In addition to our technical support (e.g. via chat), you’ll find resources on our website that may help you with your design using Dlubal Software.
Frequently Asked Questions (FAQ)
Search FAQ
Further Information
Customer Support 24/7

Answer
Yes, that is possible.
First, RFSTABILITY (or RSBUCK in RSTAB 8) can be used to determine the effective lengths for a particular structure and loading.They can then be imported in the 'Effective Lengths' of the RF/TIMBER Pro dialog box. 
Answer
RSTAB is a FEM program that uses trigonometric trial functions for the members. For this reason, members do not have to be subdivided for sufficiently accurate results and the calculation speed is correspondingly higher.
RSBUCK determines the eigenvalues of the stiffness matrix and can thus linearly calculate the critical load and buckling mode of the structure.

Answer
The critical load factor specifies the factor by which you can increase a load until the system fails. If it is smaller than one, a calculation according to the secondorder analysis is usually unstable because the system is already stressed by the critical load. This factor is also taken into account in standardization. For example, Eurocode 3 specifies that a calculation according to the secondorder analysis is no longer necessary from a critical load factor of 10.The critical load factor can be determined by the RSBUCK module or RFSTABILITY. 
Answer
The easiest way to do this is to use the addon modules RSBUCK (RSTAB) or RFSTABILITY (RFEM).RSBUCK and RFSTABILITY perform an eigenvalue analysis for the entire model with a certain state of normal force. The axial forces are increased iteratively until the critical load case is reached. This stability load is characterized in the numerical calculation by the determinant of the stiffness matrix becoming zero.If the critical load factor is known, the buckling load and the buckling curve are determined from this. The effective lengths and effective length factors are then determined for this lowest buckling load.The result shows, depending on the required number of eigenvalues, the critical load factors with the corresponding buckling curves and for each member  according to its mode shape  effective length about the strong and the minor axis.Since usually, every load case has a different normal force state in the elements, a separate corresponding effective length result for the frame column arise for each load situation. The effective length whose buckling mode causes the column to buckle in the corresponding plane is the correct length for designing the respective load situation.Since this result may be different for each analysis due to the different load situations, the longest effective length of all calculated analyzes  equal for all load situations  is assumed for designing on the safe side.Example for manual calculation and RSBUCK/RFSTABILITY
There is a 2D frame with a width of 12 m, a height of 7.5 m and pinned supports. The column crosssections correspond to I240 and the frame beam to IPE 270. The columns are loaded with two different concentrated loads.l = 12 mh = 7.5 mE = 21000 kN/cm²I_{y,R} = 5790 cm^{4}I_{y,S} = 4250 cm^{4}N_{L} = 75 kNN_{R} = 50 kN$EI_R=E\ast Iy_R=12159\;kNm^2$$EI_S=E\ast Iy_S=8925\;kNm^2$$\nu=\frac2{{\displaystyle\frac{l\ast EI_S}{h\ast EI_R}}+2}=0.63$This results in the following critical load factor:$\eta_{Ki}=\frac{6\ast\nu}{(0.216\ast\nu^2+1)\ast(N_L+N_R)}\ast\frac{EI_S}{h^2}=4.4194$The effective lengths of the frame columns can be determined as follows:$sk_L=\pi\ast\sqrt{\frac{EI_S}{\eta_{Ki}\ast N_L}}=16.302\;m$$sk_R=\pi\ast\sqrt{\frac{EI_S}{\eta_{Ki}\ast N_R}}=19.966\;m$The results from the manual calculation correspond very well with those from RSBUCK or RFSTABILITY.RSBUCK
$\eta_{Ki}=4.408$$sk_L=16.322\;m$$sk_R=19.991\;m$RFSTABILITY
$\eta_{Ki}=4.408$$sk_L=16.324\;m$$sk_R=19.993\;m$ 
Answer
The defined stiffness modifications are only considered in the stability analysis in RF‑STABILITY if the "Activate Stiffness Modifications from RFEM" option under the "Options" section in Window "1.1 General Data" is selected. 
Answer
RSBUCK/RF‑STABILITY calculates at least one critical load factor or one critical load and the assigned buckling shape. The effective length is then counted back from the critical load (see here ). Since this analysis is not carried out for the individual local components, but for the entire structure only, the resulting critical load factors refer to the global structure and not to the local elements. However, it may happen that the structure fails globally (and also locally) for some critical load factors (depending on the stiffness and the axial force state).
Therefore, the calculated effective lengths should only be used by the members that buckle in the respective buckling mode. In the case of the global failure of a structure (see the example in Figure 01), it is thus difficult to draw conclusions regarding the buckling behavior of the individual members.
Figure 02 shows a structure where the rear columns are buckling. Therefore, it is recommended to only use the effective lengths calculated for both of these columns.
General summary: The effective lengths from the RSBUCK module are only valid for a structural component in the respective direction if the related buckling shape clearly "bulges" the member in relation to the other in the respective direction. It is clear that the axial forces also have an impact on the results here.

