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Why Dlubal Software?
RFEM and RSTAB are able to cover a large number of branches in the field of building and construction industry with their generally usable structural frame analysis and FEM programs. The design of cable structures is thus also possible in both software solutions. In the following, some assistance tools for modeling and design will be presented.
In theory, an ideal gas consists of freely moving mass particles without extension in a volume space. In this space, each particle moves at a speed in one direction. The collision of one particle with another particle or the volume limitations lead to a deflection and a change in the speed of the particles.
Cable and tensile membrane structures are regarded as very slender and aesthetic building constructions. The partly very complex double-curved shapes can be found using suitable form-finding algorithms. A possible solution is, for example, to search the form via the equilibrium between the surface stress (provided prestress and an additional load such as self-weight, pressure, etc.) and the given boundary conditions.
This paper is focused on the specific aspects of designing membrane structures. These structures have specific requirements such as form-finding and cutting patterns generation. These topics are the focus of many research works and there are many methods proposed to achieve the desired results. However in this paper these issues will be discussed more from the practical point of view than from the theoretical one. The paper will be followed by examples, which will complement the discussed topic and show the special attention required during the design process.
The RF-FORM-FINDING add-on module determines equilibrium shapes of membrane and cable elements in RFEM. In this calculation process, the program searches for such a geometric position in which the surface stress/prestress of membranes and cables is in equilibrium with natural and geometric boundary conditions. This process is called form-finding (hereinafter referred to as FF). The FF calculation can be activated in RFEM globally in the "General Data" of a model, "Options" tab. After selecting the corresponding option, a new load case or a calculation process called RF-FORM FINDING is created in RFEM. An additional FF parameter is available for defining surface stress and prestress when entering cables and membranes. By activating the FF option, the program always starts the form-finding process before the pure structural calculation of internal forces, deformation, eigenvalues, etc., and generates a corresponding prestressed model for further analysis.
During the form‑finding process, the elasticity of a substructure is also taken into account when searching for the equilibrium state. You can also consider large deflections of supporting trusses or pure bending deformation of the edge beams when determining the membrane shape.
The form-finding process in RFEM seeks an equilibrium state, where the defined prestress of membranes and the prestress or length changes of cable elements with boundary reactions are in balance. For this, the program provides the option to define an isotropic or an orthotropic prestress state for membranes.
In RFEM, there is a file‑based and direct DXF interface. The file‑based DXF interface allows you to export the data in a DXF file that is transferred directly into an open AutoCAD file. In the interface dialog box, you can select which data are to be exported (results as isolines, result values, finite element mesh with boundary and integration lines).
The RF‑FORM‑FINDING add‑on module can be activated in the “Edit Model - General Data” window, “Options” tab. By activating the module, a new RF‑FORM‑FINDING load case is created and an additional menu appears in the main program, allowing for the definition of prestress conditions for membrane and cable elements.
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