| 摘要 |
<p>A groove-like outer curve section (21A) is provided in the outer peripheral surface of the vertically-oriented intermediate section of a round body (18) so as to extend around the entire circumference of the round body, and a groove-like inner curve section (21B) is provided in the inner peripheral surface of the vertically-oriented intermediate section of the round body (18) so as to extend around the entire circumference of the round body. The configuration enables the plate thickness (t) of an hourglass-shaped thin-walled section (21) located in the intermediate section in the vertical direction to be less than the plate thickness (T) of upper and lower thick-walled sections (19, 20) located at opposite ends of the round body (18) in the vertical direction. As a result, the weight of the entire round body (18) can be reduced in comparison with a prior art round body having a uniform plate thickness along the vertical direction. Also, when a load in the vertical direction is applied to the round body (18), stress can be dispersed to the outer curve section (21A) and the inner curve section (21B), and as a result, localized stress concentration at the joint between the round body (18) and an upper plate (15) of a center frame (14) is minimized.</p> |
