A plane truss is a rigid framework satisfying the following conditions:

1. The members of the framework all lie in the same plane.
2. The members are connected at their ends by frictionless pins.
3. All of the external loads lie in the plane of the framework and are applied at the joints only. This means that the bending effect of the weights of the bars is neglected, since all of the external loads, including the weights of the members, are assumed to act at the joints.

It will be seen that each of the members of a plane truss is acted upon only by forces at its two ends and hence is a two-force member. The lines of action of the forces applied to the members will thus lie along the centerlines of the members.

It will be evident that many actual structures will depart considerably from these conditions. The members, for example, are often welded together at their ends rather than being joined by smooth pins or hinges, so that an additional rigidity is introduced into the structure. In some structures the external loads may be applied at such a location that an appreciable amount of bending occurs in the members. The stresses (force per unit area) in the structure which are due to the axial forces computed on the assumption that the structure may be considered as a plane truss are called the primary stresses. Those stresses which are due to departures from the conditions for a plane truss are called secondary stresses. In most practical problems the primary stresses are so much larger than the secondary stresses that these secondary stresses can be neglected. In the following paragraphs we shall discuss several methods of determining the axial forces in plane trusses.