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Frequently Asked Questions (FAQ)
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The calculation can be terminated due to an unstable structural system for various reasons. On the one hand, it can indicate a 'real' instability due to system overload, but on the other hand, modeling inaccuracies may also be responsible for this error message. The following is a possible way to find the cause of the instability.
1. Checking the modelingFirst, it should be checked whether the system is okay with respect to the modeling. It is recommended to use the model controls provided by RSTAB/RFEM [Tools → Model Check]. Using these options, it is possible to find, for example, identical nodes and overlapping members and delete them, if necessary.Furthermore, it is possible to calculate the structure e.g. under its dead load according to 1st order theory in a load case. If results are subsequently displayed, the structure is stable concerning the modeling. If this is not the case, the most common causes are listed below (see also the video 'Model Check' in the 'Downloads' section):
- Incorrect definition of supports/lack of supports
This can lead to instabilities because the system is not supported in all directions. Therefore, it is necessary for the support conditions to be in equilibrium with the system as well as with the external boundary conditions. Statically overdetermined or kinematic systems also lead to calculation aborts due to a lack of boundary conditions.
Figure 02 - Kinematic system - Single-Span Beam Without Rigid Support
- Torsion of members about its own axis
If members twist about its axis, i.e. the member is not supported about its axis, this can lead to instabilities. Frequently, the cause is the settings of the member end hinges. For example, it is possible that torsional releases have been introduced at both the start node and the end node. A message box when starting the calculations, however, draws the attention of the user.
Figure 03 - Entering Torsional Releases on Start and End Nodes
- Missing connection of members
Especially for larger and more complex models, it can happen quickly that some members are not connected and thus 'float in the air'. Also, forgetting crossing members that should intersect can lead to instabilities. A solution is the model check 'Crossing Not Connected Members', which searches for members that cross each other but do not have a common node at the point of intersection.
Figure 04 - Result of Model Check for Crossing Members
- No common node
The nodes are apparently in the same place, but on closer inspection, they differ slightly from each other. Frequent causes are CAD imports, which can be corrected using the model check.
Figure 05 - Result of Model Check for Identical Nodes
- Formation of articulated chain
Too many member end hinges at a node can cause an articulated chain, which can lead to a calculation abort. For each node, only n-1 releases can be defined with the same degree of freedom relative to the global coordinate system, where 'n' is the number of connected members. The same applies to line releases.
Figure 06 - Kinematic System Resulting from Articulated Chain
2. Check of stiffeningA missing stiffening also leads to calculation aborts due to instabilities. Therefore, it should always be checked whether the structure is sufficiently stiffened in all directions.
3. Numerical problemsAn example of this is shown in Figure 08. It is a hinged frame that is stiffened by tension members. Because of post shortenings due to vertical loads, the ties receive small compressive forces in the first calculation run. They are removed from the system (since only tension can be absorbed). In the second calculation run, the model is unstable without these tension members. There are several ways to solve this problem. You can apply a prestress (member load) to the tension members to 'eliminate' the small compressive forces, assign a small stiffness to the members or have the members removed one by one in the calculation (see Figure 08).
4. Identifying the causes of instability
- Automatic model check with graphical output
To obtain a graphical representation of the cause of instability, the RF-STABILITY (RFEM) add-on module can help. With the option 'Determine Eigenvector of Unstable Model, ...' (see Figure 09), it is possible to calculate supposedly unstable systems. With the model data, the module performs an eigenvalue analysis so that the result of the instability of the affected structural component is shown graphically.
Figure 09 - Graphical Representation of Instability
- Stability problem
If it is possible to calculate load cases/load combinations according to the first-order theory and the calculation only starts from the second-order theory, there is a stability problem (critical load factor less than 1.00). The critical load factor specifies the factor by which the load must be multiplied so that the model becomes unstable under the corresponding load (e.g. buckling). It follows: A critical load factor of less than 1.00 means that the system is unstable. Only a positive critical load factor greater than 1.00 allows us to state that the loading due to the specified axial forces multiplied by this factor leads to the buckling failure of the stable system. To find the 'weak point', we recommend the following approach, which requires the add-on module RSBUCK (RSTAB) or RF-STABILITY (RFEM) (see also the video 'Stability Problem" in the 'Downloads' section):
First, the load of the affected load combination should be reduced until the load combination becomes stable. The load factor in the calculation parameters of the load combination serves as an aid. This also corresponds to a manual determination of the critical load factor if the RSBUCK module or the RF-STABILITY module is not available. For purely linear structural elements, it may already be sufficient to calculate the load combination according to the first-order theory and select it directly in the add-on module. Then, based on this load combination, the buckling or buckling mode can be calculated in the corresponding add-on module and displayed graphically. Through the graphical output, the "weak point" is located in the system and can then be specifically optimized. By default, the RSBUCK or RF-STABILITY modules determine only global mode shapes. To record the local eigenmodes as well, the member division should be activated (RF-STABILITY) or the division of truss members should be increased to at least '2' (RSBUCK).
