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  • Answer

    RSBUCK / RF-STABILITY calculates at least one critical load factor or one loading load and an associated buckling mode. The buckling length is then recalculated from the critical load (see here ).

    The effective lengths calculated in this way should only be used by the members that buckle in the buckling mode. Figure 1 shows a structure where the rear columns buckle. Therefore, only the effective lengths calculated for these two columns should be used.

  • 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, they are not. In the add-on module RSBUCK, no stability analyzes for lateral-torsional buckling are performed.

  • 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 determine effective length factors of sets of members in RSBUCK.

    You can only use the results of individual members. As the governing member for the set of members, you can assume the one with the smallest critical buckling load Ncr.

    Alternatively, it also helps to look at the axial forces in the individual members. If they are constant over the set of members (with a constant cross-section), then the effective length factors are also constant, so that you can use them also for the set of members.
  • Answer

    RSBUCK uses a momentary representation of the axial force distribution in the respective load state. The axial forces are increased iteratively until the critical load case occurs. In the numerical analysis, the stability load is indicated by the fact that the determinant of the stiffness matrix becomes zero.

    If the effective length factor is known, the buckling load and buckling mode are determined based on this. For the lowest buckling load, all effective lengths and effective length factirs are determined.

    Example: Hinged column with a length of 20 m, cross-section HE‑B 500, self-weight load

    For the first buckling mode, you obtain the effective length factor of kcr,y = 2.92 for the buckling about the major axis. For the buckling about the minor axis with a buckling load of 651.3 kN, you obtain an effective length factor of 1.00.

    If you set the expression for determining the buckling load Ncr = π² * E * I / Lcr² to Lcr and apply Ncr = 651.3 kN and Iy = 107,200 cm4, you obtain the Lcr,y of 58.4 m, which results in the effective length factor kcr,y of 2.92.

    In RSBUCK, there are two effective length factors determined for each buckling mode and buckling load.

    In order to obtain the correct effective length factor for the deflection perpendicular to the y-axis (buckling about the major axis), it is necessary to calculate several buckling modes (mode shapes). The correct value is displayed in Window 2.1. In the example, it is the third buckling mode with a buckling load of 5485.5 kN. For this load, the effective lengths and effective length factors are determined as follows: kcr,y = 1.0 and kcr,z = 0.345. 

    In the case of a quadratic cross-section, two equal effective lengths result as the stiffnesses in both directions are the same.

  • Answer

    In RSBUCK and RF‑STABILITY, the lowest critical load is calculated first. This is obtained, for example, for a hinged column (Euler buckling mode 1, IPE cross-section) for the buckling about the z-axis. With this buckling load, the effective length Lcr,y is determined retrospectively.

    In order to obtain the correct effective lengths for Lcr,y, it is necessary to also consider the second buckling mode (mode shape). For this, specify at least two or more buckling modes for the calculation in the calculation parameters. In the second buckling mode, you obtain a higher buckling load (sway about the y-axis), from which you obtain the correct buckling load Lcr,y.

    As shown in the example, RSBUCK or RF‑STABILITY requires the calculation of several buckling modes (mode shapes). Thus, you can obtain results for the individual directions (see Figure).

  • Answer

    Check if the settings for considering the favorable effect by tension forces are the same in RSTAB and RSBUCK.

    RSTAB determines the critical load factor according to the nonlinear calculation method: The loading is increased gradually by the value of the load factor increment Δk until the system becomes unstable. On the other hand, RSBUCK performs a linear eigenvalue analysis. Therefore, the elements acting nonlinearly, such as failing members or supports, may have different effects in RSTAB and RSBUCK.

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