FEA Consulting Part 5: Generating Results

Figure 1: Displacement results for our concrete slab simulation.  The slab is supported in the center by a square column, and on the sides by a theoretical wall.

Time now for the last in our blog series on FEA.  I’ve previously discussed how to set up CAD for FEAhow to mesh that CAD model and the different types of analysis that FEA programs can run.  The next step in the process is generating results from the FEA simulation, like the displacement illustration in Figure 1.  These are fantastic tools for generating useful reports, and are also very useful in helping an FEA consultant or engineer check the accuracy of the own results.

As a reminder, for an example I’ve been using a recent project we worked on involving punching shear in reinforced concrete.  We were examining the effectiveness of reinforcing a column-supported concrete slab against the possibility of that column punching through the concrete.

Results generation

Figure 2: Some of Autodesk™ Simulation’s results options

After iteratively solving partial differential equations for displacement, stress and strain in every finite element, the FEA program retains the final values for these parameters at every element.  These values can be used to calculate a wide variety of different stress and strain results, depending on the type of scenario the engineer is evaluating and the kind of results they’re looking for.  These include common parameters like the von Mises stress, maximum and minimum principle stresses, engineering strain, or overall displacement or displacement in a specific X, Y or Z direction, as shown in the toolbar in Figure 2.

Results Display

Figure 3: Some of Autodesk™ Simulation’s graphing options

FEA programs can output any of the above in a few different useful formats.  For numerical purposes, that output might be a simple spreadsheet showing the values at selected nodes, or a line graph showing the change in stress values over an area (shown in Figure 3).  In the scenarios involving physical deformation and movement, the FEA program can warp and twist the CAD model as a physical sample would under the same loads; often this has to be scaled up, since deformations are usually small compared to the actual size of the part.

Our concrete slab, for instance, had a maximum displacement of about 1 mm in the worst case scenario; this obviously wouldn’t be visible if it was kept to the same scale as the 6 meter-long sides.  Figure 4 shows the shear stress on our concrete slab, shown side-on, with the displacement scaled up by a factor of five.  The column is supporting the concrete slab at the bottom left corner, and with the displacement scaled up it is easy to see how this will be the most troublesome spot in the model.

Figure 4: Shear Stress, with displacement scaled up by a factor of 5x