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Fixed Support

The Fixed support constraint restricts all the translation degrees of freedom over the assigned entities to be zero. It is used to model a portion of the geometry that is connected to a rigid body. This rigid body can be the ground, an anchor point, the foundation, or another structure that is relatively much stiffer and will undergo negligible deformation under-supported loads.

fixed support boundary condition panel
Figure 1: Fixed support boundary condition panel.

The parameters of the constraint are:

  • Assignment: Set of faces or volumes where the following displacement conditions will be applied overall degrees of freedom:

$$ DX = 0 $$

$$ DY = 0 $$

$$ DZ = 0 $$

where \(DX\) is displacement in the X-direction.

As can be noticed, these conditions are equivalent to applying a fixed value constraint where all the displacements are explicitly set to zero.

Stress Concentration and Less Rigid Conditions

When using the fixed support constraint, it can be found that stress concentration and high-stress gradients might occur in the regions close to the assigned faces. According to Saint Venant’s principle, the stress distribution close to faces of load application (such as fixed faces), will always differ from the ideal distribution from an equivalent load.

Some actions to take in this case are:

  • Apply a mesh refinement over the fixed faces. This will help to smooth the stress gradients and get precise stress values at concentration points.
  • Move to a less rigid boundary condition, such as remote displacement with free rotations or deformable behavior, or elastic support to model the rigidity of the connected body.

Last updated: October 12th, 2020

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