Strengthening can increase or recover the bearing capacity of steel constructions of buildings and structures in operation. Besides well-known strengthening techniques, including an increase in the sectional area by means of attaching steel plates, angles, channels, pipes etc. to a strengthened element, other methods, that involve the use of carbon fiber-reinforced composite materials, have strong prospects. So far, the structural behaviour of steel constructions, strengthened with carbon fiber-reinforced composite materials, is understudied, and this fact restrains the practical application of this strengthening method. The article presents the results of complex experimental, theoretical and numerical studies of the features of the operation of steel-stretched elements reinforced with glued carbon fiber. The emphasis is on the load-bearing capacity of the reinforced element, and not on the mechanism of destruction of the glue. This is due to the use of an adhesive joint performed using the glue and gluing technology recommended by the manufacturer of carbon fiber. It has been experimentally established that, in this case, the stresses in carbon fiber cannot exceed a certain value. Theoretical dependences for the calculation of CFRP-reinforced steel stretched elements are proposed. The scientific novelty of this research project is a set of basic principles and methods, developed to identify the bearing capacity of steel rods, strengthened with carbon fiber-reinforced composite materials, taking into account the joint strength performance of a steel rod and adhesively bonded carbon-fiber-reinforced composite material, as well as the new findings thus obtained, such as the theoretical dependencies needed to identify the bearing capacity of steel rods strengthened with carbon-fiber-reinforced composite materials; experimental data on the joint strength performance of carbon-fiber-reinforced composite lamellas attached to a steel rod by an adhesive; experimental data on the performance and the bearing capacity of steel rods strengthened with carbon-fiber-reinforced composite lamellas; development of finite element models of steel rods strengthened with carbon-fiber-reinforced composite materials, and computational studies of steel rods strengthened with carbon fiber-reinforced composite materials.

When reinforcing metal structures, an increase of bearing capacity is achieved due to inclusion in operation which assume part of the current load. Most often, such an element is a rolling corner, channel, pipe, or steel strip. As an alternative, carbon fiber composite elements may be used for reinforcement. In this case the reinforcement element is best performed from a carbon fiber lamel. Lamel can be used as reinforcement of steel structures working both for tension and bending. When reinforcing the bending elements, the carbon fiber lamel is glued to the stretched side of the beam. The article presents the test results of beams reinforced with carbon fiber. Experimental samples are made of square steel pipes with a section of 50x3 mm, length of 1000 mm. Reinforcement is performed by carbon fiber lamel with section of 25x1.2 mm, which is glued with two-component glue. The choice of parameters of the investigated construction is due to the characteristics of the test equipment. Samples cut from the steel pipe were pre-tested to determine the mechanical characteristics of the steel. Also, the properties of the carbon fiber lamel were clarified by the tests of the samples. When evaluating the experimental load, were used the previously obtained data on the lamel inclusion in the work before the exhaustion of the strength of the adhesive layer. Method of carrying out tests of reinforced beam is presented, test installation, used equipment and step-by-step progress of experiment is described. Analysis of the experimental data showed a significant effect of the gain on the bearing capacity of the beam. It has been found that due to reinforcement of carbon fiber, beam strength increases in comparison with elastic stage of operation by 9-17%.

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