Member stiffness with offset hinges

In the most simple case of plastic collapse, it is said that a hinge forms in a member when the moment capacity is reached. In the more complex case of plastic collapse, when the effect of axial load on the moment capacity is considered, it is argued here that an "offset hinge" is formed since as you move along an interaction or limit surface, the incremental moment is proportional to the incremental axial load. In this note the stiffness of a plane, elastic, straight member with offset hinges at either or both ends is derived. An example of plastic collapse taken from the European literature is included.