| 摘要 |
One plate having different thicknesses (18) is formed by welding from both sides in the plate thickness direction both faces of a first front lower plate (17A) having a plate thickness equal to the plate thickness of a lower joining plate (33F) of an arm-side mounting member (33) and a second front lower plate (17B) having a plate thickness equal to the plate thickness of a third front lower plate (17C). Thus, a single face weld that uses a backing material (23) can be performed from the exterior of a box-type structure (12) on the lower joining plate (33F) of the arm-side mounting member (33) and the first front lower plate (17A) constituting the plate having different thicknesses (18). Meanwhile, a single face weld that uses a backing material (26) can be performed from the exterior of the box-type structure (12) on the third front lower plate (17C) joined to left and right side plates (13, 13') on the bottom side and the second front lower plate (17B) constituting the plate having different thicknesses (18). |
