Simple two-dimensional (2-D) truss structures are formed by combining one-dimensional linear members to create a triangular pattern. One starts with a triangular unit, and then adds a pair of members to form an additional triangular unit. This process is repeated until the complete structure is assembled.below Figure illustrates this process for the case where all the members are contained in a single plane. Such structures are called plane trusses; the nodes are also called “Joints.”
Three members connected at their ends form a rigid structure in the sense that, when loaded, the change in shape of the structure is due only to the deformation of the members. It follows that a structure constructed in the manner described above is also rigid provided that the structure is suitably supported.
Simple three-dimensional (3-D) space trusses are composed of tetrahedral units. Starting with a tetrahedral unit, one forms an additional tetrahedral unit by adding three linear elements, as illustrated in below Figure When the structure is suitably supported to prevent rigid body motion, the assemblage is rigid.