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Frequently Asked Questions (FAQ)
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To display the mode shape, the calculation must be successfully performed. If this is not possible, there are either invalid boundary conditions or no stability problem was found (convergence not reached). No critical buckling shape can be considered then.
The 'Mode Shape - Overview' window allows you to check the plausibility of the eigenvalue analysis. You can open this window by clicking the button marked in the figure.
The graphical control of the mode shape graphic is available if the eigenvalue solver was used in the calculation, for example, in the case of the analysis according to General Method in compliance with EN 1993‑1‑1.
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).
AnswerIn accordance to the EN 1998.-1 you need to calculate so many mode shapes that the sum of the effective modal masses is at least 90 % of the effective total mass (this is usually the overall structural mass). This can be regulated differently in other earthquake standards.
In the module RF-DYNAM Pro you find those values together with the frequencies in the Dynamic Load Cases (DLCs) in a tab called 'Mode Shapes', please see attached picture. You can check whether or not you calculated a sufficient number of eigenvalues before starting the calculation, it might be that you have ti increase the number of eigenvalues.
After calculation you find the effective modal masses and the factors in Table 5.7 5.7 Effective Modal Mass Factors.
In accordance with DIN 18800 part 2 as well as EN 1993-1-1, you have to specify geometric equivalent imperfections for the calculation according to second-order analysis in order to consider geometrical and structural imperfections.
The course of the predeformation should be applied corresponding to the lowest buckling or lateral-torsional buckling eigenmode.
Therefore, in the program FE-LTB, the eigenmode belonging to the smallest eigenvalue is calculated (preliminary eigenvalue analysis) and then chosen as initial deformation mode.
Now, the application of the imperfection is done by the user by scaling the eigenmode.
As the predeformation in FE-LTB is selected according to the eigenmode, the imperfections entered in RSTAB cannot be imported. They will be ignored.
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