Frequently Asked Questions (FAQ)

Turbulence modeling is a critical aspect of computational fluid dynamics (CFD) that seeks to predict the behavior of turbulent flows. Turbulence models are essential for designing efficient and safe engineering applications, such as wind-structure interaction in order to structural analysis and design. Among the various approaches to turbulence modeling, three popular models are the Reynolds-Averaged Navier-Stokes (RANS), Unsteady Reynolds-Averaged Navier-Stokes (URANS), and Delayed Detached Eddy Simulation (DDES). Each model has its own unique features and applications.

RANS (Reynolds-Averaged Navier-Stokes)

The RANS approach is one of the most common methods used in turbulence modeling. It involves averaging the Navier-Stokes equations over time, which effectively smooths out the fluctuations of turbulence to provide a steady-state solution. This method simplifies the computational requirements significantly and is particularly useful for applications where the flow is steady or mildly unsteady. RANS models are extensively used in industrial applications due to their robustness and low computational cost. However, they can be less accurate in predicting complex flows with significant separation or strong unsteadiness.

URANS (Unsteady Reynolds-Averaged Navier-Stokes)

URANS extends the RANS approach by allowing for time-dependent changes in the flow field, making it capable of capturing unsteady phenomena. It still utilizes the Reynolds averaging of the Navier-Stokes equations but does not average the flow in time as strictly as RANS. This means URANS can model larger-scale transient flow features and oscillatory behaviors, which are typical in many practical engineering systems, such as vortex shedding from building corners. While URANS improves upon RANS in terms of capturing unsteadiness, it still employs eddy-viscosity models that may not adequately resolve finer turbulent structures.

DDES (Delayed Detached Eddy Simulation)

DDES is a hybrid approach that combines RANS and Large Eddy Simulation (LES) methodologies. In regions of the flow where the boundary layer is attached, DDES behaves like a RANS model, providing computational efficiency. In regions where the flow detaches and larger turbulent structures dominate, DDES switches to an LES mode, which resolves these structures more accurately. This method is particularly useful in complex flows involving flow separation, reattachment, and wake regions, such as building edges and corners. DDES offers a good balance between computational expense and accuracy, particularly in simulating high Reynolds number flows with significant unsteady and separated regions.

Conclusion

Choosing the right turbulence model depends largely on the specific requirements of the problem at hand, including the flow characteristics, accuracy needs, and available computational resources. RANS models are suitable for simpler, steady flows, while URANS provides better handling of unsteady phenomena. DDES, although computationally more demanding than RANS or URANS, offers superior accuracy in cases involving complex, unsteady separated flows. Each of these models has contributed significantly to advancements in fluid dynamics simulations, supporting engineers and researchers in developing more effective and efficient technological solutions.

In the standards, torsion is not considered in the stability designs for the equivalent member method or the general method. The warning message confronts the user about this fact.

First of all, the magnitude of the torsional stress or moments should be analyzed. Very small torsional moments in the members to be designed often prevent certain design formats. In order to neglect them and suppress the warning, you can change the limit values in the ultimate configuration. There is no normative basis for this limit value and therefore, no general specification. More information can be found here:
Neglecting Torsion

In the case of open cross-sections, it can generally be assumed that the acting torsional moments are not transferred by primary torsion, but by secondary (warping) torsion. In RFEM 6 and RSTAB 9, the internal forces are initially only determined with six degrees of freedom without considering warping. For small acting torsional moments, it seems obvious to neglect the torsional stresses in the design. For large torsional moments, however, a calculation should be carried out with seven degrees of freedom in order to also consider the influence of the warping moment and the secondary torsional shear stresses. This is, for example, with the add-on Torsional Warping (7 DOF) for RFEM 6 / RSTAB 9 version will be issued to each company.

We usually deliver a new version of RFEM 6/RSTAB 9 on the Monday of every week; that is, usually 4 versions per month.

About a new version is displayed in the Extranet and in Dlubal Center. you will also find the links to download the available versions in the

In RFEM 6 and RSTAB 9, the calculation parameters are no longer managed in a dialog box with the same name, but in the form of Static Analysis Settings in the navigator category Load Cases and Combinations.

