Form-Finding Stage

RFEM and RSTAB are able to cover a large number of branches in the building and construction industry with their generally usable structural frame analysis and FEM programs. Designing cable structures is thus also possible in both software solutions. Some assistance tools for modeling and design will be presented in the following text.

Cable and tensile membrane structures are regarded as very slender and aesthetic building structures. The partly very complex double-curved shapes can be found using suitable form-finding algorithms. One possible solution is to search for the form via the equilibrium between the surface stress (provided prestress and an additional load such as self-weight, pressure, and so on) and the given boundary conditions.

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.

• Form-finding of:
  • tension-loaded membrane and cable structures
  • compression-loaded shell and beam structures
  • mixed tension- and compression-loaded structures
• Consideration of gas chambers between surfaces
• Interaction with supporting structure (substructure design according to various standards)
• Surfaces as a 2D and members as a 1D element
• Definition of different prestress conditions for surfaces (membranes and shells)
• Definition of forces or geometrical requirements for members (cables and beams)
• Consideration of individual loads (self‑weight, inner pressure, and so on) in the form‑finding process
• Temporary support definitions for the form-finding process
• Automatic preliminary form-finding of membrane surfaces (more information...)
• Definition of isotropic or orthotropic material for structural analysis
• Optional definition of free polygon loads
• Transformation of form‑found shape elements into NURBS surface elements
• Possibility of combined form-finding by integration of preliminary form-finding
• Graphical evaluation of the new form using colored coordinates and inclination plots
• Complete documentation of the calculation including user-defined adaptive evaluation figures
• Optional export of the FE mesh as a DXF or Excel file

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.