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Load Distribution

I wonder, how NX CAE is distributing force loads to nodes. When I checked the Nastran solver syntax and the contour, I noticed that after I applied a force onto a surface, some nodes belonging to the surface got no fraction of that force. What's the reason for that and why is the force not distributed equally? (e.g. I have quadratic surface with 25 nodes and a force of 5000 N, but I see only 16 nodes which got a fraction of that force. 8 nodes with 208.333 N and 8 nodes with 416.667 N = 5000 N) Why not 200 N on each node? (I know I could achieve this with a Node ID table)

 

In the manual I only found this:

"Select Geometric Distribution to distribute the total force or moment over all the selected items based on the area. All the nodes on the selected items then get a fraction of the force..."

5 REPLIES

Re: Load Distribution

Alex,

 

The force on a surface is distributed to the underlying mesh in a manner that yields a uniform pressure across the mesh. The forces are distributed to the nodes using the element shape functions.

 

For linear elements, you get what you are expect - each element face has a force proportional to the element face area relative to the total area. The linear shape functions distribute that in a manner that results in each node on that face getting an equal percentage of that faces load (1/3 for trias, 1/4 for quads).

 

With parabolic shape functions, the forces end up being 0 at the corner nodes so you only see values on the midnodes.

 

Take your model and apply the 200N to each node (no need to use a Node ID table - just select the nodes related to the face rather than the face itself). The stress results on the loaded face will be much more uniform with the distributed force from the face. The equal magnitude on each grid results will be very non-uniform.

 

The total load imparted to the structure will be the same, so the results away from the loaded face (and spc force resultants, etc.) should be the same.

 

Regards,

Jim

Re: Load Distribution

That explains the resultant distribution. But I still don't understand, why NX CAE doesn't simply distribute the force uniformly across all nodes. I mean if I would write some force into my input nastran file manually, I would just divide the force with the number of nodes. Why to use shape functions to apply boundary conditions?

Solution
Solution
Accepted by topic author AlexH
‎08-26-2015 04:32 AM

Re: Load Distribution

Again, the method of distributing the forces provides smoother stress results in the vicinity of the load.

 

If you hand edit a deck, you do not have all of the information available to do this distribution, so you compromise and divide the load by the number of nodes and assign the same magnitude to every node. NX has this information readily available so the distribution calculation is trivial. Given that, I would turn your question around: Why shouldn't NX CAE do this? Isn't one of the reasons you are paying for a pre-processor to make things like this easier?

 

If you really want the same magnitude to be applied to every node, select the nodes themselves into the BC target set rather than the polygon faces.

 

Regards,

Jim

 

Re: Load Distribution

out of curiosity:

 

What does NX do if I select all nodes of a face and select geometric distribution to apply a load? Is the method the same as if I would have selected the face?

 

Regards,

 

Elonora

Re: Load Distribution

ALexH,

 

I think I have more 'human' explanation.

Not allways mesh is very regular (ex. quads m x m). Be aware of fact that usually elements do not have the same areas and sometimes you use mix of quad and tria elements (or solids respectively) so applying a load in the way you propose will yield to non uniform distribution, because some elements can be longer or bigger than another.

The method described by JimB gives you this opportunity and yours do not.

 

See simple example: plates with the same total area but different meshes. In case you will use 'your' method for right-side structure, it will be 'overloaded' in the 'middle part' due to larger number of nodes in this area.

In case you will use built-in method for both meshes, the load distribution will be the same.

 

I think that is the reason. 

example.jpg

 

BR, Tomek