[EN] FAQ 004134 | How can I influence the sag of the found shape in RF‑FORM‑FINDING?

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.

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 geometric position where 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.

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.

The Ponding load type allows you to simulate rain actions on multi-curved surfaces, taking into account the displacements according to the large deformation analysis.

This numerical rainfall process examines the assigned surface geometry and determines which rainfall portions drain away and which rainfall portions accumulate in puddles (water pockets) on the surface. The puddle size then results in a corresponding vertical load for the structural analysis.

For example, you can use this feature in the analysis of approximately horizontal membrane roof geometries subjected to rain loading.

Go to Explanatory Video

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
• Additional 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 types "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