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Documentation

Boundary Condition: Fixed Value

In structural analysis (except for heat transfer), the fixed value boundary condition sets a fixed displacement on the assigned face or volume. The user can use this boundary condition to restrict a movement in only one or two directions. Furthermore, the value of the displacement can be a function of coordinates or time added using the table or through a formula.

fixed value boundary condition in a structure analysis to apply a fixed displacement
Figure 1: Fixed value boundary condition for structure analysis types. Besides the usual entry you can input data through formula or table.

Any value entered basically means the maximum allowed displacement of that face/volume in that particular direction. The value entered can be negative as well and signifies the maximum allowed displacement in the opposite direction. As mentioned before, the displacement can also be defined with a table or a formula. Usually, this is used when the displacement varies over time or as a function of the length of the model.

The user can allow an unconstrained movement by clicking the checkbox fixed value boundary condition checkbox to allow free movement (and toggling it off) in the direction where movement is necessary. For example, a pure movement in the vertical direction will need complete restriction in the horizontal axes.

Example

The picture below is a slice of an annulus that is subjected to volume load.

slice of an annulus
Figure 2: Slice of an annulus subjected to volume load

However, since the movement of the annulus will only be in the x- and y-axes the displacement needs to be fixed in the z-direction to zero. Hence, the fixed value boundary condition will be necessary in this case.

displacement fixed in the z-axis but unconstrained otherwise.
Figure 3: Displacement in the z-direction fixed to zero. The annulus is free to move in the other two axes as its displacement is set to unconstrained.

This boundary condition is applied to the top and bottom face of the annulus and the movement is restricted to the x- and y-axes. This is why the displacement in the x and y-axes is unconstrained while the displacement in the z-axis is fixed at 0 \(m\). The deformation in each direction can be seen in the figure below.

deformation in each respective direction
Figure 4: Comparison of deformation in x-, y- and z-axes

The picture above shows that the deformation in the z-axis is insignificant when comparing it to the deformation in the x- and y-direction.

Note

If the user wants to restrict all movement, then the fixed support boundary condition is recommended.

Last updated: May 25th, 2021

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