The paper focuses on presenting the methodology of testing corner joints along with the presentation of obtained results. The objective of the conducted research was to experimentally determine the behaviour of welded knees with slender webs under similar loading condition that occur in the knee joints of portal steel frames. Laboratory tests were carried out on a specially prepared testing stand. During the laboratory work, load tests were performed on three identical knees. The testing stand was equipped with measurement equipment, including two data acquisition systems: the SAD 256 for displacement and force sensors, and the Pontos/Aramis – a digital image correlation system. The first knee test served as a verification of the testing stand and the adopted data collection method. After implementing certain changes to the stand, the remaining load tests on the knees were conducted. Based on the conducted studies and data analysis, moment-rotation M - ϕ characteristics of the joints were determined, along with parameters such as initial stiffness S, elastic bending resistance Mel and ultimate bending resistance Mu.