Thus, membranes are considered separately and meshed automatically with triangles in comparison with all other elements. This global setting is covered in FE Mesh Settings.
Membrane Meshing
The form-finding process in RF-FORM-FINDING displaces the corner nodes of FE elements of a membrane surface in space until the defined surface stress is in equilibrium with the boundary conditions. This displacement is independent of the element geometry. In the case of elements with four corner nodes, the free displacement may cause spatial drilling in the element plane and thus exceed the validity limits of the calculation; therefore, triangular elements are generally recommended for form‑finding systems. Triangular elements remain independent of the corner node displacement and stay within the calculation limitations.
Author
Mr. Niemeier is responsible for the development of RFEM, RSTAB, RWIND Simulation, and in the area of membrane structures. He is also responsible for quality assurance and customer support.
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Windbreak structures are special types of fabric structures which protect the environment from harmful chemical particles, abate wind erosion, and help to maintain valuable sources. RFEM and RWIND are used for wind-structure analysis as one-way fluid-structure interaction (FSI).
This article demonstrates how to structural design windbreak structures using RFEM and RWIND.
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.
![Basic Shapes of Membrane Structures [1]](/en/webimage/009595/2419502/01-en-png-png.png?mw=512&hash=6ca63b32e8ca5da057de21c4f204d41103e6fe20)
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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.
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.
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- 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.
Although I modeled two identical systems, I got a different shape. Why?
It seems that the members stay not deformed after my RF‑FORM‑FINDING calculation. What did I wrong?
Can I export my cutting pattern?
How do I model a suspended membrane roof structure with line supports?
How do I model a tent roof with two cone tips?
How is an inflatable object simulated in RFEM?
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