Further Information

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

The difference between both 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, the plastic deformation is considered.

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

The Isotropic Nonlinear Elastic 1D material model allows for the anti-symmetric input of the stress-strain diagram (different for the positive and negative zone), whereas the isotropic Plastic 1D model only allows for symmetric input.

• 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 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-reinforced concrete. The internal forces and deformation can be determined in the subsequent nonlinear FE calculation.
• 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 area corresponds to the Orthotropic Elastic - 3D material model. For the plastic area, the yielding according to Tsai-Wu applies:

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

where:

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

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

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

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

You can imagine the yield criterion as an elliptical surface in a six-dimensional stress space.
If one of the three stress components is applied as a constant value, the surface can be projected onto a three-dimensional stress space. The projection of yield surfaces for normal stresses according to Tsai‑Wu: 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; the values greater 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.
• How is it possible to determine which plastic deformation remains in an RFEM model with a plastic material model after the relief?

It is important to use the "Plastic 2D/3D" or "Plastic 1D" material model. How it works in practice is shown in this recording of a Dlubal Info Day.

• How is Young's modulus calculated for the "isotropic damage" material model?

Young's modulus is calculated for each step of the defined diagram according to Hooke's law:
ε = σ / E

The value of the current step is displayed beneath the diagram on the right side (see Figure 01).

• I have purchased the add-on module RF-MAT NL, but I cannot 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
• Do I have to calculate according to the second-order or large deformation analysis when using plastic material?

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No, it is not absolutely necessary to calculate according to the second-order or large deformation analysis when using a nonlinear material model. The material nonlinearity is also considered in the case of the calculation according to the linear static analysis.

The calculation according to the second-order analysis or the large deformation analysis means that the equilibrium is set on a deformed structure. So it is geometric nonlinearity.

The difference between the second-order and large deformation is that large rotation may occur in the case of the large deformation analysis.

Thus, if there is no stability problem or if the stability problem is further analysed, the calculation according to the linear static analysis is sufficient.

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First Steps

We provide hints and tips to help you get started with the main programs RFEM and RSTAB.

Wind Simulation & Wind Load Generation

With the stand-alone program RWIND Simulation, wind flows around simple or complex structures can be simulated by means of a digital wind tunnel.

The generated wind loads acting on these objects can be imported to RFEM or RSTAB.

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