3 Replies Latest reply on Aug 24, 2009 7:42 AM by Jim Sculley

    Which is the right way to simulate application of force on a body

    Prashanth Srinivasan

      Hi, this is a pretty basic question which has got me stumped as to which methodology I need to adapt to get an accurate result:

       

      I need to determine the optimum dimensions of an aluminum platform that will withstand the load of a person weighing, say 80kg or 176lbs. I'm running an FEA analysis to check if, for the given boundary conditions & platform dimensions, the Von Mises Stress and Displacement of the platform fall within the required limits. But I'm confused as to which of the following three methods would be the right way to simulate the load (weight of the person) acting on the platform:

       

      1. Applying a point load equal to the person's weight, in the form of Force applied on a particular point on the platform.

       

      2. Determining the area of contact the person makes with the platform (i.e. the foot patches) & applying Force directly on the patches.

       

      3. On the foot patches, instead of Force, I apply Pressure (equal to Weight divided by total patch area).

       

      I need your advice on this, because the each method gives drastically different results from the other.

        • Re: Which is the right way to simulate application of force on a body
          Derek Bishop
          Not sure how accurate you want to be. I'd create a circular  split surface in the centre of the platform and apply the wight of the person (force) to that suface. This assumes that the platform is stationary. If it moves up and down you would need to consider the acceleration effects. You could use the pressure but you'd need to calculate this from the area and force. If the area is the same the results should be the same for force and pressure. If you apply the force as a concentrated load at a point I'd expect to get some higher stresses local to that point.
          • Re: Which is the right way to simulate application of force on a body
            Jim Sculley

            31799 wrote:

             

            Hi, this is a pretty basic question which has got me stumped as to which methodology I need to adapt to get an accurate result:

             

            I need to determine the optimum dimensions of an aluminum platform that will withstand the load of a person weighing, say 80kg or 176lbs. I'm running an FEA analysis to check if, for the given boundary conditions & platform dimensions, the Von Mises Stress and Displacement of the platform fall within the required limits. But I'm confused as to which of the following three methods would be the right way to simulate the load (weight of the person) acting on the platform:

             

            1. Applying a point load equal to the person's weight, in the form of Force applied on a particular point on the platform.

             

            2. Determining the area of contact the person makes with the platform (i.e. the foot patches) & applying Force directly on the patches.

             

            3. On the foot patches, instead of Force, I apply Pressure (equal to Weight divided by total patch area).

             

            I need your advice on this, because the each method gives drastically different results from the other.

             

             

            You analyze the worst case.  Anything else can give you a false sense of security.  If the worst case results are very obviously skewed due to a very unrealistic loading scenario, the temptation will be to choose a more realistic scenario.  The problem is that at some point someone may create a loading scenario somewhere between the realisitc and unrealistic and injury can result.  For example, your methods 2 and 3 assume that no one will ever stand on one foot or on tiptoe.

             

            You say that the stress and displacement have to fall within the required limits.  Do those limits already have a factor of safety calculated into them?  If not, you need to carefully consider what the factor of safety should be.

             

            I designed a manlift once and carefully considered all of these things.  Everything went well, until I saw one of the workers on the job standing on top of a 1/16" thick aluminum toekick plate because he was too lazy to hit the button and raise the lift.

             

            You must imagine how someone with absolutely no common sense would utiliize your platform and design for the person who graduated *below* that person in school.

             

            Jim S.