| 摘要 |
PROBLEM TO BE SOLVED: To enhance fatigue strength of a box type structure and to improve operability in welding a plate material closing the box type structure.SOLUTION: A first front lower plate 17A having a plate thickness t3 equal to a plate thickness t4 of a lower joint plate 33F of an arm-side fitting member 33, and a second front lower plate 17B having a plate thickness t3b equal to a plate thickness t3c of a third front lower plate 17C have both their surfaces welded together from both plate thickness-directional sides so as to form one different-thickness plate 18. One-side welding using a backing material 23 from outside the box structure 12 can be performed on a lower joint plate 33F of the arm-side fitting member 33 and the first front lower plate 17A constituting the different-thickness plate 18. Further, one-side welding using a backing material 26 from outside the box type structure 12 can be performed on the third front lower plate 17C joined to lower end sides of left and right side plates 13, 13' and a second front lower plate 17B constituting the different-thickness plate 18. |
