Deployment aspects of a tensegrity-ring pedestrian bridge
Tensegrity structures are spatial systems that are composed of tension and compression components in a self-equilibrated prestress stable state. Although tensegrity systems were first introduced in 1950s, few examples have been used for civil engineering purposes. In this paper, tensegrity-ring modules are used for a deployable pedestrian bridge. Ring modules belong to a special family of tensegrity systems composed of a single strut circuit. Assembled in a “hollow-rope” structure, ring modules were shown to be a viable system for a tensegrity pedestrian bridge. Furthermore, ring modules are deployable systems that can change shape by adjusting cable lengths (cable actuation). This paper focuses on the deployment of a tensegrity-ring pedestrian bridge. A geometric study of the deployment for a single module identified the solution space that allows deployment without strut jamming. The optimal deployment path is identified amongst hundreds of possible solutions. Moreover, the number of actuators required and their placement in the module are determined by the deployment path that is applied. Cable-based actuation often has the drawback of having to control too many cable elements. Therefore, a deployment path that minimizes the number of actuated cables was found. The number of actuated cables is further reduced by employing continuous cables. A first generation prototype made of aluminium struts and steel cables was used to verify experimentally both findings. The structural response during unfolding and folding is studied numerically using a modified dynamic relaxation algorithm. A well-known dynamic-relaxation algorithm is extended to accommodate clustered tensegrity structures (tensegrity systems with continuous cables). The deployment-analysis algorithm applies cable-length changes first, to create mechanisms allowing deployment and then, to find new equilibrium configurations. Deployment is thus carried out through an equilibrium manifold. The deployment-actuation step size is identified as a critical parameter for successful deployment. Large deployment steps lead to instable configurations while small steps are computationally expensive. Due to mechanism-based deployment, the total energy in the structural system remains nearly constant during deployment. Elastic potential energy due to cable tension is the highest energy identified while kinetic energy and the work of torque friction on strut-to-strut joints are relatively low. Finally, internal forces increase during deployment but remain low compared with service self-stress values showing that deployment is not a critical phase for the design of the bridge.