You can use the selection options in the printout report to receive the detail results (in short or long form) to illustrate the individual buckling modes with the relevant buckling analysis.
The "4.0 Results - Summary" table displays the infinity norm at the end of the load case results. The norm is used to estimate the largest eigenvalue of a structure. The largest eigenvalue of a structure is estimated by numerical analysis, as accurate determination can be very time-consuming.
The RF-STABILITY add-on module determines any critical load factors, effective lengths, and eigenvectors of RFEM models. Stability analyses can be carried out by various eigenvalue methods, the advantages of which depend on the structural system as well as computer configurations.
Utilizing the RF-STEEL AISC add-on module, steel member design is possible according to the AISC 360-16 standard. The following article will compare the results between calculating lateral torsional buckling according to Chapter F and Eigenvalue Analysis.
The article titled Lateral-Torsional Buckling in Timber Construction | Theory explains the theoretical background for the analytical determination of the critical bending moment Mcrit or the critical bending stress σcrit for the lateral buckling of a bending beam. This article uses examples to verify the analytical solution with the result from the eigenvalue analysis.
With the RF-STABILITY and RSBUCK add-on modules for RFEM and RSTAB, it is possible to perform eigenvalue analyses for member structures in order to determine the effective length factors. The effective length coefficients can then be used for the stability design.
In a multi-modal response spectrum analysis, it is important to determine a sufficient number of eigenvalues of the structure and to consider their dynamic responses. Regulations such as EN 1998‑1 [1] and other international standards require activation of 90% of the structural mass. This means: to determine so many eigenvalues that the sum of the effective modal mass factor is greater 0.9.
Critical load factors and the corresponding mode shapes of any structure can be determined efficiently in RFEM and RSTAB, using the RF-STABILITY or RSBUCK add-on module (linear eigenvalue solver or nonlinear analysis).
The RF‑STABILITY and RSBUCK add‑on modules for RFEM and RSTAB allow you to perform eigenvalue analysis for frame structures in order to determine critical load factors, including the buckling modes. Several buckling modes can be determined. They provide information about the model areas bearing stability risks.
In RF-/DYNAM Pro – Equivalent Loads, a signed result option in accordance with the dominant eigenmode has been available since version X.06.3039. For the modal combination of results corresponding to the single eigenvalues, a quadratic combination rule has to be used. In RFEM and RSTAB, the SRSS and the CQC rule are available. Only results, not loads, are allowed to be combined directly. The reason is the mode shapes, which are arbitrarily scaled and signed.
The RF-FORM-FINDING add-on module determines equilibrium shapes of membrane and cable elements in RFEM. In this calculation process, the program searches for such geometric position where the surface stress/prestress of membranes and cables is in equilibrium with natural and geometric boundary conditions. This process is called form-finding (hereinafter referred to as FF). The FF calculation can be activated in RFEM globally in the "General Data" of a model, "Options" tab. After selecting the corresponding option, a new load case or a calculation process called RF-FORM-FINDING is created in RFEM. An additional FF parameter is available for defining surface stress and prestress when entering cables and membranes. By activating the FF option, the program always starts the form-finding process before the pure structural calculation of internal forces, deformation, eigenvalues, etc., and generates a corresponding prestressed model for further analysis.
In the DYNAM Pro add‑on module for RSTAB, you can now neglect masses that may have a negative effect on the equivalent mass factor when calculating eigenvalues. To do this, you can disable the masses under [Details]. These include primarily mass points located in the support of the structures.
In the following example, the stability analysis of a steel frame can be performed according to the General Method in compliance with EN 1993‑1‑1, Sect. 6.3.4 in the RF‑/STEEL EC3 add-on module. The first of my three posts shows the determination of the critical load factor for design loads required by the design concept, which reaches the elastic critical buckling load with deformations from the main framework plane.
When accidental torsion is considered in the RF-/DYNAM Pro - Equivalent Loads module, the module exports two load cases for each eigenvalue: one with positive torsional moment, the other with negative torsional moment. The generated equivalent loads themselves do not differ in these two load cases.