Problem description

When defining prescribed displacements in PLAXIS 3D, these are assumed to be prescribed total displacements compared to the undeformed mesh. This is also described in the documentation.
However, in PLAXIS 3D 2010 and 3D 2011, these prescribed displacements are taken as prescribed phase displacements: the applied prescribed displacement will be added as an incremental displacement.

Example 1

In phase 1, after the K0-phase, we activate a prescribed displacement of u_z,phase1 = -0.01 m. In phase 2, we do not change this and the program keeps this value: u_z,phase2 = -0.01 m. In the results, according to the documentation, it should reach a total displacement of u_z = -0.01 m, but the calculation gives you u_z = u_z,phase1 + u_z,phase2 = -0.02 m.

Example 2

In a project, we calculated phase 1 and phase 2, and the result of that phase’s surface displacement is  u_z = -0.003 m. In phase 3, we do not reset the displacements to zero, and we activate a prescribed displacement at the surface with u_z,phase3 = -0.01 m. In phase 4, we do not make a change and the prescribed displacement value is kept at u_z,phase4 = -0.01 m.
Now we get these results:

• Phase 2: u_z = -0.003 m
• Phase 3, u_z = -0.013 m, while according to the documentation is should be u_z = -0.01 m
• Phase 4, u_z = -0.023 m, while according to the documentation is should be u_z = -0.01 m

Solution

In PLAXIS 3D2011, you can overcome this problem by setting the prescribed displacement to zero (u = 0 m) for the desired direction. In that case you will not have any additional displacement at that location. However, if you want to let the model deform freely, you can switch of the prescribed displacement.

In these cases, in this new version (PLAXIS 3D2012), it will give different results compared to projects calculated in the 3D2011 version when:

• prescribed displacements are used in subsequent phases;
• or when prescribed displacements are activated on an already deformed model.

The issue is fixed in PLAXIS 3D2012.