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• ### How would you best model hold down elements within panels?

In a short overview, creating hold down elements involves modeling rigid links and adding in nodal supports with non-linearity settings that allows the supports to take only tension forces. A line support is added at the bottom of the wall that only takes compression forces. The individual nodal supports connected with rigid members only take tension forces.

A more detailed look on how these elements can be model can be seen in the video below.
• ### How is the friction set on a member hinge via the COM interface?

Friction is a nonlinearity and can therefore only be modified via the interface to the member hinge.

For this, it is first necessary to create the member hinge, if not already available. Then, the IMemberHinge interface is brought to the member hinge and then to the nonlinearity (here IFriction). Then, you can use the methods GetData and SetData to modify the data (here Friction):

Sub SetMemberHingeFriction ()    Dim model As RFEM5.model    Set model = GetObject(, "RFEM5.Model")    model.GetApplication.LockLicense    On Error GoTo e    Dim data As IModelData    Set data = model.GetModelData    Dim hinge(0 To 0) As RFEM5.MemberHinge    hinge(0).No = 1    hinge(0).RotationalConstantX = 1    hinge(0).RotationalConstantY = 2    hinge(0).RotationalConstantZ = 3    hinge(0).TranslationalConstantX = 4    hinge(0).TranslationalConstantY = 5    hinge(0).TranslationalConstantZ = 6    hinge(0).Comment = "Member Hinge 1"        hinge(0).TranslationalNonlinearityX = FrictionAType    data.PrepareModification    data.SetMemberHinges hinge    data.FinishModification        ' get interface for member hinge    Dim imemhing As IMemberHinge    Set imemhing = data.GetMemberHinge(1, AtNo)        ' get interface for nonlinearity 'friction'    Dim iFric As IFriction    Set iFric = imemhing.GetNonlinearity(AlongAxisX)        ' get friction data    Dim fric As Friction    fric = iFric.GetData        fric.Coefficient1 = 0.3        ' set friction data    data.PrepareModification    iFric.SetData fric    data.FinishModification        e:  If Err.Number <> 0 Then MsgBox Err.Description, , Err.Source    Set data = Nothing    model.GetApplication.UnlockLicense    Set model = NothingEnd Sub

In the case of the friction Vy + Vz, the Coefficient2 is used to set the second coefficient. The spring constant in the Friction dialog box is controlled by the translational spring of the member hinge. In this particular case, this is TranslationalConstantX for the X‑direction (see Figure 01).

• ### I have modeled a continuous beam. One of the supports should only act as of the deformation of 10 mm. How can I set such a nonlinearity?

In this case, it is recommended to use the definition of slippage. For this, select Partial Activity as a "Nonlinearity" in the "Edit Nodal Support" dialog box. In the "Nonlinearity - Partial Activity" dialog box, you can define the slippage in the respective zone. For checking purposes, there is a diagram, see Figure 01.

• ### I would like to define a line support with ineffective tension and apply the tension force on this line by using a nodal support instead. 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, the line supports and nodal supports are treated on each FE node. If there is a nodal support located on a line support, an FE node thus receives several support definitions. If the defined directions of the supports are not equal, this is not critical and the warning message can be ignored. If the same directions are defined several times, discrepancies may occur.
In the case of a line support that is failing under the tension and a 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 a support definition in the area of each nodal support. It may also be useful to model a tension bracket by using a newly defined member. The force transmission can then be adjusted by using the support of the member, the member type itself, and the member end release.

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

• ### There are two different add-on modules for a response spectrum analysis in RF‑/DYNAM Pro. What can be the reasons for the different results of both add-on modules?

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

In the case of the same settings, there should also be the same results calculated in both add-on modules. However, this does not apply to the existing nonlinearities. The reason is that there are no nonlinearities considered in the RF‑/DYNAM Pro add-on module. If displaying the results in the Forced Vibrations add-on module, all nonlinearities are thus ignored. In contrast, the equivalent loads are calculated on a linear structural system, but the exported load cases are then calculated on a real structure in RFEM and RSTAB, that is, with all nonlinearities. This may lead to inconsistent results.

If you deactivate the nonlinearities for the exported load cases, you should obtain the same results.

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

• ### Is it also possible to use nonlinear material behavior in a time history analysis?

To be able to consider nonlinearities in dynamics, the RF-/DYNAM Pro - Nonlinear Time History add-on module is required in RFEM/RSTAB. This module is different in RFEM and RSTAB, depending on the type of nonlinearities to be applied.

###### RSTAB – Nonlinear Time History Analysis

• Nonlinear member types, such as tension and compression members as well as cables
• Member nonlinearities, such as failure, tearing, and yielding under tension or compression
• Support nonlinearities, such as failure, friction, diagram, and partial activity
• Hinge nonlinearities, such as friction, partial activity, diagram, and fixed if positive or negative internal forces

###### RFEM – Nonlinear Time History Analysis

In addition to the nonlinearities mentioned above, it is also possible to use nonlinear material behavior.

