#### Further Information

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• ### After dividing a column with intermediate nodes, the results in the RF‑/CONCRETE Columns add-on module are different from those before the division. What is the reason?

The reason for the different results is probably the settings of effective lengths for the stability analysis. According to the default setting, these are determined automatically in the RF‑/CONCRETE Columns add-on module. However, this method does not work for divided members, and thus no stability analysis is performed (a corresponding warning message appears).

If you have entered the effective lengths manually, this only applies to a single member. In order to perform a correct stability analysis of the column as an entire structural component, it is necessary to define it as a set of members.

• ### 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.

• ### 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 can I export the effective lengths from the STEEL EC3 add-on module to EXCEL?

It is possible to export the effective lengths from the add-on module to EXCEL or import it from 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
$\eta_{Ki}=4.408$
$sk_L=16.324\;m$
$sk_R=19.993\;m$
• ### Is it possible to use Excel to modify the effective lengths in the add-on modules TIMBER Pro, STEEL EC3, and others?

Yes, it is possible. However, it is very important to use the correct format and the same assignment of the corresponding members in the Excel import. This works great if you export the lengths to EXCEL first and then import them again. The attached video shows this process.

There are more options to change the effective lengths in the RF‑COM/RS‑COM add-on module. The add-on modules mentioned above are linked to this application.
• ### I cannot set a shear panel and rotational restraint in the RF‑/STEEL EC3 add-on module. Why are these options inactive?

The options for shear panel and rotational restraint are available after activating the stability analysis in Details of the add-on module. The settings of effective lengths and intermediate supports will become available as well.
• ### I have calculated the effective lengths in my model by using RSBUCK/RF‑STABILITY.Which effective lengths are relevant for further analyses?

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.

• ### I would like a stair railing, for which I have created a set of rods, measured. The profile is a hollow profile. Can the proof of stability of the whole system be made for this purpose, or do I have to transfer this to all stock in STAHL EC3 in a replacement bill?

For a vault-free cross-section, a proof of the complete system is recommended using theory II. Order - internal forces and local imperfections. The entry of the effective lengths and nodal bearings (with staff sets) can thus be omitted. However, please activate gamma_M1 for cross-section verification.
• ### Is there a way how to prepare this EC3 buckling check without setting the intermediate support manually? Or is there a workaround that can speed up this work?

Unfortunately not. Intermediate supports and nodal supports have to be set manually. The default configuration for effective lengths is 1,0*member length and for set of members, one nodal support with definition u_y' and phi_x' is defined on each side of the set of member.

To speed up the process of defining nodal supports, you can pick more nodes in one step by using the highlighted function in the attached picture.

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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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