5.1 Double-Layer Tensegrities: Introduction
For most of the tensegrities discussed so far, the tensile members compose a single continuous spherical layer.1 Such structures are resilient, but are not very rigid and tend to vibrate too much for many practical applications. Also, it seems likely that large-frequency realizations of these structures, as can happen with geodesic domes, have little resistance to concentrated loads, so that it would be difficult to suspend substructures from the their roofs, and they might cave in excessively under an uneven load like snow.
These considerations are a strong motivation for the development of a space truss configuration for tensegrity structures. Such a configuration would be analogous to the space truss arrangements developed for the geodesic dome, like the Kaiser domes of Don Richter,2 or Fuller and Sadao's Expo '67 Dome,3 and serve the same purpose. Tensegrity space trusses are characterized by an outer and inner shell of tendons interconnected by a collection of struts and tendons. The result is a more rigid structure which is more resistant to concentrated loads.
Designs for tensegrity trusses have been developed in a planar context by several authors. The trusses described in this book, especially the geodesic one described in Section 5.3, are akin to those experimented with by Kenneth Snelson in the 1950's.4 Appendix A compares the truss of Section 5.3 with an example from Snelson's work and another similar approach from other authors.
In Section 5.2, a general approach to the design of tensegrity trusses is outlined. Then, in Sections 5.3 and 5.4, two examples are given of geometries which implement this approach. The second example demonstrates incidentally how icosahedral symmetries can be handled within the Cartesian framework.