Indeterminacy of reciprocal diagrams: 3-valent primal

Based on the assumption that all loading is vertical, two-dimensional diagrams can be used to describe and control the three-dimensional equilibrium of a network of branches and nodes. The horizontal projection of the network serves as a form diagram. The reciprocal force diagram describes the in-plane equilibrium of the form diagram and therefore the horizontal equilibrium of the three-dimensional network. In a 3-valent network, the forces are uniquely defined, i.e. it is statically determinate. This is clearly visible in the force diagram: it is triangulated and therefore locked, except for an overall scale factor of the system. Decreasing the scale factor of the force diagram results in overall lower horizontal forces in the system and hence a deeper solution.