• ### What is the sign definition for the line release nonlinearity?

The determination of the direction is always determined by the line release axis system and by the position of the cut-free object. You can activate the display of the line-release axis system in the Project navigator - Display under Model -> Line releases (see Figure 1).

To specify the direction for the determination of nonlinearities, it is important to know, how the released object moves relative to the line release axis system of the original element.
In Figure 2, surface 2 is defined as a released object with the line release uz if vz is defined as negative. The loading shown in FIGS. 1 and 2 would move the released surface 2 counter to the z-axis of the line release axis system. Thus, the line release would not be effective for these loads, i.e. surfaces 1 and 2 would be firmly connected.

• ### Why do I need the RF‑CONCRETE NL add‑on module for the stability design in RF‑CONCRETE Members? EC 2, Section 5.8.6, only speaks about geometric nonlinearity.

For the stability design of compression elements, you need the combination of RF‑CONCRETE Members and RF‑CONCRETE NL. The reason is the following:

First, the internal forces of the individual load combinations (second-order analysis + imperfection) are subjected to the linear-elastic calculation. For this, you basically only need RFEM.

Then, the cross-section design is performed in RF-CONCRETE Members with these internal forces determined linearly-elastically, and the required bending reinforcement is determined from these internal forces.

This bending reinforcement is then compared with the user-defined entries concerning the existing basic reinforcement or the minimum reinforcement and based on this, the reinforcement concept is generated (dialog box "3.1 Existing Longitudinal Reinforcement" of the module).

This existing longitudinal reinforcement is then used for the nonlinear design.

According to Section 5.8.6 (1), geometric nonlinearities must be taken into account according to the second-order analysis. However, the general rules for nonlinear methods according to 5.7 also apply.

In Sec. 5.7(1), "an adequate non-linear behaviour for materials is assumed." According to 5.7(4)P, the use of material characteristics which represent the stiffness in a realistic way but take account of the uncertainties of failure shall be used when using non-linear analysis.

This requires the RF-CONCRETE NL add-on module. Thus, the geometric and material nonlinearities are considered and the requirements of EC 2 regarding the ultimate limit state design are fulfilled.

Similarly, this method is also available in RSTAB in the CONCRETE add-on module.

• ### How can I model a hole bearing in the model?

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In the case of a hole bearing, the forces are only transferred by compression. Tensile forces do not occur. These effects could be modeled as follows (for an example, see the figure):

1. Create an opening in a surface.

2. Create a beam member with the nonlinearity failure under tension and apply it in the opening. It is important to use a beam member and no compression member (a compression member is a truss member, which would make the structural system kinematic).

3. Rotate the member in the opening and copy it several times.

Instead of the member, you could also fill in the opening by a surface with the stiffness membrane without tension. When using this surface stiffness, the following happens:

The calculation is performed in several iterations. After the first iteration, it is checked, in which surface element for the main membrane stress the tension occurs. These elements will fail in the next iteration (to be more precise, the stiffness is greatly reduced).

Both modeling variants should give you roughly the same results.

• ### I've got a mechanical system that behaves nonlinearly, and I want to analyze it via direct time step integration (in time range / dynamically). Which method is best used for this?

In RFEM 5 or RF-DYNAM Pro - Nonlinear Time History, there are two different methods (also called "solvers" hereafter) available to you for nonlinear, dynamic analyses: the explicit central difference method and the implicit NEWMARK method of mean acceleration (γ = ½ and β = ¼).

In the case of linear systems, the implicit solver is preferable in most cases, because numerically it is absolutely stable, regardless of which time step length is selected. Of course this statement has to be somewhat relativized, given the fact that if the time steps are selected too crudely, substantial inaccuracies in the solution are to be expected. The explicit solver also has only limited stability in linear systems; it becomes stable, when the selected time step is smaller than a specific, critical time step:

$\triangle t\leq\triangle t_{cr}=\frac{T_n}\pi$

In this equation, Tn represents the smallest natural vibration period of the FE mesh, which leads to the following statement: The finer the FE mesh gets, the smaller the selected time step should become, in order to ensure numerical stability.

The calculation time of a single time step of the explicit solver is very short, but countless, very fine time steps may just be necessary to get a result at all. For that reason, the implicit NEWMARK solver for dynamic loadings that act over a long period of time, is preferable most of the time. The explicit solver is preferred, when you need to select very fine time steps anyway to get a useful (converging) result. This is the case, for example, in short-term and erratically variable loadings such as loads from shock or explosion.

In nonlinear systems, both methods are "only" numerically stable, but the implicit NEWMARK solver is still more stable than the central difference method in most cases. For that reason, the same statements as for linear systems apply to nonlinear systems. When the loads are erratically variable and short-term, the explicit solver is preferable, but in most other cases the NEWMARK solver of mean acceleration is preferred.

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#### First Steps

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#### 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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