This paper presents the variational calculation method for open profile thin-walled bar systems strengthened under load. The main ratios of the theory are given. The calculation of the stress-strain state of the strengthened structure consists of the following stages: 1. Determining repair stress and strain in the elements of the bar system under the influence of repair loadings; 2. Calculating the stressstrain state of the structure after strengthening, allowing for the effect of operational loadings and repair stresses. Based on the stated method a computer program was developed. The program can be used to simulate the work of the strengthened structure and monitor its stress-strain state at different strengthening stages. The results of of the strengthened thin-walled bar system calculations are presented, allowing for stress and strain at different strengthening stages.