# Frequently Asked Questions (FAQ)

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• ### Can I simulate the cracked state of a concrete cross-section for a bending beam with the "Isotropic Nonlinear Elastic 1D" material model?

No, the "Isotropic Nonlinear Elastic 1D" material model is not suitable for a bending beam because the nonlinear stress distribution over the height of the cross-section cannot be modeled here. The reason for this is that there are no stress points/FE mesh points over the height of the cross-section. Thus, it is not possible to simulate the cross-section cracking.

On the other hand, the "Isotropic Nonlinear Elastic 1D" material model would be suitable for a cracking of the entire cross-section subjected to a pure axial force loading, but not for bending and compression.

For the simulation of a cross-section subjected to bending in the cracked state, it is recommended to perform a nonlinear analysis with RF‑CONCRETE Members and the RF‑CONCRETE NL module extension. Creep and shrinkage can be considered by using the module extension in RF‑CONCRETE Members.

After the calculation, the nonlinear stiffness of the cross-section can be imported back into RFEM (see Image 01) and the internal forces can be determined again, taking into account the cracked concrete cross-section.

https://www.dlubal.com/en/support-and-learning/support/faq/002881

https://www.dlubal.com/en/support-and-learning/support/knowledge-base/000992

• ### When defining my material, I get Warning No. 1136 and cannot continue working. How can I prevent this warning?

Warning No. 1136 ("During the calculation of material nonlinearity, the material with a decreasing branch of the diagram can be calculated with one load increment only.") refers to the entries in the global calculation parameters.

In our example file, the number of load increments has been set globally to 10 for load cases and load combinations:

For this material, one load increment can only be expected. If you adjust the number of load steps for load cases and load combinations globally to 1, you can define your material:

• ### I have edited the material in my model. When starting the calculation, I get an error message. What can be the reason for this?

Materials are required to define surfaces, cross-sections, and solids. The material properties affect the stiffnesses of these objects.

There are 13 material models available if you have a license for the RF‑MAT NL add-on module.

In the case of the abundance of material models, it is necessary to make sure that you assign the corresponding material model to the members and their surfaces/solids.

In the example shown here, surfaces have been generated from a member for a detailed analysis. There is still an unused cross-section defined (marked in blue) and the material is entered for the member cross-section as well as for the surfaces. When editing an existing material to Isotropic Nonlinear Elastic 2D/3D , the 2D/3D material model is also defined for the created member cross-section, which leads to the error message.

When working with members and surfaces / solids, it is recommended to create more than one material.

• ### When entering a new material using the Diagram definition type, the first row is locked. How can I define the material?

For the first point, the modulus of elasticity of the material is assumed in the material model, so that there is quasi an initial state for the solver to ensure the numerical stability.

If there is no material selected, the first point is not calculated as expected when creating the material model using the Diagram definition type, and cannot be adjusted reasonably.

To avoid this, you have to select the material in advance. Then, you can create the material model as usual, and also adjust the points. The first point can now be adjusted with regard to the stress.

• ### What is the difference between the materials Isotropic Plastic 1D and Isotropic Nonlinear Elastic 1D?

The difference between the two material models is as follows:

• In the Isotropic Nonlinear Elastic 1D material model, no plastic deformations are considered. This means that the material returns to its initial state after the load relief.
• In the case of the Isotropic Plastic 1D material model, however, the plastic deformation is taken into account.

For both material models, you can define the nonlinear properties in an additional dialog box. When entering data by means of a diagram, it is possible for both models to define a distribution after the last step.

For the 'Isotropic Nonlinear Elastic 1D' material model, an antimetric input of the stress-strain diagram is possible (different for the positive and negative range), while for the 'Isotropic Plastic 1D' model, only symmetric input is allowed.

• ### It is not possible to enter a nonlinear stress-strain curve. The diagram is initially displayed correctly, but then an error message appears saying that the positive values have to be only entered in ascending order. How can I avoid this message?

When using a diagram in the program, the first strain is always given (initial strain). It depends on the resulting modulus of elasticity and cannot be controlled directly. For this, you can use a trick in the program and adjust the first strain to a desired value anyway. To do this, you have to calculate the initial modulus of elasticity and enter it in the material parameter. In your case, it would be possible to use the following procedure.

• ### What is the best way to consider steel fiber concrete in the structural analysis software RFEM?

The "RF‑MAT NL" add-on module allows you to use the nonlinear material model "Isotropic Damage 2D/3D" in RFEM to define the stress-strain diagram for the steel fiber concrete. The internal forces and deformation can be determined in the subsequent nonlinear FE calculation.

The two links of the FAQ contain two interesting technical articles about the steel fiber concrete.
• ### Is it possible to perform a seismic analysis with the masonry material model?

The RF-/DYNAM Pro - Equivalent Loads add-on module only contains a linear analysis of structures. If you now apply a nonlinear model for the calculation, RF‑/DYNAM Pro - Equivalent Loads will modify it internally and treat it as a linear model. The nonlinearity in your model is the masonry, which cannot absorb any tensile forces.

