Space Structures:

A space structure is a structure in a three dimensional space, for example space trusses, the basic element of which consists of six bars joined at ends to form tetrahedron. There are six fundamental motions possible, viz., translation in directions of x, y and z and rotations around x, y and z axes. Based on the constraints provided by the supports, the number of unknown reactions varies from one to six.

Frictionless surfaces, cables and ball supports prevent translation in only one direction and thus exert a single force of known line of action. The magnitude of the reaction is the only unknown in such of the cases. Wheels on the rails or rollers on rough surfaces include two unknown reaction components like motion in two directions is limited. In case of ball and socket joints or rough surfaces in direct contact, there are three unknown as they stop translation in three directions. A universal joint which does not permits rotation about one axis will involve 4 unknowns, viz., three components along x, y and z directions & a couple. A fixed support does not permits any motion, neither translation nor rotation. It includes maximum number of reaction components: three force components and three couples.