This doctoral thesis by Rostislav Lang deals with the design and analysis of membrane structures using FEA software.
RF-FORM-FINDING Add-on Module for RFEM
Form-Finding of Membrane, Cable, Shell and Beam Structures
“RFEM is the best I have used. I have experience with RISA, STAAD, ETABS, Visual Analysis, and others. In the tensile/fabric structure world, I've tried NDN, Forten, etc. Once you get used to RFEM's interface, it has no comparison to the others. Even with typical structures, it's much easier.”
Doctoral Thesis on Membrane Structure Design According to FEA
The RF-FORM-FINDING add‑on module searches for shapes of structures subjected to tension or compression, such as membrane and cable structures, shells, and so on. The shape is calculated by the equilibrium between the surface stress (specified prestress and additional load such as self‑weight, pressure, etc.), and the given boundary conditions.
The new prestressed shape is afterwards used as the initial state for the structural analysis. Cutting patterns for the membrane structures can be determined by the RF‑CUTTING‑PATTERN add‑on module.
- 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
- Form-finding of:
The form-finding function can be activated in the General Data dialog box, Options tab. Prestresses (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 (e.g. cable)
- 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
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 (e.g. membranes) 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
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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