1. General
Sometimes users are presented with a warning message about a large nodal displacement or rotation, like one of these:
Figure 1. Possible error messages
In big models it can be tricky to find out where the problem is. Here are some steps to help identify the location and reason of these errors.
2. Equilibrium error or not
If there is an equilibrium error, then it is easier to spot in which load case or combination the problem occurs. After the analysis is done, open the Equilibrium tool:
Figure 2. Equilibrium tool location
Then make sure you check the load cases or load combinations you want and find the problematic case/combination:
Figure 3. Equilibrium tool has drop-down list with options
This dialogue window indicates us the problematic load case and which component it is. In this example the problem arises in load case ll2. It is here, where we see an error of 92.61% in Mz. You can then focus on this load case/combination only and follow the next steps.
Sometimes there is no equilibrium error. It is common with large deformations when there is no rotation. The only way here is to check all the load cases/combinations.
3. Check the numeric value
Next step is to check the Translational displacements and/or Rotational displacements.
Idea. Do not forget that the large deformation might also happen in load case, so check the load cases, too!
Figure 4. Possible results to check
If you have selected a result to look, turn on the Automatic numeric values and from there Global maximum/minimum:
Figure 5. Automatic numeric values
With this function, it is easy to see the numeric value in the problematic place.
In this example, I knew from last step that the there is a large rotation in load case ll2, so I checked the deformations only there and found a large rotation in one of the columns:
Figure 6. Large rotation
4. Check the load case/combination where problem occurred
Now it is possible to check the location and load case/combination that showed the large deformation.
In this example, I looked at the Load tab to see what might be wrong with load case ll2. In first glance there seems nothing wrong with it:
Figure 7. Everything seems right
But when zoomed in, I can see a moment applied to the column. Also, there is an edge connection defined for the plate so the moment might not be restrained by it:
Figure 8. Moment applied to column
5. Check the model and boundary conditions
In this example the moment on a column it not a problem by itself. The model in Structure tab must be also checked to see the boundary conditions for the column and the plate, because it can be seen from the Rotational displacement value that the column can rotate freely. In model tab check the support for column:
Figure 9. Support conditions
Support is fully rigid, so it can take the moment. No problem here.
Then check the plate edge connection:
Figure 10. Moment-released edge on plate
In this example, the plate edge is set to be free in rotation. This means that it will not restrain the moment and the moment might only affect the column.
Next step is to check the column itself:
Figure 11. One end of column
Here the column seems to be adequately supported. But please note that the checkmark before “The same at both ends” is unchecked. This means that we need to check the other end as well:
Figure 12. Other end of column
Here we find a problem. The column can freely rotate around its axis even if the support was set to fully rigid. Since the plate is also not restraining the moment then the column will definitely rotate.
We need to fix the model now by either modifying the boundary conditions (column ends, or plate edge) to take the rotation or by removing the point moment from the model. In this example the moment load was accidentally placed on the column (all the other columns have point loads on them, but this one has moment).