Automation has recently extensively developed in bridge fabrication. No less than 70 % of welding work for assembly of girders of I-section is performed by automatic welding at present. 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, and the feasibility of use of the automatic welding has been regarded to be too low. Instead, manual welding has generally been used for this purpose, but welders are obliged to work under very adverse circumstances in this case. In this paper the authors propose a method using robots for welding in the interior of box-girders, for which simulation by 3-D/CAD is conducted and also various suggestions related to the assembly lines equipped with robots are presented.
