# Frequently Asked Questions (FAQ)

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• ### I have purchased the add-on module RF-MAT NL, but I can not find it anywhere.

The add-on module RF-MAT NL is already integrated in RFEM 5. It is assumed that the following material models can be used:

Isotropic Plastic 1D
Isotropic Plastic 2D/3D
Isotropic Nonlinear Elastic 1D
Isotropic Nonlinear Elastic 2D/3D

Orthotropic plastic 2D
Orthotropic 3D plastic

Isotropic Masonry 2D

Isotropic damage 2D / 3D
• ### Why does the calculation take a long time after changing the surface rigidities in "membrane-free"? How are these calculated?

With the property "Membranzugftrei" the model is calculated completely nonlinear. The program initially sets 5 load levels automatically (see Figure 1) for better convergence.

In the numerical calculation, it is impossible to reduce the rigidity to exactly zero. This would very quickly lead to singularities in the solution of the equation system. The program calculates the surface stiffness as follows:

1. The area is first considered in the calculation as an isotropic material with the compressive elastic modulus of the defined material.

2. In the first iteration, which is a normal linear calculation, the mean stress state is determined in each finite element.

3. The voltage is transformed in each element in the direction of the main voltages. In each direction is checked whether train occurs. If so, or more specifically, if a certain threshold of deformation is exceeded, the corresponding coefficients of the stiffness matrix are reduced to 10%.

4. Such an element then becomes an orthotropic element with the orthotropic directions identical to the directions of the principal stresses. For such a modified structure, another iteration is performed and step 3 is repeated.

5. The stiffness reduction of the surface must not fall below a certain limit; this limit is on by default 1/1000 set.

6. If in the iteration the sign changes to pressure in one of the directions, the material rigidity in this direction is increased again. However, the initial stiffness must not be exceeded.

7. The iteration stops when a stiffness change no longer needs to be made in any element or when the user-defined maximum number of iterations has been reached. It often happens that the sign of the normal voltage in an element reverses periodically. Then it is expected to reach the maximum number of iterations. In this case, some elements may also experience higher tensile stress, but the global stiffness of the surface will not be significantly affected.

8. The bending stiffness is not affected.
• ### In the material library for masonry there is the ratio "g / t". What does this mean exactly?

The ratio g / t is the ratio between the total width of the mortar strip and the thickness of the masonry wall. The ratio of butt joint mortaring should not be less than 0.4 according to the standard specification.
• ### I would like to select a material of the masonry type in the material library. Why is the library empty in the Masonry category?

The property of the material masonry depends on a large number of input parameters. It would be very extensive to list all possible combinations. To create a material, click the "Create new masonry material ..." button. Once created, the material remains in the library for repeated use.
• ### How can I model a fall?

The modeling can not initially be modeled using an offset, as this is generated internally via a rigid rod from the source node to the rod and thus the protrusion is not taken into account in the surface. Instead, extensions must be generated on both sides. In the end, the fall consists of three bars, as shown in Figure 1.

• ### When defining the material as "Isotropic Masonry 2D", I receive an error message saying the authorization of the program failed.

FAQ 002054 EN-US Add-on Modules RFEM RF-MAT NL 5.xx

In order to use the "Isotropic Masonry 2D" material, the RF‑MAT NL add-on module has to be licensed. Without the RF‑MAT NL license, only the material models "Isotropic linear elastic" and "Orthotropic elastic 2D/3D" are available.