Second-Order Rigidity and Prestress Stability for Tensegrity Frameworks

This paper defines two concepts of rigidity for tensegrity frameworks (frameworks with cables, bars, and struts): prestress stability and second-order rigidity. We demonstrate a hierarchy of rigidity--first-order rigidity implies prestress stability implies second-order rigidity implies rigiditymfor any framework. Examples show that none of these implications are reversible, even for bar frameworks. Other examples illustrate how these results can be used to create rigid tensegrity frameworks. This paper also develops a duality for second-order rigidity, leading to a test which combines information on the self stresses and the first-order flexes of a framework to detect second-order rigidity. Using this test, the following conjecture of Roth is proven: a plane tensegrity framework, in which the vertices and bars form a strictly convex polygon with additional cables across the interior, is rigid if and only if it is first-order rigid. Key words, tensegrity frameworks, rigid and flexible frameworks, stability of frameworks, static stress, first-order motion, second-order motion AMS subject classifications. Primary, 52C25; Secondary, 70B15, 70C20