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Frequently Asked Questions (FAQ)
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No, it does not. In the RSBUCK add‑on module, no stability analysis is performed for lateral-torsional or torsional-flexural buckling.
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.
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).
The effective length factors that you can change in the 'Effective Lengths' tab are not considered in the determination of internal forces in RFEM or RSTAB. These values are presettings for add-on modules used for performing stability analyses, for example RF-/STEEL EC3 or RF-/TIMBER Pro. The values will be considered in the modules only.
However, the 'Check exceeding of critical buckling load' check box has an influence on the calculation: if the the critical load is reached, the member fails. In this case, the program shows a message about the instability.
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