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• ### Is there a possibility in RFEM to consider friction for the horizontal reactions of  surface support?

For surface supports, this option is only available if a non-linearity in the local z direction has also been defined (failure if contact stress in z is negative/positive). In the dialog box where you can edit the non-linearity, there is the option 'Friction in plane xy'.

This option works as shown in the graphic dialog box: The support in x and y direction is only completely assumed when the contact stress Tau (contact stress Sigma by friction coefficient) is reached. It has to be reduced linearly before this.

In order to use this option, it is necessary to define a support in the horizontal directions. It can be defined as fixed or with an elastic spring. If the spring is defined with 0, no support is considered even when a friction coefficient is entered.

• ### In a nonlinear system, some load cases do not converge. The load combinations, however, can easily be calculated. What is the cause of it?

FAQ 004114 EN

When defining non-linearities, for example, failure of support under tension, it may happen that some load cases cannot be calculated. If these are loads that cannot exist without other stabilizing loads, solving the problem is simple: You can set the load cases to 'Not To Be Calculated'. As a result, only the load combinations are considered in the option 'Calculate All' of the calculation process. This is possible because, for example, some loads can never appear without having a self-weight.

In the attached example, it is clearly evident that the system would buckle in the load case wind, and thus no convergence is found. In contrast to this, it is possible to calculate the load combination in which self-weight and wind are combined without a problem because the self-weight stabilizes the system.
• ### 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.

• ### Is it also possible to perform a deformation analysis in cracked state for a 2D position in RF-CONCRETE Surfaces?

In this case, the Calculation Method and the type of the 2D position are important.

When using the analytical method (RF-CONCRETE Deflect), it is possible to perform a calculation in 2D positions. When using the nonlinear method (RF-CONCRETE NL), the calculation for 2D XY (uZXY) is not possible. In the nonlinear calculation, shrinkage is represented internally as strain load, which is not possible in this type of 2D position due to the limited degrees of freedom.

Convert 2D to 3D Position
In the general data, it is possible to simply convert a 2D position into a 3D position. For the supports, all degrees of freedom not contained in the 2D position are fixed when converting to a 3D position (see the video).
• ### I get different results when comparing the deformation analysis in the RF-CONCRETE Modules and another calculation program. What could be the reason for this?

Differences resulting from the determination of the deformation in the cracked state can have different causes. The following points should be checked for deviations:

Is the same calculation method applied?
RF-CONCRETE Deflect uses an analytical analysis approach according to EN 1992-1-1 7.4.3.
RF-CONCRETE NL uses a physically nonlinear analysis approach.
More detailed information about the calculation methods can be found, for example, in the RF-CONCRETE Surfaces Guide in Chapter 2.7 and 2.8.

Is the same initial structure available?
The results of the linear calculation provide the best tool for consideration if the underlying system should be taken for equivalent. The linearly determined deformation of the underlying combination should be approximately equal. Any possible differences in the linear deformation may be increased in the cracked state in connection with the deformation analysis.

Are the same effects taken into account?
When performing a comparison, make sure that the same effects as for example creep and shrinkage are taken into account (Figure 02).

Are the same input values available?
Furthermore, it should be checked in connection with the deformation analysis if the same input values are available. In this case, it is necessary to pay particular attention to whether the applied reinforcement (Figure 03) and the lever arm or concrete cover are the same.

• ### I would like to define line support with ineffective tension and include, instead, the tension force via nodal support on this line. Why does the line support still receive a tension force?

If nodal supports are modeled on supported lines, this may lead to problems and incorrect definitions. Therefore, the following warning message appears in the plausibility check.

Internally, line supports and nodal supports are treated on each FE node. If a nodal support is located on a line support, an FE node thus receives several support definitions. If the defined directions of the supports do not match, this is not critical and the warning message can be ignored. If the same directions are defined several times, contradictions may occur.
In case of line support which is failing on the tension and nodal support on this line, the tension force thus results in the FE node which however counts to the line support and to the nodal support.

To avoid this behavior, it is possible to insert a short line without support definition in the area of each nodal support. It may also be useful to model the tension bracket using a newly defined member. The force transmission can then be adjusted by means of the support of the member, the member type itself, and the member end hinge.

In general, the support stiffenings should be estimated realistically; in the example, rigid supports were assumed as a simplification.

• ### When converting from manual definition of the reinforcement areas to automatic arrangement of the reinforcement according to Window 1.4, the result of the deformation calculation changes, although the basic reinforcement has not been modified. What is the reason for this change?

When using the automatic arrangement of the reinforcement, make sure that, in addition to the basic reinforcement, additional reinforcement is applied, if applicable, for the serviceability limit state designs. The default setting in the add-on module is such that the required reinforcement from the ultimate limit state is also applied for the SLS.

You can select the reinforcement that is applied for the SLS in 'Additional Reinforcement for Serviceability State Design'. The Info button explains the individual options (Figure 02).

The reinforcement applied for the designs can be reproduced via the function 'Selected Reinforcement for SLS Check' (Figure 03).

• ### There are two different modules for the response spectrum analysis in RF-/DYNAM Pro. What are the reasons if the results of both modules differ?

The differences between the two modules are explained in this FAQ .

In general, you should also calculate the same results for both add-on modules if the settings are identical. However, this does not apply to existing nonlinearities. This is because no nonlinearities are considered in the RF-/DYNAM Pro add-on module. If the results are output via the Forced Vibrations add-on module, all nonlinearities are ignored. In contrast to this, the equivalent loads are calculated on a linear system, but the exported load cases are then calculated on the real system, that is, with all nonlinearities in RFEM or RSTAB . This may lead to inconsistent results.

If you deactivate the nonlinearities for the exported load cases, they should have identical results.

The way of considering nonlinearities in the response spectrum analysis is described using the tension members in this FAQ.

• ### 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.

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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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