A structure may consist of a truss or a frame pin-connected or rigidly secured. A truss is an assemblage of slender bars fastened together at their ends by smooth bolts or ball-and socket joints acting as hinges. A truss, by definition, is a connected structure.
The bar members, therefore, act as two-force members which can either be in tension or in compression: there can be no transverse force in a member of a truss. A frame structure, on the other hand, consists of members which may be subjected to a transverse load in addition to the axial load.
A simple structure is thus a pin connected frame or truss. A truss consists of slender -bar members which can carry no transverse loads. It follows that the loading in a truss must be at the joints only. A truss consisting of members which lie in plane and are loaded in the same plane is called plane truss.
If a truss is made of noncoplanar members, it is refer red to as space truss. Similar ly, a frame may be a plane frame or a space frame depending upon its structure.