Form-Finding in RFEM 6
Technical Article
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Prestress Load Case
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Surface Load - Form-Finding Load Type
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Membrane Input Axes
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Geometric Form-Finding Load on Member
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Form-Finding Load on Member Introduced as Force
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Analysis Settings
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Global Deformations
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Membrane’s Basic Internal Forces ny
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Consideration of Initial State from Form-Finding Load Case
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Snow Load
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.
Input
Once the structures are modeled in RFEM, the form-finding process can be initiated. In RFEM 6, this is done by assigning a load of the form-finding type to elements such as membranes and cables. The load should be defined in an exclusive load case with the load case category "Prestress", as shown in Image 1.
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Prestress Load Case
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Surface Load - Form-Finding Load Type
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Membrane Input Axes
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Geometric Form-Finding Load on Member
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Form-Finding Load on Member Introduced as Force
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Analysis Settings
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Global Deformations
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Membrane’s Basic Internal Forces ny
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Consideration of Initial State from Form-Finding Load Case
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Snow Load
The input data for the surface load of the form-finding type applied to membranes includes the calculation method (standard or projection), the form-finding definition (force or stress), and the magnitude of the associated load (Image 2). As a matter of fact, the standard method describes a vector that can move freely in space until it reaches the target position, whereas the projection method describes a vector that can move partially in space and is fixed to its XY coordinates. In general, the standard method should be preferred. The projection method is appropriate for models which are spanned around a central axis (conical shapes).
It is important to mention that to apply an orthotropic surface prestress, the Specific Axis option in the Main tab of the surface window should be checked, and the input parameters in Image 3 must be adjusted accordingly.
The member load of the form-finding type, on the other hand, can be geometric or introduced as a force. The former can be defined in terms of length, unstressed length, and sag (including maximum and low-point vertical sag). This is illustrated in Image 4. The latter can be defined as a force in member, tension at the ends, horizontal tension component, or force density (Image 5).
Analysis Settings
The analysis settings can be defined as shown in Image 6. In this way, the user can specify in how many iterations the form-finding calculation should apply the prestress to the elements with the previously defined value. When this limit is exceeded, the program stops repeatedly, applying the prestress with the start value during the form-finding calculation. In fact, increasing the allowable iterations can lead to better convergence.
Next, the Speed of Convergence that controls the calculation stability can be adjusted. The form-finding calculation applies the absolute stiffness to the membrane surfaces, but the user can increase the stiffness by defining a value lower than 1. This results in slower convergence, but higher calculation stability.
The option to integrate the preliminary form-finding is also available in the settings. The preliminary form-finding displaces the FE surface elements by using the simple linear force-density method. Thus, the path between the initial position and the target position is usually reduced for the actual iterative form-finding process. That being the case, a certain amount of computing time is saved, and the global form-finding process is faster.
Results
Once the load case has been calculated, deformations as well as member and surface results can be graphically displayed via the Results tab of the Navigator. The former describes the deformation between the initial input and the determined equilibrium shape, whereas the latter includes the force or stress conditions for the equilibrium shape, considering the defined form-finding parameters. Examples of these results are provided in Images 7 and 8, respectively.
Subsequent Load Application
At this point, the available results represent a new model configuration. 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. Thus, if a certain load should be applied afterwards, the initial state from the form-finding load case must be considered. In other words, the deformation in terms of subsequent load cases applies to the previously determined equilibrium shape.
The option to consider the initial state from the form-finding load case can be activated as shown in Image 9. An example of such a load application is provided Image 10, where snow is applied with respect to the equilibrium shape of the lightweight structure.
Author
Irena Kirova, M.Sc.
Marketing & Customer Support
Ms. Kirova is responsible for the creation of technical articles at Dlubal and supports our users in customer support.
Keywords
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Contact our free e-mail, chat, or forum support or find various suggested solutions and useful tips on our FAQ page.
Using the "End Prestress" Load Type
Until now, the prestress load type had always been an initial prestress in Dlubal Software programs. The defined load magnitude was applied and, depending on the stiffness of the surrounding system, prestress remained more or less as an axial force in the cable.
New
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?
- My aim is to mesh a circular hole plate in a mapped way. Is such a meshing possible in RFEM?
- It seems that the members stay not deformed after my RF-FORM-FINDING calculation. What did I wrong?
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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 "temporary 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?
Associated Products
Main Program
RFEM 6 structural analysis software is the basis of a modular software system. The main RFEM 6 program is used to define structures, materials, and loads of planar and spatial structural systems consisting of plates, walls, shells, and members. The program also allows you to create combined structures as well as to model solid and contact elements.
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