# Frequently Asked Questions (FAQ)

#### Customer Support 24/7

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.

#### Newsletter

Receive information including news, useful tips, scheduled events, special offers, and vouchers on a regular basis.

• ### Is it possible to import the effective lengths from RF-STABILITY or RSBUCK in RF-/TIMBER Pro?

New FAQ 004113 EN-US

#### Answer

Yes, that is possible.

First, RF-STABILITY (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.

• ### Which calculation methods are used by RSTAB and RSBUCK?

FAQ 003564 EN-US

#### 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.

• ### What is the critical load factor and how is it possible to determine it?

FAQ 003561 EN-US

#### 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 second-order 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 second-order analysis is no longer necessary from a critical load factor of 10.
The critical load factor can be determined by the RSBUCK module or RF-STABILITY.
• ### How do I determine the effective lengths of frame columns in RFEM or RSTAB?

FAQ 003538 EN-US

#### Answer

The easiest way to do this is to use the add-on modules RSBUCK (RSTAB) or RF-STABILITY (RFEM).

RSBUCK and RF-STABILITY 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/RF-STABILITY
There is a 2D frame with a width of 12 m, a height of 7.5 m and pinned supports. The column cross-sections correspond to I240 and the frame beam to IPE 270. The columns are loaded with two different concentrated loads.

l = 12 m
h = 7.5 m
E = 21000 kN/cm²
Iy,R = 5790 cm4
Iy,S = 4250 cm4

NL = 75 kN
NR = 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 RF-STABILITY.

###### RSBUCK
$\eta_{Ki}=4.408$
$sk_L=16.322\;m$
$sk_R=19.991\;m$

###### RF-STABILITY
$\eta_{Ki}=4.408$
$sk_L=16.324\;m$
$sk_R=19.993\;m$
• ### Why are the stiffness modifications not taken into account when determining critical load factors in the RF‑STABILITY add-on module?

FAQ 003108 EN-US

#### 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.
• ### I have calculated the effective lengths in my model by using RSBUCK/RF‑STABILITY.Which effective lengths are relevant for further analyses?

New FAQ 003007 EN-US

#### 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.

• ### In the stability analysis in RSKNICK, I get the following error message: "The model is not coherent because there are independent submodels." What does that mean and what can I do about it?

FAQ 002857 EN-US

#### 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.
• ### Does RSBUCK take into account the stability problem of lateral-torsional or torsional-flexural buckling?

FAQ 002602 EN-US

#### Answer

No, it does not. In the RSBUCK add‑on module, no stability analysis is performed for lateral-torsional or torsional-flexural buckling.

• ### My model is unstable. What can be the cause?

FAQ 002257 EN-US

#### 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 self-weight 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 RF-STABILITY (RFEM) add-on 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 second-order 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 RF-STABILITY (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 RF-STABILITY (RFEM) add-on module. With the graphical output, it is possible to find the "weak point" in the structure and then specifically optimize it.

#### Videos

Video 1-en.wmv (16.52 MB)
Video 2-en.wmv (23.97 MB)
• ### I would like to calculate effective lengths of a frame column in the frame plane using RSBUCK or RF-STABILITY. Which load case LC or load combination CO should I enter?

FAQ 002016 EN-US

Add-on Modules RF-STABILITY RSBUCK 5.xx 8.xx

#### 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.

1 - 10 of 14

#### Contact us

Did you find your question?
If not, contact us via our free e-mail, 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 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.

#### 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.”