The modal relevance factor (MRF) can help you to assess to which extent specific elements participate in a specific mode shape. The calculation is based on the relative elastic deformation energy of each individual member.
The MRF can be used to distinguish between local and global mode shapes. If multiple individual members show significant MRF (for example, > 20%), the instability of the entire structure or a substructure is very likely. On the other hand, if the sum of all MRFs for an eigenmode is around 100%, a local stability phenomenon (for example, buckling of a single bar) can be expected.
Furthermore, the MRF can be used to determine critical loads and equivalent buckling lengths of certain members (for example, for stability design). Mode shapes for which a specific member has small MRF values (for example, < 20%) can be neglected in this context.
The MRF is displayed by mode shape in the result table under Stability Analysis → Results by Members → Effective Lengths and Critical Loads.
Compared to the RF‑/STABILITY (RFEM 5) and RSBUCK (RSTAB 8) add-on modules, the following new features have been added to the Structure Stability add-on for RFEM 6 / RSTAB 9:
Activation as a property of a load case or a load combination
Automated activation of the stability calculation via combination wizards for several load situations in one step
Incremental load increase with user-defined termination criteria
Modification of the mode shape normalization without recalculation
Due to the integrated RF-/STEEL Warping Torsion module extension, it is possible to perform the design according to Design Guide 9 in RF-/STEEL AISC.
The calculation is performed with 7 degrees of freedom according to the warping torsion theory and enables a realistic stability design, including consideration of torsion.
The determination of the critical buckling moment is carried out in RF-/STEEL AISC by using the eigenvalue solver which allows an exact determination of the critical buckling load.
The eigenvalue solver shows a display window of the eigenvalue graphics, which enables checking of the boundary conditions.
In STEEL AISC, it is possible to consider lateral intermediate supports at any location. For example, it is possible to stabilize only the upper flange.
Furthermore, user-defined lateral intermediate supports can be assigned; for example, single rotational springs and translational springs at any location at the cross-section.
The add-on module evaluates the pre-deformation of a load case as well as mode shapes of stability or dynamic analysis. Based on this initial deformation, it is possible either to pre-deform the structure or to create a load case with equivalent imperfections of members.
The pre-deformed initial model is useful especially for structures consisting of surface and solid elements (RFEM) as well as members. It is necessary to specify only the maximum value to which the deformation is to be scaled. All FE or model nodes will be scaled with regard to the initial deformation.
Equivalent imperfections are particularly useful for beam structures. You can define inclinations and precambers of members and sets of members in the additional window. They can be generated automatically, according to standards, or defined manually. The following standards are available:
EN 1992:2004
EN 1993:2005
DIN 18800:1990-11
DIN 1045-1:2001-07
DIN 1052:2004-08
Only the imperfection resulting from the initial deformation on the relevant member is applied. In addition, you can consider the reduction factors. This way, it is possible to apply the imperfection efficiently.
The first results presented are the critical load factors. This allows for an evaluation of stability risks. For member models, the effective lengths and critical loads of the members are output in tabular form.
In the next result windows, you can check the normalized eigenvalues sorted by node, member, and surface. The eigenvalue graphic allows for evaluation of the buckling behavior. The graphical display makes it easier to take countermeasures.
Several methods are available for the eigenvalue analysis:
Direct Methods
The direct methods (Lanczos, roots of characteristic polynomial, subspace iteration method) are suitable for small to medium-sized models. These fast methods for equation solvers benefit from a lot of the computer memory (RAM). 64-bit systems use more memory so that even bigger structural systems can be calculated quickly.
This method requires only a small amount of memory. Eigenvalues are determined one after the other. It can be used to calculate large structural systems with few eigenvalues.
The RF-STABILITY add-on module can also perform the non-linear stability analysis. Also for nonlinear structures, results close to reality are provided. The critical load factor is determined by gradually increasing the loads of the underlying load case until the instability is reached. The load increment takes into account nonlinearities such as failing members, supports and foundations, and material nonlinearities.
First of all, it is necessary to select a load case or combination whose axial forces are to be used in the stability analysis. It is possible to define another load case to, for example, For example, you have to consider an initial prestress.
Then, you can select the linear or non-linear analysis to be performed. Depending on the application, you can use a direct calculation method, such as according to Lanczos or the ICG iteration method. Members not integrated in surfaces are usually displayed as member elements with two FE nodes. It is not possible to determine the local buckling of single members on these elements. Therefore, you have the option to divide members automatically.
