#### Further Information

In addition to our technical support (e.g. via chat), you’ll find resources on our website that may help you with your design using Dlubal Software.

Receive information including news, useful tips, scheduled events, special offers, and vouchers on a regular basis.

• ### Why do I get much higher design ratios in RF-/STEEL compared to cross-section design in RF-/STEEL EC3?

In the RF-/STEEL add-on module, an equivalent stress design is performed according to von Mises. An elastic stress design (EL-EL) is to be made. In RF-/STEEL EC3, a classification is carried out before the design. If the cross-section is classified as class 1 or class 2, the design is performed against plastic limit internal forces. An EL-PL design is performed. If you do not want to use the plastic load reserves, you can switch the design to EL-EL in the details of the RF-/STEEL EC3 add-on module. The results are then comparable with RF-/STEEL.
• ### 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 when the load is released.

Whereas in the case of the material model Isotropic Plastic 1D, 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 to define a distribution in both models after the last step.

For the material model Isotropic Nonlinear Elastic 1D, it is possible to enter the stress-strain diagram (different for the positive and negative zone) in an anti-metrical way, whereas for the model Isotropic Plastic 1D, only symmetric input is possible.

• ### Why are the equivalent member designs grayed out in the Stability tab when the plastic designs are activated by means of the partial internal force method (RF-/STEEL Plasticity)?

Since the equivalent member designs of Eurocode 3 have different interactions than are the case for the designs according to the partial internal forces method and a mixture of these different designs is not desired for reasons of clarity, RFEM deactivates the equivalent member designs when using the RF-/STEEL Plasticity add-on.
• ### How does the material model Plastic 1D work?

FAQ 003578 EN

The plasticity for 1D elements currently only works in relation to the normal stresses in a member. This means that only interaction between axial force and moment is possible. The shear force interaction is not taken into account. In addition, the stresses from shear force are only calculated elastically.

When applying a plastic material model, it is also important to ensure a sufficient division of the elements, because a cross-section is internally generated at each Gauss point on the member element where the stress is calculated and a reduction of the stiffness to the re-distribution of the internal forces is performed, if necessary. If, for example, the number of divisions is increased, the model may become unstable because the redistributions of stresses can no longer be carried out and thus the cross-section's loading is too high.

It is generally recommended to use a division of '50' for member elements when using the plastic material model (see the figure).

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

The material model according to Tsai-Wu unifies plastic with orthotropic properties. This way, you can enter special modelings of materials with anisotropic characteristics such as plastics or timber. When the material is plasticized, stresses remain constant. A redistribution is carried out according to the stiffnesses available in the individual directions. The elastic zone corresponds to the material model Orthotropic - 3D. For the plastic zone, the yielding according to Tsai-Wu applies:

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

with:

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

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

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

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

The yielding condition can be thought of as an elliptical surface in a six-dimensional space of tension.
If one of the three stress components is applied as a constant value, the surface can be projected onto a three-dimensional stress space. Projection of yielding surfaces for normal stresses according to Tsai-Wu If the value for fy (σ) is smaller than 1, the stresses lie within the elastic range. The plastic zone is reached as soon as fy (σ) = 1; values greater than 1 are not allowed. The model behavior is ideal-plastic, which means no stiffening takes place.
• ### Is the plastic section modulus of cross-sections implemented in your program?

FAQ 003280 EN

The plastic section modulus for cross-sections is implemented in the library. Among other things, it is used for the limiting moments of plastic member hinges. There is also an interesting technical article for this.
• ### How is it possible to determine which plastic deformation remains in an RFEM model with a plastic material model after the relief?

FAQ 003031 EN

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 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 (Figure 1).

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

This Link provides you with a full set of verification examples to this material model.

The material model itself is explained in several Knowledge base articles.

• ### 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
• ### When trying to calculate a material from the "Aluminum" category in the RF- / EL-PL add-on module, I get an error message. What is the reason for this?

The RF- / EL-PL add-on module is designed only for the elastic-plastic design of steel cross-sections. As a result, you can only calculate materials from the "Steel" category in the add-on module.

1 - 10 of 20

If not, contact us via our free e-mail, chat, or forum support, or send us your question via the online form.

#### 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.

#### Your support is by far the best

“Thank you for the valuable information.

I would like to pay a compliment to your support team. I am always impressed how quickly and professionally the questions are answered. I have used a lot of software with a support contract in the field of structural analysis, but your support is by far the best. ”