Three analysis types are preset:

• Geometrically linear
• Second-order (P-Δ)
• Large deformations

You can adjust the parameters of these analysis types or create more types.

Then, in the 'Load Cases & Combinations' dialog box, assign the relevant static analysis setting ( SA ) to the load cases and load combinations.

The Cutting Pattern add-on is still under development. Therefore, it cannot be activated in the Base Data.

The Timber Joints add-on is still under development. Therefore, it cannot be activated in the Base Data.

The Concrete Foundations add-on for foundation design is still under development. Therefore, it cannot be activated in the Base Data.

No, the module has not yet been implemented in RFEM 6/RSTAB 9. Es befindet sich aktuell noch in der Entwicklung.

Performance problems may be caused by the Fault Tolerant Heap (FTH). This is a Windows subsystem that is responsible for monitoring application crashes to prevent future application crashes.

To check whether the application (RFEM, RSTAB, RSECTION, and so on) is in the list of the applications to be checked, it is worth taking a look at the Windows registry. You will find the list in the following key:
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\FTH\State

As an alternative, you can also view the entries in the Windows Event Viewer. You will find it in the following path:
Applications and Services Logs → Microsoft → Windows → Fault Tolerant Heap

To reset the list of affected applications, run the command prompt as an administrator and enter the following command:
Rundll32.exe fthsvc.dll,FthSysprepSpecialize

The list should then be cleared and the application can be restarted without restrictions. If the problem occurs frequently, you can disable the FTH completely. The procedure and a summary of the FAQ can be found on the Microsoft Website.

Yes, you can control the load distribution by setting the limit stresses for tension as very high or small.

The "Independent Mesh Preferred" option allows you to create FE meshes between different objects that are independent of each other.

The only requirement is that these objects are integrated into one another.
This method allows you to create an individual FE mesh for different objects. This way, you can reduce the number of FE elements and thus save the calculation time.

A floor slab on a soil massif can be mentioned here as a very good example. In this case, the solid of the soil massif can be modeled with a coarser FE mesh, and the FE mesh of the integrated floor slab can be refined at the same time.

In RFEM 6 or RSTAB 9, you can use plastic hinges to simulate the nonlinear behavior of steel structures subjected to a load. You can model a plastic hinge by arranging a hinge with nonlinear properties on a member end. Proceed as follows:

• Select the member or members to which you want to assign the hinge.
• Open the "Edit Member" dialog box by double-clicking the member or using the member shortcut menu.
• Select the Hinges option in the "Main" tab.
• In the "Hinges" tab, click the button for the member start or the member end, to define a new hinge type.
• In the "New Member Hinge" dialog box, select one of the four plastic options for the relevant degree of freedom in the Nonlinearity list; for example, a plastic hinge according to FEMA 356 or a bilinear hinge according to EN 1998‑3.

• In the second tab, you can specify the yield properties, acceptance criteria, and yield strengths. The parameters are set by default based on the member properties and standards. If necessary, adjust it to describe the behavior of the hinge; for example, the bilinear yield curve including the yielding and hardening regions.

• Click OK to save the hinge.
• Clicking OK again in the "Edit Member" dialog box to assign the hinge to the member end.

In order to specify a uniaxial load transfer for a plate, you can assign orthotropic properties to the surface. This way, you can control the stiffnesses for specific directions so that the loads are mainly transferred in one direction. You have two options.

Orthotropic Thickness for Isotropic Material

• Double-click the surface to open the dialog box for editing the slab.
• In the Thickness with Material section, click the button to the right, to create a new thickness.
• In the "New Thickness" dialog box, select the Shape Orthotropy thickness type.

• Switch to the "Shape Orthotropy" tab. The Effective Thicknesses orthotropy type is set by default.
• For the uniaxial load transfer, enter the effective thicknesses as orthotropic properties in such a way that the stiffness is very high in the desired main support direction and very low in the other direction.

• To consider the self-weight, you can apply a user-defined fictitious thickness that corresponds to the full surface thickness.
• Click OK to save the thickness.
• Click OK again in the "Edit Surface" dialog box to assign the thickness to the plate.