If there is a load case or load combination in the program, the stability calculation is activated. You can define another load case in order to consider initial prestress, for example.
For this, you need to specify whether to perform a linear or nonlinear analysis. Depending on the case of application, you can select a direct calculation method, such as the Lanczos method or the ICG iteration method. Members not integrated in surfaces are usually displayed as member elements with two FE nodes. With such elements, the program cannot determine the local buckling of single members. That's why you have the option to divide members automatically.
You can select several methods that are available for the eigenvalue analysis:
Direct Methods
The direct methods (Lanczos [RFEM], roots of characteristic polynomial [RFEM], subspace iteration method [RFEM/RSTAB], and shifted inverse iteration [RSTAB]) are suitable for small to medium-sized models. You should only use these fast solver methods if your computer has a larger amount of memory (RAM).
In contrast, this method only requires a small amount of memory. Eigenvalues are determined one after the other. It can be used to calculate large structural systems with few eigenvalues.
Use the Structure Stability add-on to perform a nonlinear stability analysis using the incremental method. This analysis delivers close-to-reality results also for nonlinear structures. The critical load factor is determined by gradually increasing the loads of the underlying load case until the instability is reached. The load increment takes into account nonlinearities such as failing members, supports and foundations, and material nonlinearities. After increasing the load, you can optionally perform a linear stability analysis on the last stable state in order to determine the stability mode.
As the first results, the program presents you with the critical load factors. You can then perform an evaluation of stability risks. For member models, the resulting effective lengths and critical loads of the members are displayed to you in tables.
Use the next result window to check the normalized eigenvalues sorted by node, member, and surface. The eigenvalue graphic allows you to evaluate the buckling behavior. This makes it easier for you to take countermeasures.
When generating a pre-deformed FE mesh in RFEM, the displacement data of each individual node are saved in the background. This can be used for the calculation of load combinations in RFEM. In order to check the generated data, the pre-deformation is displayed in tables and graphically.
If the nodes of the model are to be displaced, the node coordinates are modified directly after the generation. When generating equivalent imperfections, the module creates a normal load case, including member imperfections. To facilitate the data check, generated imperfections are displayed in result tables as well as graphically.
RSBUCK determines the most unfavorable buckling modes of a structure. It is generally not possible in terms of the calculation method to exclude lower eigenvalues from the analysis and at the same time, to attempt to determine higher eigenvalues. With RSBUCK, you can determine up to the 10,000 lowest eigenvalues of a structural system.
By default, RSBUCK uses the average value of the axial forces occurring on the individual members to calculate the eigenvalues/critical load factors. Optionally, the module can also process the most unfavorable axial force of a member. The determination of buckling modes is performed by an eigenvalue analysis for the entire structure. For this, an iterative equation solver is used.
You only have to specify the following two values:
the maximum number of iterations
the break-off limit
Since an exact solution is approximated as close as desired, but never reached, RSBUCK terminates the calculation process after the specified number of iteration steps. In the case of a convergence problem, the break-off limit determines the moment when an approximate solution can be considered as an exact result. Divergence problems have no solution.
RSBUCK is distinguished by easy handling, clear data arrangement, and great user-friendliness. With only a few mouse clicks, you can define the number of buckling modes to be calculated, as well as the load case to be considered.
Structural data and boundary conditions set in the selected load case are imported automatically from RSTAB. Alternatively, you can edit the imported axial forces or enter new values manually. It is also possible to create further RSBUCK cases in order to perform several analyzes each with different boundary conditions.
For a better result display, it is possible to set the units individually in RSBUCK. If the RSTAB internal forces are not available when starting the RSBUCK module, the program calculates the required internal forces automatically before determining the buckling values.
The results of the buckling analysis are displayed in clearly arranged result tables and graphics. Since RSBUCK is fully integrated in RSTAB, you can adjust all results in detail in the printout report according to your individual requirements.
Furthermore, you can easily export all result tables to MS Excel or in a CSV file. A special transfer menu defines all specifications required for the export.
It is possible to use previously created pre-deformed FE mesh in load combinations. To do this, select the corresponding RF-IMP case in the calculation parameters of the load combination. Then, the calculation of internal forces is performed for the imperfect structure.