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  1. Foil Cushion with Material Model Isotropic Plastic 2D/3D

    Nonlinear Material Law for Membranes

    RFEM offers the 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" (RF-MAT NL add-on module required).

    This functionality enables the simulation of the nonlinear strain behaviour of, for example, ETFE foils.

  2. RF-FORM-FINDING | Features

    • 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
    • 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, etc.) in the form‑finding process
    • Temporary support definitions for the form-finding process
    • 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 coloured coordinates and inclination plots
    • Complete documentation of the calculation including user-defined adaptive evaluation figures
    • Optional export of the FE mesh as DXF or Excel file
  3. Prestress parameters for membranes

    RF-FORM-FINDING | Input

    The form-finding function can be activated in the General Data dialog box, tab Options. Prestress (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, etc.)
    • Setting of required prestress for membranes and force or length/sag for members
    • Optional consideration of other loads for the form-finding process in special form‑finding load cases (self‑weight, pressure, steel node weight, etc.)
    • Setting of loads and load combinations for further structural analyses
  4. Calculation parameters

    RF-FORM-FINDING | Calculation

    After starting the calculation, the program performs form‑finding on the entire structure. The calculation takes into account the interaction between the form‑found elements 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. In this way, shapes in equilibrium are obtained considering the pre‑defined prestress.

    Furthermore, this method allows you to consider the individual loads such as self‑weight or interior pressure for pneumatic structures in the form‑finding process. The prestress for surfaces can be defined by two different methods:

    • Standard method - prescription of required prestress in a surface
    • Projection method - prescription of required prestress in a projection of a surface, stabilization especially for conical shapes
  5. Deformed FE mesh after form-finding

    RF-FORM-FINDING | Results

    The results of the form‑finding process are a new shape and corresponding inner forces. 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 the 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.

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