I have a Validation > Analysis > General > “Settlement loads applied in unfixed direction” error. Why do I get this and how do I resolve it?

Question

I have a Validation > Analysis > General > “Settlement loads applied in unfixed direction” error. Why do I get this and how do I resolve it?

Answer

Before we look at the Settlement loads validation error let us consider a support in more detail.

A support can have 6 degrees of freedom, Translational F_x, F_y, F_z and Rotational M_x, M_y, M_z. Each degree of freedom (DOF) can be either Fixed or Free.



When a degree of freedom is set as Free, it can either be Released or act as a Linear or Non-linear spring.



If you set the support as a spring then you define the Stiffness to be adopted in the analysis.




So, considering translation in the vertical direction (F_z) as an example. If the support is set as:

• Fixed - then the support acts as a knife edge support with no movement in that direction permitted.
• Free / Released - then the support does not act at all in that direction and it acts as though there is no support at all - allowing infinite displacement.
• Free / Spring - the stiffness is the slope of the load / deflection curve. Hence, assuming a specified stiffness (K) and applied force (F) the deflection (x) can be determined from F = K x

The Settlement Load command works differently to a spring support in that you are stipulating the absolute deflection required to occur at the support. For this reason when you assign a Settlement Load to a support the support must be set as Fixed. The support is then adjusted to allow the specified displacement in the analysis model.

If we hover over one of the problem items a tooltip is displayed “Settlement loads are applied in unfixed direction at a node.”



We can double click the “Settlement loads applied in unfixed direction” error or items listed under the branch to highlight the selected items with the issue. Alternatively, you can right click and choose Highlight Items from the context menu. The view will automatically switch to the Structure 3D scene view with the selected items highlighted and its properties displayed in the Properties window.




So, to resolve the issue, review the Settlement load to determine the direction it has been applied and then select the support and ensure the same DOF is set as Fixed.





Once this has been set to Fixed the validation error no longer appears following an analysis.

Example to demonstrate the settlement load command

Let us consider a propped cantilever beam (encastre support to the left hand end and a pinned support to the right hand end). The only applied load is a settlement load. This is applied to the pinned support in the vertical F_z direction to provide a forced vertical deflection at the propped cantilever tip of 50mm.

Deflection

After running the analysis we can clearly see the induced forces from the applied settlement load.

Moment

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