Orthotropic Material Model for Constant Thickness

• Double-click the surface to open the dialog box for editing the slab.
• In the Thickness with Material section, click the button to the right, to create a new material.
• In the "New Material" dialog box, select the material from the library as usual. However, change the material model to Orthotropic | Linear Elastic (Surfaces) (1). Also, activate the User-defined material option (2).

• Switch to the "Orthotropic | Linear Elastic (Surfaces)" tab.
• For a uniaxial load transfer, change the modulus of elasticity of the secondary load direction to a very small value. You can also specify very small values for the shear moduli in the secondary support direction. Furthermore, set the Poisson's ratios to zero.

• Click OK to save the material.
• Click OK again in the 'Edit Surface' dialog box to assign the material of the plate.

If a Rhino surface cannot be created by using the geometry type, it will not be transferred. This is the case, for example, of curved surfaces with more than four boundary lines. In the Rhino command history (_CommandHistory or the F2 key combination), you can find a list of warnings and errors displayed during the transfer process:

Requirements

• RFEM 6 / RSTAB 9 is installed
• You have a valid license of WebServices.
• The Web Services are activated in the settings of RFEM 6 / RSTAB 9.
• IDEA StatiCa is installed.
• The BIM Link to RFEM 6 / RSTAB 9 by IDEA StatiCa is installed (IDEA StatiCa → "BIM" tab → "Activate Your BIM Link" → RFEM 6 / RSTAB 9).

Procedure

1. Create and calculate your RFEM 6 / RSTAB 9 model.
2. Open the shortcut "IDEA StatiCa 2X.X for RFEM 6" or" IDEA StatiCa 2X.X for RSTAB 9".
3. Select joint nodes and connecting member(s) in RFEM 6 / RSTAB 9.
4. Import the geometry into the checkbot via "Import" → "Connections".
5. Transfer the connection or member from the checkbot to IDEA StatiCa.

In RFEM and RSTAB, you can hide certain objects and thus create user-defined visibilities. These hidden objects are displayed in gray in the background.
The intensity of the gray color can be adjusted in the "Program Options & Settings" dialog box under the "Settings" tab.

You can select a value between 0 and 1. The higher the selected value, the darker the inactive objects are displayed.

The CFD simulation (Computational Fluid Dynamics) is a valuable tool for various areas of engineering, design and research. In the following, we present some of the most important advantages of CFD simulations in more detail.

Cost Efficiency

Using CFD simulations in large or complex structural systems is much more cost-effective than performing physical experiments. For example, the digital wind simulation eliminates the costs for the production of prototypes, securing test facilities, and the use of special equipment

Saving Time with CFD Simulations

You can also save some time in the analysis process by using a CFD simulation. The setup and execution are quick and easy, requiring only a few clicks, which saves you a lot of time compared to setting up a physical test. As an engineer, you can thus benefit from iterative analysis processes. Test your designs in a virtual environment to perfect all aspects digitally before deciding on a physical prototype.

Detailed Visualization

For example, CFD provides you with flow patterns, pressure distributions, and other relevant physical phenomena that would be difficult or impossible to demonstrate experimentally. This additional information gives you an even better insight into the technical data of your project.

Scalability in CFD Simulations

Another advantage of the CFD simulations is the arbitrary scalability without great effort. Do you want to analyze different sizes or configurations of an analysis? In the digital flow tunnel, this is easily possible without any physical changes or new prototypes. A CFD simulation provides you with a wider range of scenarios and analysis parameters.

Sustainability Due to CFD Simulations

Another benefit of the CFD simulation is that it requires fewer physical prototypes and experiments. Lower energy consumption and lower waste production is ideal for achieving sustainability goals, such as reduced material costs and addressing environmental concerns.

Complexity and Precision

A digital wind tunnel gives you the opportunity to accurately model complex geometries and fluid interactions. This would be difficult to replicate in an experimental setup. The precision of CFD allows you to analyze subtle effects and interactions in a controlled environment. At the same time, the program provides you with complex insights that enhance the comprehension and optimization of structural systems.

