The example initially consists of a two-way spanning slab item supported by beams along all four edges. The beams have fully fixed supports at each end. The slab item is defined with its X-axis parallel to its long edge. A full area load is applied to the slab.

The slab is set as orthotropic by selecting the appropriate option in the Analysis parameters property grid.

The stiffness modification X and Y factors are only visible only when Orthotropic and Calculate properties options are selected.

Adjusting the factors allows you to automatically set material orthotropicity using the method described in Using the ‘Calculate properties’ functionality. Adjusting one of the factors can have a significant impact on internal forces in the slab item, as well as on the load transfer.

It can be seen below how reducing the X factor from 0.1 (left) to 0.001 (right) impacts the results.

Max deflection: 17.6 mm

Max deflection: 19.3 mm

As an alternative to calculating the orthotropic material properties using X and Y factors, the properties can be set manually if required. This can be achieved by deselecting the Calculate properties option in the Analysis parameters property grid. The properties take default values based on their state prior to deselecting the checkbox.

To increase the slab’s orthotropicity, after adjusting Young’s modulus in the local X direction (to replicate the effect of the X factor), one might consider modifying the in-plane shear modulus, Gxy, as well. For example the in-plane shear modulus could be reduced from 523 N/mm² (left) to 52.3 N/mm².

How this impacts the results is shown below. Shear modulus 523 N/mm² (left) and 52.3 N/mm² (right).

Max deflection: 19.3 mm

Max deflection: 19.5 mm

Decreasing global or sub-model slab mesh size can lead to the convergence of the FEM results with the analytical solution. It is recommended that the mesh size is no greater than 10% of the shortest edge of the slab.

The example below shows four different mesh sizes, equal to 20%, 10%, 5%, and 1% of the shortest slab edge length, respectively.

20% - Max deflection: 19.7 mm	10% - Max deflection: 19.6 mm
5% - Max deflection: 19.6 mm	1% - Max deflection: 19.6 mm

For further details, see Adjust global slab mesh properties and Apply different mesh properties at different levels.

For this comparison three versions of the model are considered: (i) isotropic slab properties with the slab supported along all four edges (ii) isotropic slab properties with the slab only supported along the two shorter edges, (the longer beams having been deleted) and (iii) highly orthotropic slab properties with the slab supported along all four edges.

The shears in the supporting beams and the slab deflections are shown below.

Isotropic slab item - 4 edges of support	Isotropic slab item - 2 edges of support
Max deflection: 14.1 mm	Max deflection: 15.9 mm
Orthotropic slab item
Max deflection: 19.7 mm

It is normal that in the case of the orthotropic slab some load is transferred onto shorter supporting beams. This has been minimized by adjusting the mesh size and the in-plane shear modulus. The majority of the load is transferred on the beams perpendicular to the principal direction of the slab, resulting in a difference of under 0.8% when compared to the case of the isotropic slab supported along two edges.

In order to ensure that the material properties as defined in the Analysis parameters property grid are passed directly to the solver, a checkbox is provided to allow you to disable the global modification factors for the slab.

For isotropic slabs, this functionality also provides a way to Override global stiffness modification factor with slab specific factors.

When the example model is set to have isotropic properties, the results below show the effect of switching the Apply modification factors option on (left), and off (right).

Max deflection: 13.8 mm

Max deflection: 1.8 mm