In recent years, there has been disucussions about reducing cost and labor in bridge fabrication. There are some ways of reducing cost, automation is one of the ways . It recently has been further developed for bridge fabrication. In the case of girders of box-section, only the panels composing them are fabricated in assembly lines with robots. The automatic welding has, however, not been fully utilized for formation of box section, because it is difficult to manipulate the welding machines inside, their interior structures are complicated . Instead, manual welding has generally been used for this purpose, but welders are obliged to work under very adverse circumstances in this case. The authors proposed a method of using robots for welding in the interior of box-girders by which diaphragm and cross rib details are improved. In this paper, i mproving diaphragm details are discussed though stress analysis using finite element method
