# Frequently Asked Questions (FAQ)

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• ### I have calculated a pre-deformed structure according to the second-order analysis with RF‑STABILITY and RF‑IMP. Why are the deformations of the CO smaller than the applied pre-deformation?

In RF‑IMP, you have selected the "Generate pre-deformed FE mesh" option. In this case, the imperfect structure is considered in the calculation by using the deformed FE mesh generated in RF‑IMP. Thus, the CO results refer to the coordinates of the pre-deformed FE mesh and not to the original position in the perfect system.

To better explain this issue, I exported the coordinates of the pre-deformed FE mesh nodes from RF‑IMP to Excel. Based on this information, an imperfect structure was created in a new RFEM file and calculated according to the second-order analysis. The left window shows the deformation of the perfect structure considering RF‑STABILITY and RF‑IMP, and the right window shows the result of the imperfect structure. The results are identical.
• ### Is it possible to perform stability analyses on reinforced concrete structures by means of RF‑STABILITY?

Since concrete has a nonlinear material behavior that can only be simulated with the CONCRETE NL module, it is not possible to analyze it by using the RF‑STABILITY add-on module.

The use of another material model such as isotropic linear elastic or isotropic plastic would not represent the crack formation correctly, and the results are therefore not usable.

The stability analysis on columns can be performed with RF‑CONCRETE Columns or RF‑CONCRETE NL. You can find a small example under Downloads.

This example includes the design of a column by the RF‑CONCRETE Columns add-on module. Make sure that the calculation of the internal forces in RFEM is performed according to the geometrically linear analysis and that no imperfections are required because the method used in the add-on module takes them into account.

The example also includes the design with RF‑CONCRETE NL. Here, it is also necessary to calculate according to the second-order analysis and it requires the imperfections in the form of inclinations. For better comparability, the layout of the longitudinal reinforcement was aligned with the result from RF‑CONCRETE Columns, as shown in Figure 01 and Figure 02. Since the reinforcement is optimized by the module after a new calculation, the desired reinforcement was saved as a template (see the red arrow).

• ### When determining the buckling modes in RF‑STABILITY, additional lines are displayed for each member perpendicular to the buckling curve. Why?

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

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

Yes, it is possible.

First, RF‑STABILITY (or RSBUCK in RSTAB 8) can be used to determine the effective lengths for a structural system and certain loading.

They can then be imported in the "Effective Lengths" dialog box in RF‑/TIMBER Pro.

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

All add-on modules are a part of the installation of the main program RFEM/RSTAB. You can open them by using the Add-on Modules menu (see the video). Some of the add-on modules must be activated in General Data (for example, RF‑FORM‑FINDING).
• ### What is the critical load factor and how is it possible to determine it?

A critical load factor specifies which factor can be used to increase the load until the structural system fails. If it is smaller than one, the calculation according to the second-order analysis is usually unstable as the structural system is already subjected to the critical load. This factor is also referred to in standards. For example, Eurocode 3 specifies that the calculation according to the second-order analysis is no longer necessary as of the critical load factor of 10.
The critical load factor can be determined by using the RF‑STABILITY or RSBUCK add-on module.
• ### How can I export the effective lengths from the RF‑STABILITY add-on module to Excel?

It is possible to export the effective lengths from the add-on module to Excel as shown in Figure 01.
• ### How are the effective lengths of frame columns determined in RFEM or RSTAB?

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

RF‑STABILITY and RSBUCK perform an eigenvalue analysis for the entire model with a certain state of the axial 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 by using this. The effective lengths and the effective length factors are then determined for this lowest buckling load.

Depending on the required number of eigenvalues, the results show the critical load factors with the corresponding buckling curves, and the effective length about the major and the minor axis for each member, depending on the mode shape.

Since every load case has usually a different state of the axial force in the elements, a separate belonging 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 the design of the respective load situation.

Since this result may be different for each design due to the different load situations, the longest effective length of all calculated analyses is assumed as equal for all load situations.

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

l = 12 m
h = 7.5 m
E = 21,000 kN/cm²
Iy,R = 5,790 cm4
Iy,S = 4,250 cm4

NL = 75 kN
NR = 50 kN

$EI_R=E\ast Iy_R=12,159\;kNm^2$
$EI_S=E\ast Iy_S=8,925\;kNm^2$

$\nu=\frac2{{\displaystyle\frac{l\ast EI_S}{h\ast EI_R}}+2}=0.63$

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 RF‑STABILITY and RSBUCK.

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

###### RF-STABILITY
$\eta_{Ki}=4.408$
$sk_L=16.324\;m$
$sk_R=19.993\;m$
• ### What is the purpose of the "Calculate eigenvector for unstable model..." option in RF‑STABILITY?

This feature is intended to detect the modeling errors in a structure that may lead to instability. Using this method, it is possible to calculate such systems and to graphically determine the instability cause.

This feature is not suitable for the following problems:
• Calculation aborts due to overloading (stability problems)
• Determination of buckling curves and buckling modes
If the system is stable and the stability problems only occur during the calculation according to the second-order analysis, this function sets all results to 0.

The problem solving of instabilities is described in detail in FAQ 3045 .
• ### Why are the stiffness modifications not taken into account when determining critical load factors in the RF‑STABILITY add-on module?

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.

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#### 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.

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