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  • Answer

    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.

  • Answer

    RSTAB is a FEM program that uses trigonometric trial functions for members. For this reason, it is not necessary to divide the members to obtain sufficiently accurate results and the calculation speed is higher accordingly.

    RSBUCK determines the eigenvalues of the stiffness matrix and can thus linearly calculate the critical load and buckling mode of the structure.

  • Answer

    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.
  • Answer

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

  • 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.
  • Answer

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

  • 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.

  • Answer

    RSBUCK does not display any buckling length factors of sets of members. You can only start with the results of the individual members. You can consider the member for which the smallest critical buckling load N cr is output as governing for the set of members.

    It may also be helpful to consider the axial forces in the individual members. If they are the same across the set of members with an unchanged cross-section, the effective length coefficients are also the same. This value can then also be used for the set of members.

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