#### Further Information

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

FAQ 004151 EN

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 clear arrangement of the graphic. 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 RSBUCK in RF-/TIMBER Pro?

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.

FAQ 003603 EN

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

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?

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?

FAQ 003538 EN

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
$\eta_{Ki}=4.408$
$sk_L=16.324\;m$
$sk_R=19.993\;m$
• ### 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:
• 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.

A detailed description of solving the instability problems is included in FAQ 2257.
• ### 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.
• ### Which modules are responsive via the COM interface RS‑COM or RF‑COM?

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

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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.

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