In field studies of one hundred under construction steel skeletons was found that over 70 percent of these frames contain operational defects in the welded connection of braces. Since in Iran, most steel skeletons are made in situ and then installed, therefore, the lack of accuracy in installing end plates of braces during the assembly causing the braces are not fully fitted on end connection plates and thus leading to form gap between braces and the end plates. Unfortunately, the welders to solve the problem, put rebar in the gap between the brace and the connection plate, and then fill it with welding. In this research to quantify the effects of the aforementioned problem in the strength of welded joint, both theoretical and experimental investigations were carried out. The standard welded connection and the welded connection with the rebar as filler were modeled by the ABAQUS finite element software and exerted by tensile force. Similar test samples were prepared and tensile tests were performed. This study shows that in both methods, theoretical and experimental, the strength of welded joint with a rebar as filler is 30 to 33 percent lower than the resistance of the standard welded joint. The result suggests that the need for care and supervision of those involved in the construction of the steel frames related to the technical principles in welding, and in particular welding of braces should be important and serious issues.