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4. How to set the mesh size?

How to set the mesh size?

Diamonds is FEM software. That means the structure will be shopped into smaller pieces, the mesh, that are logically linked together. The quality of the results, goes with the quality of the mesh. Garbage in = garbage out. So after generating the mesh, you have to verify it!

Step 1: generate the mesh

The mesh window consists of two parts: one for plates and one for bars.

Plates

• Maximum element size: 3 to 4 times the plate thickness
The maximum element size determines the general fineness of the mesh.

 Maximum element size: 0,6m Maximum element size: 0,8m Maximum element size: 1,0m
• Minimum element size: 1,5 to 2 times the plate thickness
The minimum element size determines the fineness of the mesh around small lines and points that are very close to each other (see ‘Step 2’).Don’t set the minimum element size equal to the maximum element size. Only for standard models (like a 4 sided supported plate) a minimum element size of 0m is acceptable.

Bars

• Number of divisions: 8 recommended
A beam is simply supported and loaded with self-weight. The shape of the moment line is parabolic.

 2 divisions 4 divisions 8 divisions 16 divisions

With 8 divisions the parabolic shape is well approximated.

• With more divisions the calculation time will increase, while the results don’t benefit.
• With less divisions the calculation time will decrease, but the parabolic shape will be less detailed.
In larger models (with bars mainly loaded with axial force), the number of divisons can be set 4 or 6 divisions to limit the calculation time.
• Minimum element size: 0,1m recommended
With the minimum element size, you can request less divions in shorter bars.

 8 divisions + minimum element size: 0,0m 8 divisions + minimum element size: 0,1m
• With a minimum element size equal to 0,0m, all bars have the same number of divisions (8), regardless their length.
• With a minimum element size equal to 0,1m, the number of mesh elements in the shorter beams is reduced. Resulting in less calculating time. Since the complete behaviour of the truss is imported and not the behaviour of every bar seperatly, the mesh shouldn’t be so detailed.

Step 2: Verify the mesh

2.1 Size

Play with the maximul element size to obtain a mesh that is not too big or not too small in relation to the dimensions of the model. Preferably you should have at least two triangles along each plate edge and two triangles between an opening and a plate edge.

 Good Too small Too Big

Remove unnecessary points and lines, because each point in the geometry, will become a mesh point.

2.2 Regularity

Each mesh triangle only fulfills the equilibrium equations over its surface, regular triangles give better quality of results. Avoid elongated triangles by playing the maximum and minimum size.