Plastic Subassemblage Analysis for Unbraced Frames

Plastic methods of design for unbraced multistory frames are well developed. One of the most successful is plastic moment balancing. Following the preliminary design of a frame by moment balancing, a load-drift analysis of each story of the frame is required. The theoretical development of a plastic method of analysis suitable for determining the load-drift behavior of each story of an unbraced frame is presented. The analysis (designated the subassemblage method of analysis) is particularly applicable to the middle and lower stories of an unbraced frame where the PΔ moments become significant. It is based on the concept of sway subassemblages and uses directly the results of recent studies of restrained columns permitted to sway. In the subassemblage method of analysis a one-story assemblage is isolated from the frame. The load-drift behavior of the one-story assemblage is determined either manually using charts or by computer.