This doctoral thesis by Rostislav Lang deals with the design and analysis of membrane structures using FEA software.
Form-Finding for RFEM 6
Form-Finding of Membrane, Cable, 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 Form-Finding add-on finds the optimal shape of members subjected to axial forces and tension-loaded surface models. The shape is determined by the equilibrium between the member axial force or the membrane stress and the existing boundary conditions. The resulting new model shape with impressed force conditions is made available as a universally applicable initial state for further calculation of the entire structure.
- Form-finding of tension-loaded membrane and cable structures
- Compression form-finding of member-shaped supporting structures
- Form search taking into account the entire system with subsequent design
- High quality form-finding by using the iterative URS method
- Pneumatic prestress by means of gas pressure loads
- Use of an ideal gas in a membrane
- Geometric and force-based form-finding specifications for member components
- Isotropic and orthotropic surface prestress
- Stabilized form-finding for conical shapes with tangential and radial prestress
- Organization of all form-finding specifications in one load case
- Application for isotropic and orthotropic materials with distinct warp and weft direction
- Consideration of nonlinear material laws by means of the "Nonlinear Material Behavior" add-on
- Staggered form-finding processes and temporary model supports by means of the "Construction Stages Analysis (CSA)" add-on
- Output of a universally applicable form-found initial state with impressed strains
- Graphical form evaluation by means of colored coordinate and inclination plots
- Automatic assignment of the initial state by load combination wizards
- Display of all results on the initial and deformed shape
By activating the Form-Finding add-on in the Base Data, a form-finding effect is assigned to the load cases with the load case category "Prestress" in conjunction with the form-finding loads from the member, surface, and solid load catalog. Such a prestress load case thus mutates into a form-finding analysis for the entire model with all member, surface, and solid elements defined in it. The form-finding of the relevant member and membrane elements amid the overall model is carried out via special form-finding loads and regular load definitions. The form-finding loads describe the expected state of deformation or force after the form-finding in the elements, and the regular loads describe the external loading of the entire system.
The form-finding process of the load cases with the load case category "Prestress" shifts the initial mesh geometry to an optimally balanced position by means of iterative calculation loops. For this task, the program uses the Updated Reference Strategy (URS) method by Prof. Bletzinger and Prof. Ramm. This technology is characterized by equilibrium shapes that, after the calculation, almost exactly comply with the initially specified form-finding boundary conditions (sag, force, and prestress).
In addition to the pure description of the expected forces or sags on the elements to be formed, the integral approach of the URS also enables a consideration of regular forces. In the overall process, this allows, for example, for a description of the self-weight or a pneumatic pressure by means of corresponding element loads.
All these options give the calculation kernel the potential to calculate anticlastic and synclastic forms that are in an equilibrium of forces for planar or rotationally symmetric geometries. In order to be able to realistically implement both types individually or together in one environment, the calculation specifies two ways to describe the form-finding force vectors:
- Tension method - description of the form-finding force vectors in space for planar geometries
- Projection method - description of the form-finding force vectors on a projection plane with fixation of the horizontal position for conical geometries
The form-finding process outputs a structural model with active forces in the "prestress load case". This load case shows the displacement from the initial input position to the form-found geometry in the deformation results, as well as the state for maintaining the found form in the force or stress-based results (member and surface internal forces, solid stresses, gas pressures, etc.). For the analysis of the shape geometry, a two-dimensional contour line plot with output of the absolute height and an inclination plot for the visualization of the slope situation are offered.
For further calculation and structural analysis of the overall model, the program transfers the form-found geometry including the element-wise strains into a universally applicable initial state for use in the load cases and load combinations.
RFEM 6 includes the Form-Finding add-on to determine the equilibrium shapes of surface models subjected to tension and members subjected to axial forces. Activate this add-on in the model's Base Data and use it to find the geometric position in which the prestress of lightweight structures is in equilibrium with the existing boundary conditions.
Compared to the RF-FORM-FINDING add-on module (RFEM 5), the following new features have been added to the Form-Finding add-on for RFEM 6:
- Specification of all form-finding load boundary conditions in one load case
- Storage of form-finding results as initial state for further model analysis
- Automatic assignment of the form-finding initial state via combination wizards to all load situations of a design situation
- Additionally form-finding geometry boundary conditions for members (unstressed length, maximum vertical sag, low-point vertical sag)
- Additional form-finding load boundary conditions for members (maximum force in member, minimum force in member, horizontal tension component, tension at i-end, tension at j-end, minimum tension at i-end, minimum tension at j-end)
- Material type "Fabric" and "Foil" in material library
- Parallel form-findings in one model
- Simulation of sequentially building form-finding states in connection with the Construction Stages Analysis (CSA) Add-On
- Why do the results in a modal analysis differ between the initial prestress and the surface load?
- Although I have modeled two identical structural systems, I obtain a different shape. Why?
- How is it possible to make factorized combinations of a dead load in the context of form‑finding?
- I want to create a mapped mesh for a circular hole plate. Is it possible to generate such a mesh in RFEM?
- 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 tent roof with two cone tips?
- How do I model a suspended membrane roof structure with line supports?
- I would like to calculate and design "flying structures." What do I need for this?
- I have already divided my membrane roof into individual surfaces. Can I quickly create a cutting pattern from these surfaces?
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