Frequently Asked Questions (FAQ) - Search term: Design Lo...

The setting for torsion in the Strength Configuration works together with the torsion limit set, and it is on purpose for it to be always on the safe side. Below, you find short explanations for each option:

Check torsional limit only:
In this case, the ratio torsion check is compared to the torsion limit. If the ratio is smaller than the limit, then nothing is done. If the ratio is bigger than the torsion limit, an error will be shown in the design check. The error is then the most governing design check in the graphical and tabular results.
According to Timber Construction Manual:
In this case, design is according to the Timber Construction Manual 4.6, and the result is a typical design ratio based on the calculation.
Ignore torsion:
This case is very similar to the first option. The ratio is compared from the torsion calculation with the torsion limit. If the ratio is smaller than the limit, then nothing is done. If the ratio is bigger than the limit, then a warning is shown in the design check. This warning will not be a governing design check in the results tables or graphics. The main reason it is shown at all is for safety.
If you do not want to see the warning at all, then the limit of torsion must be increased.

The following programs and add-ons are particularly suitable for structural timber 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 6 or RSTAB 9.

• 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. This program is the only one with the capability to analyze laminate or cross-laminated timber elements and curved objects.

Timber Design Add-on

Use the Timber Design add-on to design the timber components of your model. The design checks in compliance with specific standards cover both members (bending beams, columns) and surfaces (plates, walls). For columns, the stability analysis according to the equivalent member method or the second order analysis is performed.

You can perform the ultimate and serviceability limit state design checks, as well as the fire resistance design according to the following standards:

• EN 1995 – including National Annexes
• SIA 265
• NTC
• NDS
• CSA O86
• GB 50005
• AS 1720

In the fire resistance design, the charring of a cross-section is taken into account for both the stiffness and the design.

Multilayer Surfaces Add-On

With the Multilayer Surfaces add-on for RFEM, you can analyze laminate and sandwich structures with user-defined layer structures. This allows you to design cross-laminated timber panels, for example.

If certain layers have nonlinear material behavior, you can assign the appropriate properties to them. Furthermore, this add-on allows provides you with the option to analyze curved surfaces.

Additional Add-ons

In case of special requirements for your timber structure, we recommend further add-ons and additional programs to help you solve the task.

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 timber structures.

Please note that the enveloping results are usually shown when displaying the results of design situations or result combinations.

There are three options to graphically display the results of deformations, internal forces, and support forces: Max and Min-values can be displayed separately. To display both envelopes from the extreme values on the model, select Max and Min-values.

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, no torsion is taken into account in the stability analysis for the equivalent member method or the general method. The warning message is a reminder to the user of 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. Find more information 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. For this, you can use the Torsional Warping (7 DOF) for RFEM 6 / RSTAB 9 add-on, for example.

We usually release a new version of RFEM 6 / RSTAB 9 every Monday; means, four versions per month.

You are informed about a new version on the Extranet and in the Dlubal Center. There you will also find the links to download the versions.

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

There are tree analysis types preset:

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

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

In the "Load Cases & Combinations" dialog box, you can then assign the relevant static analysis setting (SA) to the load cases and load combinations.

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

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

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

The Piping Design 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 in development. Therefore, it cannot be activated in the Base Data.

The stiffening can be modeled using line releases as follows:

• Create a line with the planned beam dimensions.
• Open the "Edit Line" dialog box and assign a member with the cross-section of the planned stiffening to the line.
• Create a New Line Release in the "Special Objects" subfolder by right-clicking it.
• A dialog box appears where you can select the previously created line and then the created member (see the image).
• Create a new Line Release type and enter the appropriate support conditions in the dialog box according to your planned stiffening and fasteners.

• Select the original line and repeat Steps 2 through 5 to create a second reinforcement (for unsymmetric cross-sections, the corresponding section should be arranged with an angle of 180°).
• Assign the cross-section of the main beam to the original line.
• Now, you can assign a corresponding eccentricity to the stiffening beams.

Another option for applying stiffening is described in the following FAQ:

You can find another article on the subject of flexible timber joints on our 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.

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 6 or RSTAB 9.

