaken.

                         

                        I am also using Solidworks 2014.

                         

                        Mike

                        • Re: Reaction Force (Again)
                          Mike Armstrong

                          Hi Mark,

                           

                          I cannot attached the .CWE file as it is too large,.

                           

                          Mike

                        • Re: Reaction Force (Again)
                          James Riddell

                          Mike,

                           

                          First, if your only connection of the plate is a perimeter weld then you should not be restraining the complete under surface.  If the weld bead is mostly the thickness of the plate then you could use those edge faces.  Otherwise, I'd suggest just using the edges.  As Gregory & Mark say, look at your FBD.  I'd expect some difference but I've had good luck with using the resultant forces to get a sanity check on the solution numbers.

                           

                          Jim

                            • Re: Reaction Force (Again)
                              Mike Armstrong

                              James Riddell wrote:

                               

                              Mike,

                               

                              First, if your only connection of the plate is a perimeter weld then you should not be restraining the complete under surface.  If the weld bead is mostly the thickness of the plate then you could use those edge faces.  Otherwise, I'd suggest just using the edges.  As Gregory & Mark say, look at your FBD.  I'd expect some difference but I've had good luck with using the resultant forces to get a sanity check on the solution numbers.

                               

                              Jim

                              Hi JIm,

                               

                              I have applied the restraints to the edge faces of the doublers plates. Although not entirely correct, it will be a good approximation for this analysis.

                               

                              Mike

                            • Re: Reaction Force (Again)
                              Jared Conway

                              I think mark's got it right here

                               

                              let's look at the whole structure's input force + weight and compare it to the reaction force total (all fixed locations selected)

                               

                              check the .out file to make sure the weight is what you expect it to be

                               

                              and then do some sanity checks for yourself to show that the software is working as expected. build a really simplified model and check that force in = force out.

                               

                              something I did want to mention is that if you are grabbing restraints individually and adding them up to try and equal out to the total input load you may run into issues if there are any shared nodes. check the solidworks kb for some good examples.

                              1 person found this helpful
                                • Re: Reaction Force (Again)
                                  Mike Armstrong

                                  Jared Conway wrote:I think mark's got it right here

                                  let's look at the whole structure's input force + weight and compare it to the reaction force total (all fixed locations selected)     

                                  Thanks for the response Jared,

                                  Which faces are you referring to on the restrained plates?

                                  check the .out file to make sure the weight is what you expect it to be

                                  How do I access this?

                                  and then do some sanity checks for yourself to show that the software is working as expected. build a really simplified model and check that force in = force out.

                                  I have done this and it seems the software is producing correct results, so as thought, the issue probably lies with the operator