The CFD simulations are especially useful if you have a thorough comprehension of the underlying models and assumptions. In most cases, it is best to use a combination of CFD simulations and experimental tests to achieve comprehensive results. This gives you the opportunity to fully understand the behavior of your structures in flow situations.

Are you looking for a way to carry out CFD simulations for your projects? The powerful RWIND is a stand-alone program that allows you to perform flow simulations in various situations. Find more information about the latest program generation here: RWIND 2 .

The following programs and add-ons are perfectly suitable for your reinforced concrete projects.

Main Program

To model your structure and determine the internal forces, you need one of the main programs. Depending on the application area, you can use RFEM or RSTAB.

• RSTAB: Analysis of planar or spatial frame & truss structures
• RFEM: Finite element analysis of plate, wall, shell, and member structures

If your model has surface components, such as plates and walls, in addition to membe elements, you need RFEM.

Concrete Design Add-on

Use the Concrete Design add-on to design the reinforced concrete components of your model. The design checks according to specific standard include not only members (bending beams, columns) and surfaces (plates, walls), but also surfaces susceptible to punching shear. For columns, the stability analysis according to the nominal curvature method is integrated in the add-on.

You can specify user-defined reinforcement or determine it by using the program. With these basic conditions, you can perform the ultimate and serviceability limit state design checks according to one of the following standards:

• EN 1992, including National Annexes
• ACI 318
• CSA A23.3
• SP 63.13330

The add-on also allows for deformation analyses for cracked reinforced concrete surfaces (state II) using the approximation methods described in the standards. Tension stiffening as well as creep and shrinkage can be taken into account.

Additional Add-ons

If you have special requirements for the reinforced concrete structure, we recommend further add-ons and additional programs to help you solve the task.

To use a cross-section that is not included in the extensive cross-section library of RFEM 6 or RSTAB 9, the RSECTION program helps you when modeling the cross-section including reinforcement. You can then design this cross-section as usual in the Concrete Design add-on.

If you need to perform seismic or vibration analysis, the dynamic analysis add-ons provide you with powerful tools for your structural analysis and design. The Modal Analysis add-on enables the calculation of natural frequencies, and in combination with the Response Spectrum Analysis add-on, you can also perform seismic analysis according to numerous standards.

If you are designing a building with a complicated shape, so that the determination of wind loads according to the relevant standards reaches its limits, the RWIND RWIND program will help you: Using a CFD analysis, you can determine the wind loads even for highly complex building shapes.

If the construction stages affect the final internal forces in a structure, or if the governing internal forces for the structural design result from certain construction stages, you should consider the modeling using the Construction Stages Analysis add-on. The Building Model add-on allows you to easily enter large multi-story concrete structures.

Basic Add-ons

If you design steel structures, you need the following add-ons in addition to RFEM or RSTAB:

• Steel Design
• Stress-Strain Analysis
• Stability
• Steel Joints

With the Steel Design add-on you can perform stability analyzes (buckling and lateral-torsional buckling) in addition to the normal cross-section design.

The The Stress-Strain Analysis add-on provides you with you can also calculate strains in members, surfaces, and solids.

Steel structures are usually slender structures so the stability plays an important role. Use Add-on Structure Stability to calculate the effective lengths and critical load factors for the require an exact stability analysis.

Model the connections between members precisely and flexibly with FEA based Steel Joints Add-on. The connections need to be loaded not only in the main load-bearing plane. Even if the connection is loaded about the "minor axis", you can dimension the connection using this add-on.

Additional Add-ons

If you have special requirements for steel design checks, there are other add-ons and additional programs available to support you in solving your special tasks.

If you would like to use cross-sections that are not available in the extensive cross-section library of RSTAB 9 and RFEM 6, the add-on program RSECTION helps you when modeling the cross-sections and the calculation of the cross-section properties.

If you want to perform the lateral-torsional buckling design with the more exact and more economical method with 7 degrees of freedom, then you need the analysis/warping-torsion-7-degrees-of-freedom Torsional Warping add-on.

If it is necessary to consider the plastification of the material when determining the internal forces, you can use Use Nonlinear Material Behavior add-on.

