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.
Entering soil layers for soil samples is performed in a clearly arranged dialog box. A corresponding graphical representation supports clarity and makes checking the input user-friendly.
An extensible database facilitates the selection of soil material properties. The Mohr-Coulomb model as well as a nonlinear model with stress and strain dependent stiffness are available for a realistic modeling of the soil material behavior.
You can define any number of soil samples and layers. The soil is generated from all entered samples using 3D solids. Assignment to the structure is carried out using coordinates.
The soil body is calculated according to the nonlinear iterative method. The calculated stresses and settlements are displayed graphically and in tables.
Automatic consideration of masses from self-weight
Direct import of masses from load cases or load combinations
Optional definition of additional masses (nodal, linear, or surface masses, as well as inertia masses) directly in the load cases
Optional neglect of masses (for example, mass of foundations)
Combination of masses in different load cases and load combinations
Preset combination coefficients for various standards (EC 8, SIA 261, ASCE 7,...)
Optional import of initial states (for example, to consider prestress and imperfection)
Structure Modification
Consideration of failed supports or members/surfaces/solids
Definition of several modal analyses (for example, to analyze different masses or stiffness modifications)
Selection of mass matrix type (diagonal matrix, consistent matrix, unit matrix), including user-defined specification of translational and rotational degrees of freedom
Methods for determining the number of mode shapes (user-defined, automatic - to reach effective modal mass factors, automatic - to reach the maximum natural frequency - only available in RSTAB)
Determination of mode shapes and masses in nodes or FE mesh points
Results of eigenvalue, angular frequency, natural frequency, and period
Output of modal masses, effective modal masses, modal mass factors, and participation factors
Masses in mesh points displayed in tables and graphics
Visualization and animation of mode shapes
Various scaling options for mode shapes
Documentation of numerical and graphical results in printout report
In the modal analysis settings, you have to enter all data that are necessary for the determination of the natural frequencies. These are, for example, mass shapes and eigenvalue solvers.
The Modal Analysis add-on determines the lowest eigenvalues of the structure. Either you adjust the number of eigenvalues or let them determined automatically. Thus, you should reach either effective modal mass factors or maximum natural frequencies. Masses are imported directly from load cases and load combinations. In this case, you have the option to consider the total mass, load components in the global Z-direction, or only the load component in the direction of gravity.
You can manually define additional masses at nodes, lines, members, or surfaces. Furthermore, you can influence the stiffness matrix by importing axial forces or stiffness modifications of a load case or load combination.
As soon as the program has completed the calculation, the eigenvalues, natural frequencies and periods are listed. These result windows are integrated in the main program RFEM/RSTAB. You can find all mode shapes of the structure in tables and also have an option to display them graphically and to animate them.
All result tables and graphics are part of the RFEM/RSTAB printout report. In this way, you can ensure clearly arranged documentation. You can also export the tables to MS Excel.