In this article effectiveness of strengthening of angle member by use of additional “new” member that is attached to the “old” member in selected points was analysed. The loading force was applied at the end points of “old” member intended to be strengthened. This way of load application is a result of the solutions commonly used in practice. Single bar with initial bow imperfection was loaded before strengthening with axial force lower than its load capacity. In the presented analysis of built-up member formed thanks to new branch addition, the aforementioned initial loading force was included as well as load eccentricity, joint flexibility and plasticization of cross-section of strengthened angle. Estimated load capacities were compared with load capacity of a member that was initially prefabricated as a built-up member, eccentrically loaded. An experimental research program including problems mentioned in this article was initially mentioned.