The Modal Analysis add-on allows you to perform the calculation of natural frequencies, and the seismic design together with the Response Spectrum Analysis add-on numerous standards.

If you are planning a building with a complicated shape so that the determination of wind loads according to the relevant standards reaches its limits, the add-on program RWIND can help you here. Using a CFD analysis, it can determine wind loads even for the most complicated building shapes.

If the construction stages affect the final internal forces in a structure or if the governing internal forces for the dimensioning of the structure result from certain construction stages, model this with the Construction Stages Analysis add-on.

The Building Model add-on facilitates your input of large multi-story steel structures.

If you are planning lightweight steel structures including cables and membranes, the add-on may help you -On Form-Finding for determination of the structural shape.

A reason for the calculation time taking a while with little utilization of your computer's processor can be caused by using the Iterative instead of the Direct solver. Both options control the method used for solving the equation system.

The direct solver is a method that uses matrix decomposition techniques, such as the LU decomposition, to solve the system of equations in a single step. This approach is generally more robust and can handle any type of problem, but it may require more memory and computational resources, especially for very large systems.
The iterative solver, such as the Conjugate Gradient method or the GMRES (Generalized Minimal Residual) method, solves the system of equations by iteratively refining the solution.

The solver method that leads to the quicker results depends on the complexity of the model as well as the size of the available main memory amount of RAM in the machine. If resources are not a concern when solving large complex models, then the Direct solver is recommended for a majority of the time and will be the quickest. Make sure to check this under the Static Analysis Settings within RFEM 6.

The "Beam Panel" element will allow you to model entire buildings or only certain areas with using surfaces for timber shear wall panels.
By introducing the necessary line hinges in the rigid couplings, the "Beam Panel" element was also released, which will now be further optimized step-by-step in order to achieve the goal mentioned above.
Currently, the application is limited to very simple structures. However, we are very focused on improving the application as quickly as possible. The current state of development is summarized in this FAQ:

Current State and Planned Development:

• Modeling of single, independent surfaces
• Boundary lines orthogonal to each other and not interrupted (four boundary lines, except across surface cells)
• Openings possible via surface cells
• Automatic connection stiffness calculation according to EN 1995‑1‑1 (linear) for nails and fasteners
• More convenient input and extension for all standards (in development)
• Rotation orthotropy direction of sheathing (in development)
• Semi-rigid coupling of adjacent elements (in development)
• Different cross-sections for boundary members in the positive and negative directions (in development)
• Optional no definition of boundary members (in development)
• Connections of panel joints (corners, T-joint, wall-slab) (in development)
• Integration of modeling tools (drag and drop, copy...) (in development)

There can be several reasons for this error.

Missing Surface Reinforcement

In the surface with the punching nodes is no reinforcement available.

You can find out how to create surface reinforcement here: Surface Reinforcement

Missing Dimensions

In RFEM 6, the geometry of punching nodes is usually determined using the cross-section of the connected structural component (wall, column). If a slab or a plate is designed separately within a 2D positional analysis and thus no walls or columns in the form of surfaces or members exist in the model, RFEM cannot automatically determine the structural component dimensions. In this case, it is necessary to manually enter the dimensions:

Not Supported Nodes

In the case of punching nodes that are not supported by a nodal nor linear support, the dimensions of the connected components can be defined via the ultimate configurations.

More information can be found here: Ultimate Configurations (Punching)

Supported Nodes

In the case of punching nodes that are supported by a nodal nor linear support, which represent a connected component, you can specify the dimensions in the support settings.

More information can be found here:
Fictitious Columns (Nodal Supports) | Fictitious Wall (Line Supports)

Not Valid Nodes

In the punching nodes are located in the middle of a line load or a line support, no punching shear occurs. Therefore, these nodes are indicated as not valid.

Both features known from RFEM 5, "Area Load on Members via Planes" and "Area Load on Members via Cells", can be found in RFEM 6 in the load wizard "Member Loads from Area Load". You can either define planes via nodes (marked in green in the image) or select the cells (marked in blue in the image).

