# Frequently Asked Questions (FAQ)

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• ### I have set a variable load distribution for a generated load. This should be applied incrementally in a certain direction. However, the load is not stepped, but is displayed with a linear distribution. How can I display the load correctly?

In order to display the loads correctly, it is necessary to make some adjustments. In the case of an incrementally applied load, the boundary of the area load plane may only be defined in sections (by load increment). Otherwise, the load is distributed linearly over the entire area load plane.
• ### How is it possible to control in RF‑LOAD‑HISTORY whether the deformations of the individual load steps are determined separately in each load step or added?

This setting is available in the RF‑LOAD‑HISTORY add-on module under "Details" → "Options" → "Type of Deformation," see Figure 01.

The "In particular load step" type of deformations displays the deformations of each individual load step separately, the "On undeformed structure" type shows the sum of all load steps.

• ### How can I import an IGES or STEP file into RFEM?

For the import of the *.igs or *.stp (STEP) files, the RX‑LINK add-on module is required (see Figure 01). The export is currently not possible.

Furthermore, it is possible to import a model with the CAD/BIM option. After selecting the file to be imported, it can be activated in the Options tab of the "New Model - General Data" dialog box (see Figure 02). This option allows you to additionally convert the geometric elements directly into members, surfaces, or solids after the import (see Figure 03).

• ### Which import/export options between SolidWorks and Dlubal Software are available?

As CAD software, SolidWorks currently provides the output via the IGES and STEP interfaces, which can be imported by our software, but cannot be exported. For the greatest possible comfort, the CAD/BIM option should be activated (see the link).

Furthermore, it is also possible to exchange data via DXF files. In this case, RFEM is also able to export or import 3D structures and surfaces.

• ### I am working with the RFEM or RSTAB program for the first time. What is the easiest way to learn to work with it?

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On our website www.dlubal.com, you can find useful tips in the form of PDF files, videos, FAQ, or Knowledge Base articles. For an easy start, we have set up an additional page for beginners with all necessary information. Please follow the links below this FAQ.
• ### Is there a way how to easily prevent RF‑SOILIN from recalculating the foundation every time the load in the governing load case is changed?

No, that is not possible. The calculation of the foundation parameters in RF‑SOILIN is performed iteratively. The first iteration step requires that the start values for the foundation parameters are selected internally by the program. These start values can be used to perform a finite element analysis of the FE model in RFEM.

The result is the soil contact stress distribution. The soil contact stress of the first iteration step is included in the RF‑SOILIN calculation as the initial value. Together with the stiffness modulus of the entered soil layers, it is possible to calculate the settlement for each finite element. The settlement and the soil contact stress are then used to calculate the foundation parameters.

In the next iteration step, the new foundation parameters replace the old ones, and a new finite element analysis is started, which results in a new distribution of soil contact stress. As a convergence criterion, the new distribution of soil contact stress is compared with the old one.

As long as the deviation exceeds a certain convergence limit, the new distribution of soil contact stress in RF‑SOILIN is considered in the calculation of the new foundation parameters. In the case the the deviation of the soil contact stress distribution of two consecutive iteration steps is not reached for the first time, the iteration is ended and the foundation parameters of the last iteration step are given as a result in RF‑SOILIN.

• ### Can I display the results of the individual load increments after the calculation?

You can access the results of the individual load increments by activating the "Save the results of all load increments" option in the calculation parameters of the load case or load combination (see Figure 01).

By using the "Calculation Diagrams" window in the calculation parameters, it is possible to additionally evaluate the individual results for the selected elements (see Figure 02). It is also possible to easily create several calculation diagrams.
• ### Which time step should I select for the calculation of the time history analysis in the RF‑/DYNAM Pro add‑on module?

There are two options available: an automatic time step selection and a manual one. Especially for a structure with nonlinearities, it is always recommended to select the time step manually, because the automatic determination is only carried out on the basis of the defined accelerograms or time diagrams. For this purpose, a time step convergence study should be performed, which compares the calculation time and the accuracy.

The time step to be selected depends on many factors, including the excitation frequency, the frequency and the size of the structure, as well as the degree of nonlinearities. Thus, it is not possible to make a general statement about the size of the time step.

To achieve sufficient accuracy, the governing period T = 1/f should be divided into approximately 20 steps, that is, the time step Δt should be selected as follows:

$\mathrm{Δt}\;<\frac{\mathrm T}{20}\;=\;\frac1{20\mathrm f}\;=\;\frac{\mathrm\pi}{10\mathrm\omega\;}$

For transiently defined suggestions, such as accelerograms or tabulated time diagrams, the shortest time period should be divided into 7 steps:

$\mathrm{Δt}\;=\;\frac{\mathrm{Min}\left\{{\mathrm t}_{\mathrm i+1}\right.-\;{\mathrm t}_{\mathrm i}\}\;}7$

Regardless of the calculation, time steps are specified to save the results.

• ### When importing a .stp file into RFEM, an error message appears saying that I should select a different format. How can I import .stp files?

Unfortunately, there are two different interfaces with the same file extension - the .stp interface of the German Steel Construction Association (Deutcher STahlbau-Verband, DSTV) on one hand, and the STEP interface (STandard for the Exchange of Product model data) on the other hand, which is defined in the ISO standard 10303.

While the DSTV interface refers to the system axes of members, that is to the structural model, the STEP interface refers to the model geometry in the form of boundary lines, surfaces and simple solids. In the latter case, the models can be imported either as RFEM objects (nodes, lines, surfaces, solids) or as background layers so that the nodes can be set manually. The STEP interface does not usually transfers any system lines, but only the outer contours of structural components so it is necessary to manually create system planes afterwards. You can do this in RFEM and RSTAB, for example, by using a CAD/BIM model.

If you get the error message shown in Figure 01, you have tried to import an .stp file based on STEP using the DSTV interface. As described above, both formats are not compatible with each other.

In this case, select the STEP format for the import, as shown in Figure 02. If a message shown in Figure 03 appears, it is because this interface requires the RF-LINK add-on module.

• ### What is the effect on the internal forces if I limit the number of iterations to "1" in the calculation parameters?

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The calculation will be done faster. HOWEVER: Since only one iteration has been calculated, the results are only correct if there is also just one iteration required for the calculation. For example, this can be the case of load cases using linear static analysis. Also, object with nonlinear properties, such as tension and compression members, may not fail.

On the contrary, load combinations according to second-order analysis require an iterative calculation. Therefore, the results of one iteration only are not correct.

The number of possible iterations should always be sufficiently high. The value can also be much larger than the actually required number of iterations: Only as many iterations as necessary are run through.

Result Table 4.0 shows the number of performed iterations (see Figure).

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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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