BOX-SHAPED WELDED STRUCTURE AND POWER SHOVEL

PROBLEM TO BE SOLVED: To suppress the reduction in fatigue strength, and to reduce labor costs in manufacture.SOLUTION: An arm 1 which is in a hollow form with a rectangular cross section and has a length along a shaft center longer than that of each side: comprises an upper plate 2 and a lower plate 3 disposed to face each other across a shaft center 1a; a pair of side plates 4 and 5 disposed to each other across the shaft center 1a perpendicular to the upper plate 2 and the lower plate 3 and having upper ends 4a and 5a and lower ends 4b and 5b welded to the ends of the upper plate 2 and the lower plate 3; and stiffening members 6 and 7 for preventing torsional buckling welded to both the side plates 4 and 5. The positions of the stiffening members 6 and 7 are in an area F between a position D separated from a linear weld (that is a lower surface 2a of the upper plate 2) of the upper plate 2 and the upper end 4a (5a) of the side plate 4 (5) toward the lower plate 3 by three times a thickness t1 of the upper plate 2 or greater and a position E separated from a linear weld (that is an upper surface 3a of the lower plate 3) of the lower plate 3 and a lower end 4b (5b) of the side plate 4 (5) toward the upper plate 2 by four times a thickness t2 of the lower plate 3 or greater.