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Frequently Asked Questions (FAQ)
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Currently, there is no option to perform a stability analysis in the RF-CONCRETE Surfaces add-on module. However, it may be possible to perform a simplified design by means of a result beam by means of the nominal curvature method in RF-CONCRETE Columns.
AnswerThis function is intended to detect modeling errors in the structure that may lead to instability. Using this method, it is possible to calculate such systems and to determine the cause of the instability graphically.This function is not suitable for the following problems:
If the system is stable and only stability problems occur during the calculation according to the second-order analysis, all results are set to 0 with this function.The problem solving of instabilities is described in detail in FAQ 2257 .
- Calculation aborts due to overloadings (stability problems)
- Determination of buckling curves and buckling modes
Whether or not the download for RFEM or RSTAB is displayed in the download area of the Dlubal account depends on whether or not you have concluded a service contract for the respective product.If a service contract has been concluded for the respective product, the links for this product are visible.If no service contract has been concluded, no links are visible.If you purchase RFEM or RSTAB without a service contract at a specific time, you are entitled to use this program version. If you need the download link for the purchased program version, you can download it on our homepage in the 'Previous Program Versions' section.See the link to 'Download Previous Versions' below:
AnswerThe most common causes are listed below:
1. SingularitiesSingularities appear in a limited range due to a concentration of the stress-dependent result values. They are conditioned by the FEM methodology: Theoretically, the stiffness and/or the loading in infinite magnitude are concentrated on an infinitesimally small area. Singularities therefore occur, in particular, at punctiform supports, load application locations, reentrant corners, or in the area of stiffness changes.If the result value of the stress peak is larger and the area of this stress peak is smaller for a finer FE mesh, a singularity is very likely to occur.When handling singularity locations it is recommended to follow articles of our Knowledge Base:
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 adjacent structural components should be estimated realistically.The deformation diagram, possibly with a pronounced precamber, and the output of the support reactions or contact stresses are suitable for checking purposes. For a better overview, the simplest possible loads should be used for the check.Often, a mistake in the direction definition, for example of loads, member hinges, or line and surface releases, is the cause of unrealistic behavior. When using local or rotated coordinate systems as reference systems, particular attention must be paid to the correct definition. For example, nonlinearities defined in the opposite direction are typical for 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 conveniently displayed in the Results navigator by using the "Load Distribution" option.Furthermore, modeling inaccuracies can lead to 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 lines skewed in the wrong direction.The function "Regenerate Model" is very helpful for the treatment of minor inaccuracies.Incorrectly defined releases and hinges can usually be identified by means of the deformation graphic and the diagrams showing the distribution of internal forces. Again, it is recommended to work with simple loads for checking purposes.
4. Model does not correspond to realityOften it can 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 it happened in the wrong place. The realistic estimation of the stiffness of adjacent structural components is also important. Provided that it has been over- or underestimated, the load transfer in the model is sometimes significantly changed.However, a simple check of the deformation, possibly with a pronounced precamber, is possible.The following questions may help to find a solution if the real structure is known: Is the magnitude of the deformations realistic? Is the deformation diagram qualitatively consistent with my expectations?A good example is presented in the following Knowledge Base article:
I am trying to compare a design for flexural buckling according to the equivalent member method and internal forces according to the linear static analysis with a stress calculation according to the second-order analysis including imperfections and obtain very large differences. What could be the reason?
AnswerWhile the influence of imperfection increases significantly with increasing axial force and the design increases exponentially, it only increases linearly in the equivalent-member design to the increase of the axial force. Therefore, very strongly and very weakly loaded systems usually deviate more strongly in the ratios of the linear static analysis by means of equivalent member methods compared to second-order analysis by means of stress design.
The load application point in the detail settings of RF-/STEEL EC3 Warping Torsion refers only to the transverse loads.
AnswerThe axes to which the support rotations and support eccentricities refer are preset to the local member axes, which can be displayed with a button below the partial view. However, they can also be switched to the global axis system.
The first two reinforcements selected for display are displayed in red and blue by default. To change these and other colors, use the color scale within the panel, following the steps below:
- Double-click one of the reinforcements in the panel
- The dialog box 'Edit Isoline Value and Color Scales' will be opened: Double-click the color to be changed
- The "Color" dialog box opens: Select a new color and confirm it with OK
- The dialog box 'Save Color Scale' will be opened: Enter the name of the new color scale and confirm with OK, the new color scale is now saved
- Select the desired color scale and confirm it with OK
Figure 01 - Panel - Modifying the Color Scale
AnswerIn the fire resistance design, the acting internal forces are determined again if the thermal expansion of the concrete and the reinforcing steel and the precamber due to the temperature difference is taken into account (Figure 01).If the thermal expansions and the precamber are not taken into account, the acting internal forces should correspond to the internal forces of the calculation from RFEM or RSTAB (Figure 02).To what extent it is allowed to deactivate the thermal expansions and precambers has to be checked in connection with the underlying standard.
AnswerIf there is an insulated glass unit selected for design in RF‑GLASS, the method of the large deformation analysis is set by default. Based on the existing gas volume in the space between glass panes, this method of analysis is also required and cannot be changed.
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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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