2 Replies Latest reply on Dec 30, 2014 9:58 PM by Jared Conway

    Trouble setting up correct boundary conditions

    Ian Fay

      Hi, I am trying to setup boundary conditions on a quarter section of a symmetric steel sub-frame for a cast iron boiler. My application is I'm conducting a stress analysis on the sub frame by measuring strain with strain gauges. The strain gauges are placed close and far away on the top of a hollow box channel beam from a c-channel welded to the top of the boxed beam. the strain is measured when a forklift lifts the frame and boiler off the ground through the c-channels on the bottom of the frame to an equilibrium position. I am trying to obtain a  psi stress value at these particular points where the strain gauges are mounted using Simulation in Solidworks. I am having trouble setting up my boundary conditions properly to achieve a model that mimics this analysis. I tried placing a uniformly distributed load equal to 1/4 of the weight of the cast iron boiler resting on the top beam of the sub-frame, as well an upward force greater than the weight of the boiler and frame on the inside face of the steel c-channel that is being lifted by the forklift. As far as the fixtures are concerned, I was told any face that was cut I had to fix in the normal direction to the face. My results have not shown stress concentrations where I expect them to be and I wondering how i can improve my Simulation study. My model is attached, Thanks!

        • Re: Trouble setting up correct boundary conditions
          Mark Ankrom

          I don't completely understand the total problem but have a few suggestions.


          1.  You need to have the structure in equilibrium.  Grounding the square tube and applying the boiler load and a  > boiler load to the fork tube will cause extra bending in the square tube.

               I would include a model of the forks - use contact between the forks and your structure.  Use a displacement boundary condition at the root of the fork to raise/tilt  the structure like the real lift.

          2. I suspect the physics are only 1 plane of symmetry and not 2.

               a.  Common for forks to not extend through the structure being lifted.  Also forks are tapered.

               b. Fork lifts almost never raise straight up (they tilt to get the load closer to the lift).  this angle will change the center of pressure and put more load on the square tube near the forklift.

               I suggest using at most 1 plane of symmetry (use a split line between the forks).  For this problem it may be best  to just run the full model.

          3.  The boiler load is not likely to be uniform on the upper rail. It will mostly likely concentrate on the vertical members.  This will reduce the bending in the verticals and the gusset loads on the square tube.  You may need to add some more of the boiler model to get the right distribution.



          • Re: Trouble setting up correct boundary conditions
            Jared Conway

            can you draw up a diagram of the forces and how things are restrained in "real life"


            it would also be helpful to note where you are expecting the stresses to be and why you think they will be there.