• 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

The Steel Design add-on allows you to perform stability analysis (lateral-torsional and torsional-flexural buckling) in addition to the normal cross-section design.

The Stress-Strain Analysis add-on calculates not only stresses, but also strains in members, surfaces, and solids.

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

Model the connections between members precisely and flexibly using the Steel Joints add-on that is based on FEA. 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.

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 and calculating the section properties.

If you want to analyze the lateral-torsional buckling with the more accurate and cost-effective method with 7 degrees of freedom, you will need the Torsional Warping (7 DOF) add-on.

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

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 wind load calculation according to the relevant standards reaches its limits, then the RWIND program will help you: 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 structural design result from certain construction stages, you can model this using the Construction Stages Analysis add-on.

The Building Model add-on allows you to easily enter large multi-story concrete structures.

For your design of lightweight steel structures, containing cables and membranes, the Form-Finding add-on can help you to determine the shape of the structure.

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.

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).

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.

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.

Saving Graphic Template

Once you have assigned the concrete material and activated the design properties in the member input dialog box, the display of the reinforcement layout for the selected member is available.

In the Display Reinforcement Layout window, you can proceed as follows.
1. Use the options in the menu bar to customize the display according to your preferences.
2. This configuration can send "Directly to a printer" as a template.
3. When saving the template, it is stored globally and is also available for future models.

Multi Print of Reinforcement Layout for Selected Members

Use this graphic template to print the reinforcement of any members with a similarly formatted reinforcement layout in a multi print.

First, it is worth taking a look at the "Load Transfer" tab when editing the surface. If the load transfer direction is set to "Isotropic | FEM", the loads are not displayed until the load case or load combination has been calculated, as they must be calculated in a sub-model first. So make sure the results are available there.

The design concept of RFEM 6 has been fundamentally changed, so the design cases are no longer available in the add-ons. When defining members and surfaces, you can specify whether to perform design and assign the design properties to the objects. Therefore, it is not possible to perform alternative analyses in the same model.

If you want to design some members according to different specifications, define a new design configuration for a limit state, and assign this configuration accordingly.

If you design both members and member sets, you can use the object properties to control whether to consider each member individually or to analyze the member set as a superordinate object.

Furthermore, RFEM 6 and RSTAB 9 provide you with the option to design objects with common properties using representatives.

When creating a new model in RFEM, it is not possible to import the templates from any path. It is necessary to create a project.

When accessing a template in Revit or Rhino/Grasshopper, an error message often appears if the path of the template file used is not linked to a "Template Project" in the Dlubal Center.

Open the Dlubal Center and check whether there is a "template project" for the folder with your template file.

If not, you can add the path of the template file as a project. In the FAQ video, the Downloads folder has been created as a template project, as it contains the template file.

In RFEM 6 and the "Concrete Design", the fiber-reinforced concrete design can be carried out according to DIN EN 1992‑1‑1 in conjunction with the DAfStB guideline "Fiber Reinforced Concrete".

To activate the surface or member design using steel fiber reinforced concrete, it is sufficient to assign a material of the corresponding material type to the surface or member.

To do this, create a new material of the "Fiber Concrete" material type.

The concrete and lightweight concrete are designated with the "P1/P2" extension, which stand for Performance Class 1 and Performance Class 2.

The performance classes L1 and L2 are initially preset with the values 1.20 and 0.90 (see Image 02).

In order to be able to adapt these performance classes, you can activate a "user-defined material" in the "Main" tab of the material under the "Options".

Thus, the text boxes for L1 and L2 become accessible, so you can adjust the performance classes.

If you now perform the design with the material of the "Fiber Concrete" type in the "Concrete Design" add-on, the design checks modified or completed in the DAfStB guideline for the "fiber-reinforced concrete" design are adjusted. This modification of the design process can be found in the "Design Details", including the reference to the DAfStB guideline.

Note

For this type of design, it is necessary to specify "normal reinforcement" (member reinforcement for a member, area reinforcement for a surface), otherwise the surfaces cannot be designed. This means that you can only add steel fibers to already reinforced components.

To calculate a slab with fiber-reinforced concrete only (without reinforcing steel), use the "Damage 2D/3D" material model.