The problem is as follows: RF-/DYNAM Pro - Equivalent Loads calculates the equivalent loads linearly and exports the load cases from them. However, the load cases are subsequently calculated nonlinearly on the basis of the material model, which is not entirely correct. Furthermore, the results are superimposed according to the SRSS or CQC method, which results in tensile and compressive forces being present in the model.

In this case, you could change the masonry to isotropic linear and work with linear properties of the material model, for example. Additionally, it is possible to insert line hinges at this location, which could be used to avoid the moment restraint, for example.

• ### How does the "Orthotropic Plastic" material model work in RFEM?

The material model according to Tsai-Wu unifies the plastic with the orthotropic properties. In this way, it is possible to specifically model the materials with anisotropic properties, such as plastics or timber. If the material is plastified, the stresses remain constant. The redistribution is carried out according to the stiffnesses available in the individual directions. The elastic region corresponds to the "Orthotropic - 3D" material model. For the plastic area, the yielding according to Tsai-Wu applies:

${\text{f}}_{\mathrm{crit}}\left(\mathrm\sigma\right)=\frac1{\mathrm C}\left[\frac{\left({\mathrm\sigma}_{\mathrm x}-{\mathrm\sigma}_{\mathrm x,0}\right)^2}{{\mathrm f}_{\mathrm t,\mathrm x}{\mathrm f}_{\mathrm c,\mathrm x}}+\frac{\left({\mathrm\sigma}_{\mathrm y}-{\mathrm\sigma}_{\mathrm y,0}\right)^2}{{\mathrm f}_{\mathrm t,\mathrm y}{\mathrm f}_{\mathrm c,\mathrm y}}+\frac{\left({\mathrm\sigma}_{\mathrm z}-{\mathrm\sigma}_{\mathrm z,0}\right)^2}{{\mathrm f}_{\mathrm t,\mathrm z}{\mathrm f}_{\mathrm c,\mathrm z}}+\frac{{\mathrm\tau}_{\mathrm{yz}}^2}{{\mathrm f}_{\mathrm v,\mathrm{yz}}^2}+\frac{{\mathrm\tau}_{\mathrm{xz}}^2}{{\mathrm f}_{\mathrm v,\mathrm{xz}}^2}+\frac{{\mathrm\tau}_{\mathrm{xy}}^2}{{\mathrm f}_{\mathrm v,\mathrm{xy}}^2}\right]$

where:

${\mathrm\sigma}_{\mathrm x,0}=\frac{{\mathrm f}_{\mathrm t,\mathrm x}-{\mathrm f}_{\mathrm c,\mathrm x}}2$

${\mathrm\sigma}_{\mathrm y,0}=\frac{{\mathrm f}_{\mathrm t,\mathrm y}-{\mathrm f}_{\mathrm c,\mathrm y}}2$

${\mathrm\sigma}_{\mathrm z,0}=\frac{{\mathrm f}_{\mathrm t,\mathrm z}-{\mathrm f}_{\mathrm c,\mathrm z}}2$

$\mathrm C=1+\left[\frac1{{\mathrm f}_{\mathrm t,\mathrm x}}+\frac1{{\mathrm f}_{\mathrm c,\mathrm x}}\right]^2\frac{{\mathrm E}_{\mathrm x}{\mathrm E}_{\mathrm p,\mathrm x}}{{\mathrm E}_{\mathrm x}-{\mathrm E}_{\mathrm p,\mathrm x}}\mathrm\alpha+\frac{{\mathrm\sigma}_{\mathrm x,0}^2}{{\mathrm f}_{\mathrm t,\mathrm x}{\mathrm f}_{\mathrm c,\mathrm x}}+\frac{{\mathrm\sigma}_{\mathrm y,0}^2}{{\mathrm f}_{\mathrm t,\mathrm y}{\mathrm f}_{\mathrm c,\mathrm y}}+\frac{{\mathrm\sigma}_{\mathrm z,0}^2}{{\mathrm f}_{\mathrm t,\mathrm z}{\mathrm f}_{\mathrm c,\mathrm y}}$

The stress criterion can be imagined as an elliptical surface within a six-dimensional space of stresses. If one of the three stress components is applied as a constant value, the surface can be projected onto a three-dimensional stress space.

If the value for fy(σ) is smaller than 1, the stresses rest within the elastic area. The plastic area is reached as soon as fy(σ) = 1. Values higher than 1 are not allowed. The model behavior is ideal-plastic, which means there is no stiffening.

• ### How does the license distribution work for the RF‑MAT NL add‑on module in the case of a network dongle?

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As soon as the material with a nonlinear material model assigned has been created, the license for RF‑MAT NL is used. The license is not released until you close the file or change the material model. Therefore, the license is used for the entire duration, even if no calculation is carried out.

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