When working with Inventor Nastran or any FEA software package you need to remember that the model you design then take to manufacture is not necessarily the model that will be used for downstream processes FEA or simulation.
Features like fillets if they are for aesthetic can be left out as well as holes that might not make a difference in the overall analysis, especially if it is tiny in comparison to the entire component that you are running the simulation on. These extra features will greatly increase the the analysis time to get to a result but if not in the final model that will be used for the FEA study then it could be the difference between getting that model out on time so that manufacturing production can start or it needs to go back to the drawing board because the component is not strong enough.
The below image depicts a plate which is supported by 2 ribs. The second image depicts the shape that is created after we have run the Find Thin Bodies or create mid surfaces function in Inventor Nastran.
You will notice that where the supports meet the plate there are gaps in the main plate surface.
When creating the midsurfaces, you need to create the surface contacts to tell Nastran that the surfaces are connected and how that connection behaves. When you have many of these parts those selections will be plenty and many clicks makes the designer grumpy.
A way to simplify this process is to create the surfaces that you need for the analysis in the design model and then you idealize that. In the 3D model the surface would like the below.
As you can see there are no gaps in the plate and the selections you need to make for your surface contacts are now significantly less.