In RFEM 6 and RSTAB 8, you can freely set all keyboard shortcuts.

To do this, open the "Customize" dialog box using the menu "Options > Customize Menus and Toolbars".

Click "View" in the "Categories" list on the left, then double-click "AI Assistant Mia on/off" in the "Commands" list on the right.
Here you can define a new keyboard shortcut.

The Structure Stability add-on is not dependent on the settings made in the Steel Design add-on. It is a global stability analysis of the entire structure according to how the model is created. The setting of boundary conditions is only used for the design in the Steel Design add-on.
If you want the top flange to be supported, for example, we recommend using objects from Types for Members → Member Supports → Shear Panel. These objects are reflected in the results of the add-ons for dynamic and stability analysis.
In order to take into account the shear panel, it is necessary to activate this option of structural modification for the selected load cases or load combinations.

Navigation to the "New Member Hinge" Dialog Box
To connect a member as hinged in RFEM 6 or RSTAB 9, you need to define a member hinge first. You can open the "New Member Hinge" dialog box by using the menu bar, the navigator, the table, or the "Edit Member" dialog box. The following video shows all four options.

Creating New Member Hinge
When defining a new member hinge, select your reference coordinate system first. By selecting the hinge conditions, you release the corresponding degree of freedom. In the image below, the member hinge has been defined, for example, in such a way that free rotation about the local y- and z-axis of the member is possible. Furthermore, you can define spring constants and nonlinearities.

More information about defining a member end hinge can be found in the online manual of RFEM 6 / RSTAB 9:
RFEM 6 - Online Manual Member Hinges

Connecting Member Ends with Hinges
In the "Edit Member" dialog box, you can define member hinges for the selected members. For this, select the "Hinges" option in the "Main" tab.

In the "Hinges" tab, you can separately select the member end hinges defined previously for both member ends.

No, it is not possible to deactivate the Mia assistant. However, if the avatar is in a distracting position, you can simply move it by using the mouse.

Our AI assistant is continuously being developed. In the future, Mia will become much more integrated in RFEM and it is learning new things every day. Our goal is to make your work with RFEM even more intuitive and easier.

Mia provides you with the opportunity to use the capabilities of large language models (LLM) for the structural analysis and program operation of RFEM 6 and RSTAB 9.

Mia already offers a number of advantages:

Support 24/7, including weekends

Perhaps you know this situation, too: a project that still needs to be finished. Shortly before the end, problems arise for which you urgently need support, and the phone support has already closed.

Mia can help you to find a solution to your problem in this situation.

Better work efficiency

You will get the answers to your problems directly. You don't need to interrupt your work until the support has responded to your question.

Communication in almost any language

Mia has an integrated translation function. Communicate in the language that is easiest for you.

Easy access to information

You don't need to search through long articles to finally get the information you are looking for. Instead, you get exactly the information that is relevant to your question.

Easy to use

Simply describe your problem in natural language. It's no longer necessary to formulate complicated Google search queries and to search in countless documents for the results.

User friendliness

The AI function of Mia is directly integrated into the program interface of RFEM 6 and RSTAB 9; that is, exactly where the questions arise. It is no longer necessary to switch to the browser and open a specific website.

Free access

With Mia, you get free, fast access to the support team.

In the shortcut menu, you can find some important functions that support you when working with RFEM 6 and RSTAB 9.

Test Mia yourself. The answers are not always perfect, but are often amazingly good and effective. By the way, Mia also has a sense of humor.

More Information About Mia:

There are no technical differences between the student, trial, and full versions. The scope of functions is only controlled by the authorization.

Simply log in with your email address and password to use your student license.

RFEM 6 and RSTAB 9 require OpenGL 4.2, or a higher version, to run.

On the computer running RFEM or RSTAB, install a driver that enables OpenGL when accessed via Remote Desktop.

If this is not possible, you can install the software renderer. This FAQ describes how it works:

If you display a visibility—as described in the manual chapter Visibilities—the inactive objects are also displayed with a reduced intensity.

To hide these inactive objects completely, deactivate the Show Hidden Objects in Background checkbox in the General category of the "Navigator – Display".