#### Knowledge Base

In the Knowledge Base, you’ll find technical articles and tips & tricks that may help you with your design using Dlubal Software.

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• ### Where do I find the setting to specify the entered structural component as a "wall" or "floor"?

FAQ 002367 EN General RFEM 2D RFEM RSTAB 2D RSTAB

You find this setting in the "General Data" of the model.
You can distinguish between the model types "3D", "2D - XY", "2D - XZ" and "2D - XY".

If you want to enter a floor as a 2D model, and the axial forces within the slab should not be taken into account, set the model type to "2D - XY". If you want to enter a wall, set "2D - XZ".

Find more information in the RFEM manual, chapter 12.2.1 General - Type of Model.
• ### Can you give me a hint where to activate the design for limiting a gaping joint according to DIN 1054: 2010-12 A 6.6.5 in the FOUNDATION Pro add-on module?

FAQ 002365 EN Add-on Modules FOUNDATION Pro RF-FOUNDATION Pro

First, select the National Annex "DIN" in the "1.1 General Data" window. Then, open the [Details...]. A dialog box opens where you can find the design for limiting the gaping joint which is described as "Highly eccentric loading in the core". See Figure 01.
• ### The address and/or company name was changed in the authorization file, but this is not applied in the print header

FAQ 002364 EN Printout Report SHAPE-THIN PLATE-BUCKLING Stand-Alone RFEM 2D RFEM RSTAB 2D RSTAB COMPOSITE-BEAM CRANEWAY

The title block of the printout report is created using the authorization file, which was used in the initial installation of our programs.
A subsequent change in the authorization file does not necessarily change all report headers immediately.

For the changed title block to be created as default in the authorization file, the "old" one has to be removed first.
To do so, please delete the file "DlubalProtocolConfigNew.cfg" in the following (default) directory:
C:\ProgramData\Dlubal\Global\General Data

While deleting the file, our applications must be closed!
• ### Is it possible to define a new user-defined cross-section with inclined principal axis in RSTAB or RFEM?

FAQ 002363 EN General RFEM RSTAB

Yes, this is done by creating a user-defined cross-section in the cross-section library.
• ### How do I divide a surface in RFEM?

FAQ 002362 EN Modeling | Structure RFEM

RFEM provides two ways to divide surfaces:

1)  Split Surface

The requirements to split a surface evenly are that it has four sides and is convex; in other words, none of the four internal angles may be greater than 180°.
This function is accessed by right-clicking the surface and selecting "Split Surface". In the corresponding "Split Surface" dialog box, the number of divisions in each direction and the individual relative distances are entered (see Figure 1).

2)  Divide Surface

If any lines are drawn within the surface plane, they may be used to divide the surface (see Figure 2). First you have to check, whether the division lines are integrated in the surface. This is normally the case due to the automatic object detection (see Figure 3). Afterwards, right-click on the surface and select "Divide Surface" to use the desired function (see Figure 4).
• ### How do I take eccentric load introduction into account?

To take into account external concentrated loads that act eccentrically on a member, it is possible to model a connection via a rigid member. The rigid member is connected perpendicularly to the corresponding member, and the length of the rigid member corresponds to the value of the eccentricity.
Alternatively, the corresponding torsional moment, which is a result of the eccentric load introduction, could be entered as external loading. This way, it would not be necessary to define a rigid member and the corresponding eccentricity would still have been taken into account.
• ### How can I use SHAPE-MASSIVE cross-sections for tapered members?

FAQ 002359 EN Modeling | Structure RFEM RSTAB

To use SHAPE-MASSIVE cross-sections for tapered members, two conditions must be met.
1st In the General Data of the SHAPE-MASSIVE structure, the option "Stresses in stress points" must be deactivated. See Figure 01.
2nd The nodes (corner points) of the cross-sections used for the taper must be arranged the same way in both cross-sections. See Figure 02.
• ### Can I define average regions on curved surfaces as well?

FAQ 002358 EN Results RFEM

To define an average region on curved surfaces, the projection direction has to be clearly defined. Since this is not possible for the "Perpendicular" option, you can define the direction yourself. Either along the global axes X, Y, and Z, or with a vector.
• ### Is is correct, that you have to enter the allowable soil pressure as the characteristic value in input form 1.1 General Data in FOUNDATION Pro? Is the design value determined automatically?

FAQ 002357 EN Add-on Modules RF-FOUNDATION Pro FOUNDATION Pro

Yes, this is correct.

The characteristic value of soil pressure is entered in input form 1.1 General Data. See Figure 01.

The design value is:
${\mathrm\sigma}_\mathrm{Rd}\;=\;{\mathrm\sigma}_\mathrm{Rk}\;/\;\;{\mathrm\gamma}_{\mathrm R;\mathrm v}$

The estimated partial factor for spread foundations can be found in the parameters of the national annex you selected for the design. See Figure 02.
• ### I've got a mechanical system that behaves nonlinearly, and I want to analyse it via direct time step integration (in time range / dynamically). Which method is best used for this?

FAQ 002356 EN Calculation RF-DYNAM Pro - Nonlinear Time History

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 relativised, 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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