The scope of the work involves carrying out the process of strengthening 6-meter IPE180 beams in a simply supported beam scheme using batten plates under an existing load. It is meant to simulate the situation of reinforcing an existing building structure without relieving the load, supporting it or dismantling the structural elements. As part of the work, a test stand was designed and constructed that allowed the introduction of a concentrated force at the center of the span of the test beams. During the test, it was assumed that 3 beams would be strengthened under a load of more than 50% of their load capacity, and 3 more beams would be strengthened without subjecting them to prior loading. Before the test their real geometry and imperfections were measured using a 3D scanner and a tachymeter. Strengthening under load included welding of plates in the support area, as well as verification of the effect of a local temperature increase on deformation in the beam when the load was held constant. The geometry of the beams during the test was recorded using the ARAMIS system, and deformations were measured at key locations using measuring systems built with strain gauges. The study showed that the use of batten plates can significantly increase the load carrying capacity of I-beams by being a warping restraint, and that welding them even under existing load does not cause a definite increase in beam strain due to the location of the reinforcing plates in a region of low element strain.