Structures formed of members that are rigidly connected and designed to resist a load are known as frames, while the same members, if connected by pin connections, are known as trusses. Analytically, trusses are considered to be a special case of frame. For frames, it is assumed that there is no interaction between axial, torsional and flexural deformations and that the responses are based on uncoupled extension, torsion and bending theory.
Fixed connections: sometimes referred to as rigid joints, are capable of transferring axial forces as well as moments. It is not possible for any relative rotations between the two connected members without distortion occurring. Fixed connections demand greater attention during design and construction as they are often the source of failures.
Pinned connections: or pined joints are capable of transferring axial forces only. Because there can be relative rotations between the connected members bending moments are not transferrable.