Problem description

When using curved surfaces with features that use local axes, like plates, surface loads and surface contractions, in PLAXIS 3D2012.00, and such a curved surface is intersected with another curved or flat surface with local axes, the local axes for these curved plate elements on the intersecting edge with the other surface can be incorrect. This would lead to incorrectly defined local axes for the surface elements. Since the local axes are not correct, the results will be incorrect.

Common cases where this can go wrong are:

• intersecting tunnels
• a tunnel with a front
• a tunnel with side branches
• a vertical pile as a tube with a cap on top

How to detect incorrect local axes for plates

After mesh generation, review the mesh, select the plate elements and open this in a separate plot. Make sure that the local axes are activated in the View menu, and check if axis 1 (red axis) and axis 2 (green axis) are in the plane of the plate, and axis 3 (blue axis) is perpendicular to the plate’s surface.

Example: tunnel intersected by another tunnel

The example below shows two tunnels that intersect each other:

Now let’s focus on the tunnel that is aligned with the y-axis:

When looking at the mesh plot for this tunnel’s plate elements, it cannot directly be seen whether the problem exists:

But after the model is slightly rotated, it can be seen that a number of local axes 1 (red) point inside the tunnel at the shared edge with the second tunnel:

 

Solution

In order to still use the model in PLAXIS 3D2012.00, there are two possible workarounds

1. generate a mesh with a surface angle tolerance of 1. This will disable the use of so called framesurfaces (3d curved surfaces) in the PLAXIS 3D program. However, potentially this could lead to a bad quality mesh, so make sure to inspect the mesh quality after generating the mesh.
   
2. send the model over to the Plaxis support team. Until the release of an update, the Plaxis support team will use an internal PLAXIS 3D version with a fix to generate a mesh with correct local axes. Plaxis will generate the model as-is and send the model with the generated mesh to you: it will not optimize the mesh for a good Finite Element calculation. Please use pack project from the file menu to send all required data. You can find this option in the menu under “File/Pack project”. This will give a wizard to compress the project. You can select the option “Support” in the top right corner of the pack project window, and then disable the mesh checkbox. In that case you create a packed project with the smallest possible size with all required data to regenerate the mesh with the correct local axes.

This issue is fixed since PLAXIS 3D2012.01