Problem description

When custom connections are used in a PLAXIS 3D dynamics analysis, the calculation process will result in a severe divergence error.

In PLAXIS 3D custom connections provide the option to apply rotation- and translation degrees of freedom between the connected structural elements. By default, if no custom connection is created the elements will be rigidly connected. An illustration of the custom connection between two plates is shown below:

This means it is not possible to perform a dynamic analysis in PLAXIS 3D when you model the following custom connections:

1. Hinge between two plates;
2. Hinge between a plate and a beam;
3. Dilation joint (displacement discontinuities) between two plates or between a plate and a beam.

In the case of configuring the custom connection as rigid connection – both rotation and translation set to fixed, no error is encountered.

Solution

There is a method that could simulate a similar behavior to a custom connection which involves introducing a relatively small structural element (plate or a beam) which could act as a ‘connection’ in between the two structural elements. You can modify the stiffnesses of this element accordingly to provide a similar behavior to the custom connection. For example, use a higher stiffness for a more inflexible connection and a lower one for an element that allows movement (rotation or translation) and in effect releases bending moments.

Following the general recommendation to avoid numerical problems, you are advised not to use stiffness values that could result into a stiffness ratio higher than 1E6 when compared to other values used in both structures and soil/rock materials.

Keep in mind that this method is quite crude but can offer a rather good approximation of the results until the issue is resolved. Careful consideration should be taken to the accuracy of the results.

PLAXIS is further investigating the possibilities to overcome this issue in the future.

If you have questions regarding the workaround, you could reach out to the PLAXIS support team for further clarifications.