# Frequently Asked Questions (FAQ)

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• ### When determining the RF-STABILIZER buckling objects, additional lines are displayed for each member perpendicular to the buckling curve. What is it all about?

New FAQ 004151 EN-US

#### Answer

These lines represent the local torsion rotation (see Figure 01). By default, only torsion rotations φx with normalized values greater than 0.2 are displayed. This ensures that the graphic is clear. The graphical representation is controlled directly in the add-on module (see Figure 02).

• ### Is it possible to import the effective lengths from RF-STABILITY or RSBUCKEN 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 into RF- / TIMBER Pro in the "Effective Lengths" dialog box.

• ### Where can I download add-on modules such as RF-STABILITY?

New FAQ 003603 EN-US

#### Answer

All add-on modules are part of the RFEM / RSTAB main program installation. They can be enabled via the Add-ons menu (see video). Some add-on modules need to be enabled in the Basic Data (eg Form-Finding).
• ### What is the critical load factor and how is it possible to determine it?

New 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 to export the effective lengths from the RF-STABILITY to EXCEL module?

New FAQ 003557 EN-US

#### Answer

It is possible to export the effective lengths from the add-on module to EXCEL as shown in Figure 1.
• ### How do I determine the effective lengths of frame columns in RFEM or RSTAB?

New 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
$\eta_{Ki}=4.408$
$sk_L=16.324\;m$
$sk_R=19.993\;m$
• ### For which application is the option 'Calculate Eigenvector for Unstable Model...' suitable in RF-STABILITY?

FAQ 003338 EN-US

#### Answer

This 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:
• Calculation aborts due to overloadings (stability problems)
• Determination of buckling curves and buckling modes
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 .
• ### Why does the RF-STABILITY Add-on Module take no account of stiffness modifications when determining critical buckling load factors?

FAQ 003108 EN-US

#### Answer

The defined stiffness modifications are only considered in the RF-STABILITY Stability Analysis if the option “Activate Stiffness Modifications from RFEM” in “Options” of the menu “1.1 General Data” is activated.

• ### Which modules are responsive via the COM interface RS‑COM or RF‑COM?

FAQ 003034 EN-US

#### Answer

With the COM interface, you can access most operating elements as well as results of the following programs or add-on modules:

• RF-/STEEL
• RF-/STEEL EC3
• RF-/ALUMINUM
• RF-/CONCRETE
• RF-STABILITY
• RF-/TIMBER Pro
• RF-/DYNAM Pro
• ### 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.

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