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• ### How can I define failure criterion s such as Tsai-Wu for orthotropic material?

If you want to define failure criterion s for orthotropic material you have to define a orthotropic plastic material in the Material Model (picture 1).

The yield criterion will be done according the Tsai-Wu criterion (picture 2).

By this Link you can find a full set of verification examples to this material model.

The material model itself is explained in several Knowledge base articles.
• ### Why do not I get tensions as an orthotropic surface as a result?

No stresses are calculated for orthotropic surfaces: The different stiffness coefficients would result in "smeared" results because they refer to an average value of the thickness. These stresses do not correspond to the orthotropy model.

It is recommended to use the add-on module RF-LAMINATE here .
• ### Can I change the stiffness of a surface out of its plane?

FAQ 002558 EN General RFEM

The stiffness of a surface can be influenced both in its plane and out of its plane. For this, use orthotropic stiffnesses (see figure).
• ### Is it possible to model a pane of trapezoidal sheeting in the model?

FAQ 002468 EN Modeling | Structure RFEM

There are two ways to do this:

1. Modeling as a folded structure (see Figure 01)

2. Modeling as orthotropic surface (see Figure 02)

The first option is only useful if the stresses in the trapezoidal sheeting are of interest. It is necessary to represent each single bead with surfaces. The effort required for modeling and calculation is very high. It is a bit easier to model: The RFEM cross-section contains all common trapezoidal cross-sections. Define a member with the cross-section of a trapezoidal cross-section. Then, use the shortcut menu function to split the member into surfaces. You can then modify or copy the created surfaces with trapezoidal sheeting.

If only the stiffness of the corrugated sheet is relevant for the analysis, modeling as an orthotropic surface is recommended. When defining the surface, select the stiffness type "Orthotropic". Use the [Set Parameters] button next to the list to define the orthotropic properties. The stiffnesses can be described by substitute thicknesses or directly by the coefficients of the stiffness matrix (see manual). It is possible to specify any angle for the orthotropy direction that is related to the local surface axis system. This angle can also be checked graphically (Display navigator: "Model → Surfaces → Orthotropy Direction").

• ### I have modelled a precast slab as "orthotropic surface". Can I use these surfaces also in reinforced concrete design?

Yes, you can use "orthotropic surfaces" for the reinforced concrete design in RF-CONCRETE Surfaces.

If you only would like to carry out serviceability limit state designs, you do not have to define additional settings in RF-CONCRETE Surfaces concerning the surface type.

You need the RF-CONCRETE NL add-on module to perform the serviceability limit state designs. Select in the "serviceability limit state" tab (in the dialog box "1.1 General Data") the design method "Nonlinear...". See Figure 01.

You can then change the surface type to "Constant" in the dialog box "1.3 Surfaces" so that the serviceability limit state design is also possible here. See Figure 02.
• ### When analyzing a timber panel as an orthotropic surface, an error message appears saying that the Poisson's ratio has to be within a certain interval. What should I do?

FAQ 002017 EN Calculation RFEM 5.xx

In this case, it is not sufficient to only set the stiffness type to 'orthotropic'. You also need to select an orthotropic material model for the material, otherwise the stiffness material is defined singularly.

• ### Are the surfaces calculated as torsionally flexible or torsionally stiff plates in RFEM?

FAQ 000603 EN Calculation RFEM

The surfaces are considered in the calculation as torsionally stiff objects by default.

In the Modify Stiffness tab of the 'Edit Surface' dialog box, it is possible to assign 'multiplication factors' to a surface for each stiffness matrix element (see Figure 01). By reducing the coefficient k33 , the torsional stiffness can be controlled individually.

For orthotropic surfaces, this tab is not available. In this case, the stiffnesses can be adjusted in the 'Edit Surface Stiffness - Orthotropic' dialog box (see Figure 02).

• ### I would like to define an orthotropic plate. In order to check the influence of the changed material properties on the stiffness matrix, I would like to know how the stiffness matrix is determine. Also, how is the self-weight of an orthotropic surface calculated?

FAQ 000274 EN Calculation RFEM

In RFEM, you can define various types of orthotropic surfaces. The information about the calculation of respective coefficients of the stiffness matrix is available in the 'Stiffness Matrix' tab by clicking the [Info] button (see Figure).

The self-weight determination is based on the specific weight of the material as well as the values defined in the windows of the selected orthotropy type. To consider the self-weight, there are separate text boxes available, where you can enter the equivalent plate thickness, for example.