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Loading Visualisation Arrows in Reverse Directions

Because the finite element solution requires both the stiffness and the loading to be known at nodal points, loading the loading assigned to features needs to be transformed into "equivalent nodal loads". For instance, a constant, uniformly distributed load applied along the length of a 3-noded bar element is transformed into equivalent nodal loads which are distributed according to the ratio [1/6 : 4/6 : 1/6].

The equivalent nodal loading for a surface element is distributed according to a ratio that specifies negative coefficients at the corner nodes. It is this equivalent nodal loading that explains why visualising the loading attributes on a mesh, can produce a series of arrows that either alternate unexpectedly (linear elements) in size or reverse in direction (quadratic elements).

This behaviour is entirely valid and, indeed, essential for a correct solution of the analysis and is explained in detail here.

For discrete loads only, the visualisation may be adjusted from the menu bar in LUSAS Modeller as follows:

View > Insert layer > Attributes. Choose the "Loading" tab, select "All" and click "Settings". The option is then available to "Show discrete loading by definition" or "Show discrete loading by effect on mesh".

By default the latter is invoked and will show the equivalent nodal forces as described above, whilst the former will visualise the forces as defined in the attribute definition without additional transformation into equivalent loads.

These options, however, only apply when a pre-processing load case is set active - only the equivalent nodal loading is visible for results loadcases.

Note also that the reversal of loading arrows will only be seen when quadratic effects are selected as follows:

View > Insert layer > Mesh. Check the box beside "Show quadratic effects".

If the model is solved the deformed mesh and reactions will indicate that the load applied is actually in the correct directions. It would be possible to check that reversed arrows are caused by this equivalent loading by reverting to linear order elements.

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