A plane structure can be defined as a number of members, joined together in a single plane with the ability to support loads, forces and moments.
A structure comprising members which are pin-jointed at their ends is called a truss. The examples dealt with here are two-dimensional or plane trusses, although the theory developed here can be extended to three-dimensional structures. The important simplifying assumption in trusses is that the pin joints are frictionless and thus can transmit no moment from one member to any other. Members can only sustain tension or compression, such members being called ties or struts respectively.