Answer
Independent 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. 
Answer
No, it does not. In the RSBUCK add‑on module, no stability analysis is performed for lateraltorsional or torsionalflexural buckling.

Answer
The calculation can be terminated due to an unstable structural system for various reasons. On the one hand, it may indicate a real instability due to an overload of the system, but on the other hand, modeling errors may be responsible for the error message. Below is a possible way to find the cause of the instability.
First, it should be checked if the modeling of the system is correct. To find modeling problems, use the model controls (menu "Tools" → "Model Check").
Furthermore, you can structure z. For example, it is possible to calculate under pure selfweight in a load case according to the linear static analysis. If results are displayed subsequently, the structure is stable with regard to the modeling. If this is not the case, the most common causes are listed below (see also Video 1):
 Supports are missing or have been defined incorrectly
 Members can rotate about their own axes because a corresponding support is missing
 Members are not connected ("Tools" → "Model Check")
 Nodes are evidently in the same place, but if looked at more closely, they deviate slightly from each other (common cause for CAD Import, "Tools" → "Model Check")
 Member Hinges / Line Hinges Create a "Hinge Chain"
 The structure is not sufficiently stiffened
 Nonlinear structural elements (for example Tension members) fail
Finally, Figure 02 shows an example. It is a pinned frame stiffened by tie rods. 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 (because only tension can be absorbed). In the second calculation run, the model is unstable without these ties. There are several ways to solve this problem: You can apply a prestress (member load) to the tension members to "eliminate" the small compression forces, assign a small stiffness to the members (see Figure 02), or have the members removed one after the other in the calculation (see Figure 02).
To obtain a graphical representation of the cause of instability, the RFSTABILITY (RFEM) addon module can help. The option "Determine mode shape of unstable model ..." (see Figure 03) allows you to calculate unstable systems. In the graphic, the component that leads to the instability is usually recognizable.
If load cases and load combinations can be calculated according to the linear static analysis, the calculation is only broken when calculating according to the secondorder analysis or the second analysis. Order, then there is a stability problem (critical load factor smaller than 1.00). The critical load factor indicates the factor with which the loading must be multiplied so that the model becomes unstable under the corresponding load, for example buckles. It follows from this: A critical load factor smaller than 1.00 means that the structure is unstable. In order to find the "weak point", the following approach is recommended, which is required by the RSBUCK module (for RSTAB) or RFSTABILITY (for RFEM) (see Video 2):
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 is used as an aid (see Video 2). Then, the buckling or buckling shape can be calculated and graphically displayed on the basis of this load combination in the RSBUCK (RSTAB) or RFSTABILITY (RFEM) addon module. With the graphical output, it is possible to find the "weak point" in the structure and then specifically optimize it.
Videos
Video 1en.wmv (16.52 MB)
Video 2en.wmv (23.97 MB) 
Answer
The modules perform the eigenvalue analysis for the entire model with a certain axial force state. Depending on the number of eigenvalues required, the programs provide results of crictical load factors with the corresponding buckling shapes for an eigenvalue, and effective length about the major and minor axis for each member per mode shape.
Since each load case LC and each load combination CO often has a different axial force state in the elements available, there is a separate respective effective length result for each load situation of the frame column concerned. The effective length, which causes the column in the frame plane buckles sideway in the buckling shape, is the correct length to be used for the analysis of the load situation.
However, this result may be different because of various load situation in each analysis, the longest effective length of all analyses performed applies in the design on the safe side equally for all load situations.
Contact us
Did you find your question?
If not, contact us via our free email, chat, or forum support, or send us your question via the online form.
First Steps
We provide hints and tips to help you get started with the main programs RFEM and RSTAB.
Wind Simulation & Wind Load Generation
With the standalone 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.
Your support is by far the best
“Thank you very much for the useful information.
I would like to pay a compliment to your support team. I am always impressed how quickly and professionally the questions are answered. In the industry of structural analysis, I use several software including service contract, but your support is by far the best.”