Figure 10 - Activation of Member Division in RF-STABILITY
Figure 11 - Member Division in RSBUCK
- Incorrect definition of supports/lack of supports
This is because the effective lengths or buckling lengths of members and sets of members differ. While the effective length is used for the stability analysis for members, RFEM takes the length of the summarized members for the set of members.
The frame shown in Figure 01 consists of a horizontal beam that is divided into four equally long members. In addition, a set of members is created for the four members. The stability analysis is carried out for both cases according to the equivalent member method.
For the design of members, the program calculates with a length of 1.00 m in each case. In contrast, the set of members has a length of 4.00 m (see Figure 02). This difference in length naturally affects the stability design, which means that the capacities are also different (see Figure 03).
In addition, it is not recommended to calculate all members and sets of members in a single design case because this leads to falsified results.
AnswerThe most convenient and fastest solution is to copy a load case to the next load case. If already generated loads have been created for several load areas in a load case (see Figure 1), all loads are transferred to the next load case during copying (see Figure 2). Subsequently, only the load values of the generated loads have to be adapted to the respective load case.This method is particularly useful for plane structures whose load type remains the same.
AnswerThat's fine. Call up the program options under Options and check the box "Save after calculation". This way the model will be saved automatically after each cycle.This option is particularly useful for larger structures that take a long time to calculate.
AnswerIndependent submodels are not interconnected and are considered as separate submodels in the calculation. They are thus independent models without influencing each other (see Figure 2).It is recommended to edit submodels separately as individual files. Then a stability analysis with RSKNICK is possible.Otherwise, the partial models must be connected to each other. In this case, it should be taken into consideration that the static systems of the submodels should be retained when the submodels merge into an overall model (see Figure 3).The feature "Independent Systems" is helpful in detecting partial models. This finds all independent systems and lists them as groups (see Figure 4).One finds this function under Extras -> Model control -> Independent systems.
AnswerThe 1. Possibility is a reinstallation of the program. The first time you try to install the program again, you will be asked if you want to uninstall first. Confirm the query and then reinstall the program.The 2. Possibility is the manual integration of already existing material libraries of another computer on which the programs are already installed. For example, the following material databases must be synchronized in each program version:
By default, these databases are located in the hidden folder C: \ ProgramData \ Dlubal \ RFEM \ General DataYou will also find the material databases for the versions RFEM 5.18 and RSTAB 8.18 under the download links
- the program-installed material library "Materialien.dbd" with common materials
- The user-defined material library "Materials_User.dbd"
AnswerThis depends on the choice of model type. In this case a flat system was chosen. Thus, only for this level, the relevant results are displayed because they are reduced because of the limited coordinates and degrees of freedom. If, on the other hand, a 3D system is selected as the model type, all stresses and internal forces are displayed.
This is due to the fact that the respective corner nodes for defining the area load plane are duplicated in the selection window. If one of them is deleted, the correct values are displayed in the info window after clicking OK.
AnswerThe following causes can be responsible for this:
- In most cases, these differences can be attributed to a lack of convergence. Increasing the iterations and increments in the calculation parameters and FE mesh settings should help.
- High stiffness jumps result in numerical problems, which leads to errors in the result evaluation. In RSTAB, this is not a major problem with a full and analytical approach. In RFEM, on the other hand, approximation approaches are used, so higher stiffness jumps should rather be avoided.
- Bedded bars may well be subject to deviations as well. If the bars are not or only roughly divided, there are convergence problems. A practical solution here is to select a "finer" bar pitch in the FE mesh settings.
AnswerAt the bottom of the user interface of RSTAB and RFEM is the status bar. The center area can be used to influence the user interface. If the options FANG and OFANG are activated, grid points, nodes and objects can be caught (see Figure 1). Then a dimensioning of the system is possible.
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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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