Dynamic Changes of Mode in Rigid Plastic Structures

An account is given of some theoretical properties of structures having rigid-plastic characteristics, when acted on by time-dependent loads producing accelerated motion of the members. Mode changes are studied and conditions for change are examined, taking as an example a fixed base portal frame with two attached masses. It is shown that the dynamics of motion may be represented graphically by the use of a dynamic interaction diagram, which is analogous to the load-interaction diagram of static plasticity. The kinematics of motion can also be represented graphically on a diagram which shows the velocities of points on the frame at any instant. The reciprocal properties of these diagrams are discussed, and the use of the diagrams in the solution of simple problems is described.