1. General
Most of the time the mesh problems are actually modelling problems and the way to solve them is to remodel of fix some parts of the model. Sometimes the fixing is so extensive, that it is more simple to remodel everything. To correct the meshing problems, users must first understand the mesh settings very well. Please read this article first to fully understand the settings before continuing this article.
Read about mesh settings first!
2. Main problem with bar elements
With bars there are two common problems. First users forget that they are divided into smaller pieces by the connected objects. So, one beam may be divided into many geometrical “beams” that all use the minimum finite element division and thus that one beam may be divided into tens of pieces, although user set the minimum division to 2. There is no way to avoid it, just keep this in mind.
The other bar problem is that they sometimes look like they are connected, but they are not. This mainly happens when transferring models from other software, but it can happen in models that are only modelled in FEM-Design as well, due to snapping errors.
Figure 1. Common misaligned columns
In this example, if we look from afar, then two columns seem to be connected. If we zoom in on them, we see that there is a very small gap. This could be in x, y or z direction. The solution is to use the Correct model tool and select the Stretch lines option:
Figure 2. Correct model with Stretch lines option
Note. This problem is also common culprit of surface mesh problems. Read more below
3. Typical mesh problems
With meshes, the most common problem is misaligned elements. As it is with shells (read the Mesh settings article, reference at the top of this page) all the connected objects affect how the mesh is generated. Problem with misaligned elements is that they are usually misaligned very little. We cannot usually see it by just looking at the model, but we see the dense mesh instead.
Example 1
For example, here are two very common type of misalignments: a column and a wall:
Figure 3. Common misaligned column and wall
In this example, we can see dense mesh parts above the middle column and above the wall corner. If we zoom in on these parts, we see there are misaligned elements: a column is placed 1mm away from the edge into the plate and walls have gap in the corner. Both misalignments produce extra points on the top plate which must also be considered when generating mesh. Since the misalignments are small, the mesh that is generated next to them, must be small as well. Away from the small mesh, FEM-Design gradually calculates bigger mesh size.
Moreover, since the plate and walls are connected and connected elements affect each other’s mesh, the wall mesh also must have smaller elements near the corner, even if there is nothing specifically wrong with the wall itself.
The solution for this problem is again to use the Correct model tool. This time the Stretch to crossing regions option needs to be checked to correct the walls and Stretch lines to fix the column. Depending on how the walls are located, it may be necessary to use the Strech regions in plane option as well.
Figure 4. Correct model tool with two options
Tip. You can select all options every time you use the function, but it is important to understand exactly what each option can or cannot fix.
Example 2
Another common misalignment is walls in parallel planes. For example, here are two facade walls that are not connected to each other. If we mesh the model, then we do not see any problems and the mesh looks correct:
Figure 5. Mesh looks good at the moment, but model is not correct
But if we model the next storey with one big plate and mesh it, then we see mesh problems:
Figure 6. Errors when trying to mesh the next item
We cannot even make the mesh here since the right façade wall is actually 0.1mm inside the building (together with the bottom plate). To check whether it is the right or left wall that is “correct”, we can use the Query tool:
Figure 7. Query tool is useful for understanding element relations
and check the points of the windows for example:
Figure 8. Positive dY indicates that point 2 is more to the "inside" of the building
Here we clicked on point 1 and then on point 2. The results show that we have positive dY (so while moving from point 1 to point 2, the Y distance increased in the Y-axis direction => so the second point must be “inside” the building (on the other hand, the point 1 might be outside of the building, but we need to make some assumptions here).
This problem can be most easily fixed with Adjust analytical model tool. The Correct model tool could also work in some cases, but it might be more confusing and has more limitations.
So, let’s use the Adjust analytical model tool. We can select the first option Align any object to… and then the last option Selected objects. We also need to set the tolerance to at least the distance between the walls, so I insert 3,3 to be on the safe side:
Figure 9. Adjust analytical model tool with Align option
Now we select the fist wall (the one that is a reference, or the one that we want to keep) and then the second wall (that we want to move):
Figure 10. Message about what was done
After the command has finished, we can see an information dialogue about how many elements were changed. Here the one wall we wanted to move, was moved.
The problem is not yet solved, though, since we only moved this wall. There are two additional elements that need to be moved: the plate underneath and the staircase wall. We can use the same tool Adjust analytical model but with different options. For example, we can use the Fit to each other… and Shells with the default tolerance. Then we can select both walls and plate at the same time (so use CTRL key to select multiple elements) and press ENTER. Now we should see a message that two elements were modified (one wall and one plate). If you want to have more control over it, you can select only two elements at a time or use the other option Fit any object to…
Figure 11. Different option in Adjust analytical model tool and message about completion of task
The model may still be not fixed, because the cover in the middle with the supporting beams might still be wrong. We could check this with the Correct model tool. The trick here is to select the least number of elements necessary for the fix. I would select the bottom plate, cover and the edge beam:
Figure 12. How to make smart selections is key in tool usage
If I select more elements, it is not wrong, but I get lot of suggestions to move the walls back to the “wrong” position of the cover. With clever selection I can avoid or reduce the number of suggestions. Select at least the options Strech regions in plane and Strech lines. Of course, make sure that you don’t stretch the plate. Now the model is fixed.
Example 3
Another common mistake is when storey elements don’t align to each other. For example, these columns here are not aligned. This causes plate’s mesh to be very dense around them. One solution is to align the elements with the Adjust analytical model tool with option Align to each other… and option Column. Select the Bottommostoption to keep the bottom column in place and move every other column according to it:
Figure 13. Misaligned column on many storeys can be fixed with Adjust analytical model tool
Note! Make sure you are on the Structure tab, otherwise you might not be able to move the columns or even select them. Alternatively, you can unlock the corresponding layers.
Example 4
Yet another common mistake is shell that consists of unnecessary points. This happens mainly when elements are exported from other software or also due to using references like DWG to select shell edge lines. Here is one wall and one plate that have extra points on them. It is not initially visible, so we need to turn on the crosses on the points to actually see the problem:
Figure 14. Turn on end point indicators
From afar it is not clearly visible that the wall corner has multiple points next to it, but it is clear from the mesh that there is something wrong:
Figure 15. Many points on a line
The Correct model tool has a special function to get rid of these multiple corner points Merge region lines:
Figure 16. Correct model tool with Merge region lines option can solve the issue
Example 5
Sometimes we cannot use the Merge region lines option, because want to keep some of the points. For example in this case here:
Figure 17. More complicates setup, where automatic tools are not the best
Here we have to be careful when using the Merge region lines option, as it can easily delete the node that we manually placed on the top plate. Here we could have more control over the edges if we used the Join tool instead. The Jointool joins two lines into one, but the lines must have same direction (they cannot be at an angle to each other). For example, below we can remove some of the points by using the Joincommand, but not all of them:
Figure 18. How to manually remove extra nodes on line
4. Multi-storey building key takeaways
Small mistakes on one storey are easily carried on to others. The best practice is to model one storey first, then mesh it and correct all the problems. Only when one storey is completely fixed, copy the whole floor to the next storey. This way many errors can be avoided.
Another tip is to minimize the modelling directly from reference and model it while just looking at the reference, but not actually using all the points/lines from reference. For example, when modelling walls on a DWG and then later a floor is needed, we should use the FEM-Desing walls that we already made for modelling the plate, not the original DWG. In fact, we should turn off all snaps on the DWG (or turn off the whole layer) while we model the plate to reduce the mistakes.