Displacement fixities are not applied to isolated beam elements

ApplicationPLAXIS 3D
Issue statusOpen
First Affected VersionPLAXIS 3D 2018
Found in VersionPLAXIS 3D CONNECT Edition V21
Fixed and Released in Version-
Issue #700220
Date created24 September 2021
Date modified24 September 2021

Problem description

Global boundary conditions for a specific phase can be defined at the Model conditions subtree in the Model explorer. These default displacement fixities should apply to soil volumes and structural elements, and any user-defined conditions defined always prevail over the ones shown in the Model conditions subtree. 

However, if symmetry conditions apply and the nodes of beam elements are located at the tri-dimensional plane of symmetry, without being connected to a soil volume or other structural element, those default fixities that are defined at the Model conditions subtree will not be applied. 

Note that when structural elements extend to the model boundary (XMin, XMax, YMin, YMax, ZMin, ZMax), where at least one displacement direction is fixed, they obtain at least two rotation fixities in the points on the boundary. In the current issue, although the default fixity is not correctly applied to the beam node, the inherent rotation fixities are not affected.

For instance, in the illustrated example the default fixity in the y-direction is not being correctly applied but the rotation in x and z will still be zero (φx=φz=0, φy= free).  

Solution / Workaround

Since user-defined conditions always prevail over the ones defined in the Model conditions subtree, applying a point prescribed displacement at the end-node where the beam is located is a workaround for such cases. 

The steps to solve this are the following:

  1. Identify the beam(s) to be considered in the Input and/or Output program.
  2. In Input, switch to Structures mode.
  3. Select the relevant endpoints of the specific beam(s) and create a point prescribed displacement.
  4. Switch to Phases mode. Note that re-meshing is required.
  5. In the relevant phase where the beams are considered, activate the point prescribed displacements and configure the appropriate directional component that is perpendicular to the lateral boundary to Fixed (rest can be set as Free).
    In this example the correct configuration would be: