Plastic mechanism analysis

Design for ductile response is a well-used strategy for reducing design forces in earthquake-resisting structures, and typically requires the use of special details to enhance ductility capacity at locations expected to undergo inelastic response. This chapter describes the use of a kinematically admissible mechanism for determining the locations of the plastic hinges, and a virtual work method of analysis for determining the strengths required at these locations. It describes the method of analysis and addresses a design context. For the case with high gravity loads, superposition of gravity and seismic moments causes a negative moment hinge to develop at one end of the beam and a positive moment hinge along the beam span. By proportioning the structure to develop an intended plastic mechanism, it is ensured that the intended mechanism is the one that develops.