Definition of Plastic design
The method in which the analysis for loads is also performed considering the collapse mechanism of the structure is known as plastic design.
It is similar to LRFD to some extent. Full reserve strength of the structure due to indeterminacy and inner elastic portion of the structure is utilized. Inelastic material behavior is considered in the analysis and design. Deflections and other serviceability conditions become more important in this method.
Reference : Steel structures by Zahid Ahmad siddiqi and Muhammad ashraf