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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.

  6. RF-CUTTING-PATTERN | Features

    • Planar and geodesic cutting lines
    • Flattening of double-curved surface parts of tensioned membranes or pneumatic cushions
    • Definition of cutting patterns by using boundary lines which are not required to be connected
    • Sophisticated flattening based on the minimum energy theory
    • Uniform or linear compensation by warp and weft direction
    • Possibility of different compensations for boundary lines
    • Welding and boundary allowances
    • Adaptable data organisation (any additional modification of input data is considered up to the final ‘weld’)
    • Graphical display of cutting patterns
    • Statistical information about each cutting pattern (width, length, size)
    • Option to automatically generate cutting patterns from cells
  7. Dividing membrane surface by using the "Cut via Two Lines" line type

    RF-CUTTING-PATTERN | Input

    RF-CUTTING-PATTERN is activated by selecting the respective option in the Options tab in General Data of any RFEM model. After activating the add‑on module, a new object ‘Cutting Patterns’ is displayed under Model Data. If the membrane surface distribution for cutting in the basic position is too large, you can divide the surface by cutting lines (line types ‘Cut via Two Lines’ or ‘Cut via Section’) in the corresponding partial strips.

    Then you can define the individual entries for each cutting pattern by using the ‘Cutting Pattern’ object. Here you can set boundary lines, compensations, and allowances.

    Steps of the working sequence:

    • Creation of cutting lines
    • Creation of the pattern by selecting its boundary lines or using a semi‑automatic generator
    • Free selection of warp and weft orientation by entering an angle
    • Application of compensation values
    • Optional definition of different compensations for boundary lines
    • Different allowances (welding, boundary line)
    • Preliminary representation of the cutting pattern in the graphic window at the side without starting the main nonlinear calculation
  8. RF-CUTTING-PATTERN | Calculation

    The nonlinear calculation adopts the real mesh geometry of planar, buckled, simple curved, or double curved surface components from the selected cutting pattern and flattens this surface component in compliance with the minimization of distortion energy, assuming defined material behaviour.

    Basically, this method attempts to compress the mesh geometry in a press assuming frictionless contact and to find such a state where the stresses due to flattening the component in the plane are in equilibrium. In this way, the minimum energy and the optimum accuracy of the cutting pattern are achieved. Compensation for warp and weft as well as compensation for boundary lines are considered. Then, the defined allowances on boundary lines are applied to the resulting planar surface geometry.

    Features:
    • Minimization of distortion energy in the flattening process for very accurate cutting patterns
    • Application for almost all mesh arrangements
    • Recognition of adjacent cutting pattern definitions to keep the same length
    • Mesh application for main calculation
  9. Representation of cutting pattern on RFEM model

    RF-CUTTING-PATTERN | Results

    After the calculation, the ‘Point Coordinates’ tab appears in the cutting pattern dialog box, which shows the result in the form of a table with coordinates and a surface in the graphical window. The coordinate table presents new flattened coordinates relative to the centroid of the cutting pattern for each mesh node. Furthermore, the cutting pattern with the coordinate system at the centroid is represented in the graphical window. When selecting a table cell, the respective node is displayed with an arrow in the graphic. In addition, the area of the cutting pattern is displayed below the node table.

    Moreover, standard stress/strain results for each pattern are displayed in the RF‑CUTTING‑PATTERN load case.

    Features:
    • Results in a table including information about the cutting pattern
    • Intelligent table relating to the graphic
    • Results of flattened geometry in a DXF file
    • Results in the global printout report
    • Results of strains after flattening for the evaluation of patterns

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