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Frequently Asked Questions (FAQ)
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In RFEM, the interface application 'Simulate and Generate Wind Loads' makes it possible to exchange member, surface, and solid elements, and in RSTAB to exchange the bar elements.
To avoid generating a too fine mesh along with a corresponding long calculation time, the program simulates all members with a rectangular cross-section as standard. The size of the rectangular cross-section is selected in such a way that the real cross-section geometry is barely included.
By deactivating the option 'Export Optimized Member Topology', you can avoid this additional optimization of the model and allow consideration of the real cross-section geometry within existing cross-section settings.
If the exact representation of the cross-section geometry requires more than 1000000 elements, the interface automatically changes to the simplified rectangular section display of the cross-sections.
AnswerBy using the FE mesh refinement, the program allows you to create a mapped FE mesh. The automatic FE mesh generator can thus be controlled to a certain extent. However, it is not possible to use it for setting the given mesh geometry.
AnswerThe most common causes are listed below:
1. SingularitiesSingularities appear in a limited range due to the concentration of the result values that depend on the stress. They are conditioned by the FEM methodology: In theory, the stiffness and/or the stress in an infinite size concentrate on an infinitesimal small area. Therefore, the singularities occur especially at point supports, load application locations, reentrant corners, or in the area of stiffness peaks.If the result value of the stress peak is greater and the area of this stress peak is smaller in the case of a finer FE mesh, the singularity is very likely to occur.The recommendations for dealing with the singularity locations are included in the following technical articles of our Knowledge Base, for example:
2. Unrealistic Support DefinitionRigid supports (infinitely stiff supports) are rather unrealistic in many cases. Therefore, it is recommended to display the supports as elastic supports. In this case, the stiffness of the adjacent structural components should be estimated realistically.For checking purposes, the deformation diagram is suitable, possibly with a stronger precamber, as well as the result display of the support reactions or contact stresses. For a better overview, the simplest possible loads should be used for the check.
3. Incorrectly Defined Directions / NonlinearitiesA mistake in the direction definition, for example, of loads, member hinges, or line and surface releases, is often the cause of the unrealistic behavior. When using the local or rotated coordinate systems as a reference systems, you must pay attention to the correct definition. For example, nonlinearities defined in the opposite direction are typical for the supports that fail due to tension or compression.Incorrectly defined loads can be easily identified by displaying the loading. The loads applied for the calculation can be easily displayed in the Results navigator by using the "Load Distribution" option.Furthermore, modeling inaccuracies can also lead to the incorrect definition of directions. By importing a DXF file, you can introduce inaccuracies into the model, for example, nodes that are not on top of each other or the lines skewed in the wrong direction.The "Regenerate Model" feature is very helpful for treating the minor inaccuracies.Incorrectly defined releases and hinges can usually be identified by means of the deformation image and the diagrams of internal forces. Again, it is recommended to work with simple loads for checking purposes.
4. Model does not correspond to realityIt may often happen that not all external or internal influences from a structure to be modeled have been considered sufficiently and accurately in the model. Supports or supporting structural components may not have been modeled or they are in a wrong place. The realistic estimation of the stiffness of the adjacent structural components is also important. Provided that it has been over- or underestimated, the load transfer in the model is sometimes changed significantly.However, it is possible to simple check the deformation, possibly by using a stronger precamber.The following questions may help you to find the solution if the real structure is known: Is the magnitude of the deformations close to reality? Is the deformation diagram qualitatively consistent with my expectations?A good example is presented in the following Knowledge Base article:
Yes, it is possible.When applying a new surface load, it is possible to set "Temperature" as a load type. Instead of applying the uniform load distribution, it is possible to apply the variable load distribution (for example, linear in Z).For the load values, you can specify whether the constant temperature or the delta temperature should be applied.This results in the following load specification:And the following results (deformation in this case):
AnswerUnfortunately, we do not currently have any solutions for the design of composite bridges in the program. Therefore, there are no verification or reference projects available for this.Basically, you can create the models of composite bridges made of steel and concrete in RFEM 5 and use them for the determination of internal forces, for example. However, we are currently unable to offer any solutions for the design of such bridges according to the relevant standards.
AnswerThe function transfers the deformation from one LC or CO into another LC or CO. Internal forces and stress states are not transferred at the same time.If the load case or the load combination is based on the same structural system, no significant internal forces should result. The FE division in the structure could have an influence here, and should be checked, if necessary.If the initial strain is transferred into a load case or a load combination that is based on another structural system, additional internal forces may result due to the prevented deformation (restraint). In the example (Figure 02), the initial strain of two consecutive single-span beams is transferred to a continuous beam. The deformation on the intermediate column is prevented and a moment is generated (Figure 03). The magnitude of the resulting moment can be better illustrated by cutting free the structure and applying the prevented deformation as an imposed deformation on the load side.
AnswerThere is no general answer to this problem. In the RF‑/STAGES add-on module, however, there is a specific feature regarding the structural system. Similar to some other add-on modules, such as RF‑/STEEL Warping Torsion, it is possible to consider the structural system detached from the main program. Thus, there are some advantages regarding the definition of construction stages, and so on. However, this possibility means that the modifications in the main program RFEM or RSTAB are not updated automatically with these add-on modules. Such an update would inevitably lead to incorrect calculations and is therefore blocked.
AnswerIn RF-STAGES, the temporary loads in the respective construction stages are calculated only linearly, according to the linear static analysis. The permanent loads that are embossed into the system become nonlinear, according to Theorie III. Order, calculated. In the combinations that can be created in the add-on module, the results of the individual load cases are combined.
AnswerThis can not be implemented in RFEM on the load side. It may be possible to display the behavior on the Structure page by using orthotropic surfaces.
AnswerFor example, the standard EN 1990 + EN1991-2; Road bridges could be used (see picture 1). Alternatively, one could of course also determine the combinations by hand. Further information can be found i.a. in our manual.
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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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