Form-finding :Soap Film

Membrane geometries are defined using soap-film form-finding. In this method, all or part of the structure is modelled as having zero stiffness by assigning Soap Film Form-Finding Properties. It is useful to visualise this as making 1D elements into elastic bands, and 2D elements a soap film.

Use Quad4 and Tri3 to define fabric elements. The initial geometry of the fabric surface only needs to be an approximation of the final expected geometry. Therefore generating simplified flat grids of 2D Quad4 or Tri3 elements can be a fast method of constructing a model. The edges of these simplified grids can be modified to form the boundaries and supports as required.

During soap film form-finding, the nodes within soap film elements tend to wander and coalesce. Spacer elements and Spacer Properties should be input to control the final node positions generated during the form-finding process.

A state of prestress needs to be defined in fabric and boundary elements using Soap Film Properties to give the desired geometry and boundary conditions.

Once the model is defined store the data to a new file name as the results will overwrite the original data. Set up the analysis using the Analysis Wizard and selecting the “Form finding” solver option. An analysis case is prompted for but external loading is not always needed for form-finding. If no external loading is to be considered a blank entry is acceptable.

Once converged, the deflected shape represents a form that is in equilibrium with the specified soap film forces. The resulting forces in Elements with “real” properties will be in equilibrium with their deflected shape. These forces will differ from any prestress input for “real” elements. The option is given to overwrite the data file with the deformed geometry and to store resulting forces as a specific prestress load case.