In the case of an isotropic stress condition, the stress in both surface axes (warp and fill directions or x and y) is equal. The orientation of the surface axis system is irrelevant in this case.
In the orthotropic solution, you can define a different prestress for both axes. This requires the axes in the desired direction to display different prestresses. Since the membrane elements are moved to a new place during the form‑finding process while the axis system orientation basically depends on the position of the elements, it is necessary to define the axis system of the elements independently of the structure for an orthotropic prestress. Without this step, the input box for the elements remains inactive.
You can define the axis orientation of membrane elements in the "Edit Surface" dialog box, "Axes" tab. Prestresses of membrane elements can be entered in the "General" tab using the [Edit Parameters of Stiffness] button in the "Surface Type" section, "Form‑finding" tab.
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