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.
Have you activated the Time-Dependent Analysis (TDA) add-on? Very well, now you can add time data to load cases. After you have defined the start and end of the load, the influence of creep at the end of the load is taken into account. The program allows you to model creep effects for frame and truss structures made of reinforced concrete.
In this case, the calculation is performed nonlinearly according to the rheological model (Kelvin and Maxwell model).
Was the calculation successful? You can now display the determined internal forces in tables and graphics, and consider them in the design.
Automatic generation of FE analysis models: The add-on automatically creates a finite element model (FE) of the steel connection in the background.
Consideration of all internal forces: The calculation and design checks include all internal forces (N, Vy, Vz, My, Mz, MT) and are not limited to planar loading.
Automatic load transfer: All load combinations are automatically transferred to the FE analysis model of the connection. The loads are transferred directly from RFEM, so manual data input is not necessary.
Efficient modeling: The add-on saves you time when modeling complex connection situations. You can also save the created FE analysis model and use it further for your own detailed analyses.
Extensible library: An extensive and extensible library with predefined steel connection templates is available.
Wide applicability: The add-on is suitable for connections of any type and shape, compatible with almost all rolled, welded, built-up, and thin-walled cross-sections.
No manual editing of the FE model required by the user, the essential calculation settings can be changed via the configuration settings
Automatic adaptation of the connection geometry, even if the members are subsequently edited, due to the relative relation of the components to each other
Parallel to the input, a plausibility check is carried out by the program to quickly detect missing input or collisions, for example
Graphical display of the connection geometry that is updated in parallel with the input
The program supports you: It determines the bolt forces on the basis of the FE analysis model and evaluates them automatically. The add-on performs the standard-compliant design of bolt resistance for failure cases, such as tension, shear, hole bearing, and punching, and clearly displays all required coefficients.
Do you want to perform weld design? The welds are modeled as elastic-plastic surface elements, and their stresses are read out from the FE analysis model. The plasticity criteria is set in the way that they represent failure according to AISC J2-4, J2-5 (strength of welds), and J2-2 (strength of base metal). The design can be performed with the partial safety factors of the selected National Annex of EN 1993‑1‑8.
The plates in the connection are designed plastically by comparing the existing plastic strain to the allowable plastic strain. The default setting is 5% according to EN 1993‑1‑5, Annex C, but can be adjusted by user-defined specifications, as well as 5% for AISC 360.
You can display all essential results on the FE model. In this case, you can filter the results separately according to the respective components.
Furthemore, RFEM delivers you all design checks in a tabular form, including the display of the formulas used. If you wish, you can transfer the result tables to the RFEM printout report.
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
For each load case, the deformations can be displayed at the end time.
These results are also documented for you in the printout report of RFEM and RSTAB. You can select the report contents and extent specifically for the individual design checks.
Do you work with steel connections? The Steel Joints add-on for RFEM supports you when analyzing steel connections by using an FE model. In this case, the modeling runs fully automatically in the background. Nevertheless, you can control this process via the simple and familiar input of components. You can then use the loads determined on the FE model for your design of the components according to EN 1993‑1‑8 (including National Annexes).
Do you have great respect for the ravages of time? After all, it eventually gnaws at your construction projects. Use the Time-Dependent Analysis (TDA) add-on to consider the time-dependent material behavior of members. Long-term effects, such as creep, shrinkage, and aging, can influence the distribution of internal forces, depending on the structure. Prepare for this optimally with this add-on.
In this case, you calculate the critical load factor for all analyzed load combinations and the selected number of mode shapes for the connection model. Compare the smallest critical load factor with the limit value 15 from the standard EN 1993‑1‑1, Clause 5. Furthermore, you can make user-defined adjustment of the limit value. As a result of the stability analysis, the program displays the corresponding mode shapes graphically.
For the stability analysis, RFEM uses the adapted surface model to specifically recognize the local buckling shapes. You can also save and use the model of the stability analysis, including the results, as a separate model file.
To design a Steel connection, you must have the Steel Joints Add-on enabled. The Add-ons in RFEM 6 are activated in the Add-ons tab of the Edit Model - Base Data window. If the Add-on is active, it is displayed in the navigator.
Numerous predefined components for easy input of typical connection situations (for example, end plates, angles, web plates)
Universally applicable basic components (plates, welds, bolts, auxiliary planes) for the input of complex connection situations
Graphical display of the connection geometry that is updated in parallel with the input
The steel connection template included in the add-on allows you to select different connection types and apply them to your model
The template provides connections from three categories: Rigid, Pinned, Truss
Automatic adjustment of the connection geometry, even during subsequent editing of the structural components, due to the relative arrangement of the components to each other
Design of a frame connection with taper and stiffened members. A stress analysis and a buckling stability analysis were carried out for the connection. To display the buckling results, the connection was converted into a separate model.
In addition to other predefined components in the design add-on for steel connections, the universal base component "General Weld" can be used to enter complex connection situations.
In the case of rectangular cross-sections, you can usually achieve a direct connection by using welds. However, you can also connect them to other cross-sections in the same way. Furthermore, other components such as end plates help you to connect the rectangular cross-sections to other structural components.
If a weld seam connects two plates with different materials, it is possible to select from a combo box in the Steel Joints add-on which one of both materials should be used for the weld seam.