Structure Stability for RFEM 6 / RSTAB 9

Add-on for Stability Analysis According to Eigenvalue Method and Incremental Method

Additional Analyses

★ ★ ★ ★ ★

"The RFEM add-on module RF-STABILITY is a perfect combination with RWIND Simulation. Using RF-STABILITY, I can perform a buckling analysis to get accurate effective lengths. Using RWIND Simulation, I can get accurate wind loads. For unusually shaped structures, it would be a wild guess if calculating wind loads from the standard code... either not conservative or too conservative. My client is happy with the results and impressed!"

Additional Analyses

The Structure Stability add-on analyzes the stability of structures. It determines critical load factors and the corresponding stability modes.

Features

Calculation of models consisting of member, shell and solid elements
Non-linear stability analysis
Optional consideration of axial forces from initial prestress
Four equation solvers for effective calculation of various structural models
Optional consideration of stiffness modifications in RFEM/RSTAB
Determination of stability mode greater than the user-defined load increment factor (Shift method)
Optional determination of the mode shapes of unstable models (to identify the cause of instability)
Visualization of stability mode
Basis for determining imperfection

Input

The stability calculation is activated in a load case or load combination. It is possible to define another load case in order to consider initial prestress, for example.

Then, you can select the linear or non-linear analysis to be performed. Depending on the case of application, you can select a direct calculation method such as the Method by 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 local buckling of single member on these elements. Therefore, there is the option to divide members automatically.

Calculation

Several methods are available for the eigenvalue analysis:

Direct Methods
The direct methods (Lanczos (RFEM), roots of characteristic polynomial (RFEM), subspace iteration method (RFEM/RSTAB), shifted inverse iteration (RSTAB)) are suitable for small to medium-sized models. These fast methods for equation solvers benefit from a lot of memory (RAM) in the computer.
ICG Iteration Method (Incomplete Conjugate Gradient (RFEM))
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.

With the Structure Stability add-on, you can also perform a nonlinear stability analysis with the incremental method. It provides realistic results even 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, as well as material nonlinearities. After the load increment, a linear stability analysis can optionally be carried out on the last stable state in order to determine the stability mode.

Results

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 graphical output of the eigenvalues allows an evaluation of the buckling behavior. It makes it easier to provide countermeasures.