User Area > Advice
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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