The objective of this research was to analyze the elastoplastic behavior of welded connections of steel tubes of square section, subjected to monotonic load by means of the comparative theoretical - experimental analysis of the distortion of the geometry of the cross-section of the elements of the beam-column joint. A rigid support frame designed with STAAD software was constructed for the tests. Pro v8i software, to fix and assemble the gantries and load application system of the universal machine of the materials laboratory of the Engineering Faculty of the University of Zulia, Venezuela. Each test consisted of imposing progressive load on the upper connection of the gantry, to measure in displacements and distortion of the geometry of the elements junction beam column of the lower connection. The theoretical model was made using the finite element program ANSYS Workbench. The results showed: the welded connection presents local failure by punching in the column, reducing the theoretical plastic moment capacity of the Mp gantry by 46%, a scenario of concern for being this capacity used in the design under the Design by Load and Resistance Factors theory. The welded connection (node) does not comply with the strong beam column criterion established by COVENIN Standard 1618:1998. The column of the welded joint must be reinforced to avoid the formation of the plastic ball-and-socket joint and a possible collapse mechanism in the gantry.