With the activated option 'Topology on Form-Finding Form' in Project Navigator - Display, the model display is optimized based on the form-finding geometry. For example, the loads are displayed in relation to the deformed system.
Activating 'Show Form-Finding' in the shortcut menu leads to an automatic preliminary form-finding according to the saved form-finding properties when you change the structure of membrane surfaces. This interactive graphics mode is based on the force density method.
In RFEM, there is an option to couple surfaces with the stiffness types "Membrane" and "Membrane Orthotropic" with the material models "Isotropic Nonlinear Elastic 2D/3D" and "Isotropic Plastic 2D/3D" (add-on module RF-MAT NL is required).
This functionality enables simulation of the nonlinear strain behavior of ETFE foils, for example.
The results of the form‑finding process are a new shape and corresponding inner forces. The usual results, such as deformations, forces, stresses, and others can be displayed in the RF‑FORM‑FINDING case.
This prestressed shape is available as the initial state for all other load cases and combinations in the structural analysis.
For more ease when defining load cases, the NURBS transformation can be used (Calculation Parameters/Form-Finding). This feature moves the original surfaces and cables into position after form‑finding.
By using the grid points of surfaces or the definition nodes of NURBS surfaces, free loads can be situated on selected parts of the structure.
The form-finding function can be activated in the General Data dialog box, Options tab. Prestresses (or geometrical requirements for members) can be defined in the parameters for surfaces and members. The form‑finding process is performed by calculation of an RF‑FORM‑FINDING case.
Steps of the working sequence:
Creation of a model in RFEM (surfaces, beams, cables, supports, material definition, and so on)
Setting of required prestress for membranes and force or length/sag for members (for example, cable)
Optional consideration of other loads for the form-finding process in special form‑finding load cases (self‑weight, pressure, steel node weight, and so on)
Setting of loads and load combinations for further structural analyses
After starting the calculation, the program performs form‑finding on the entire structure. The calculation takes into account the interaction between the form‑finding elements (membranes, cables, and so on) and the supporting structure.
The form-finding process is performed iteratively as a special nonlinear analysis, inspired by URS (Updated Reference Strategy) by Prof. Bletzinger / Prof. Ramm. This way, shapes in equilibrium are obtained considering the pre‑defined prestress.
Furthermore, this method allows you to consider individual loads such as self‑weight or internal pressure for pneumatic structures in the form‑finding process. The prestress for surfaces (for example, membranes) can be defined using two different methods:
Standard method - prescription of required prestress in a surface
Projection method - prescription of required prestress in the projection of a surface, stabilization especially for conical shapes