Answer:
Form-finding warps surfaces into each other because the defined prestress on the boundary line between two membranes does not coincide at the same angle, thus the affected FE nodes are not in equilibrium with the boundary conditions. In this case, the FE nodes of the membrane surfaces move in the direction of the remaining resultants until the form-finding process finds a solution within the tolerance limit. Since it is usually only possible to achieve this solution with large displacement of FE nodes in space, the surfaces assigned to the FE nodes seem to "swim" into each other.
This behavior results in an unfavorable FE mesh arrangement in conjunction with a singular membrane force distribution. In this case, it is necessary to adjust the prestress in order to reach the equilibrium, or to set an element at the limit situation in order to transfer the forces due to the disequilibrium.
