The disclosure herein provides a technique that provides increased strength to a jointed site of ends of two channel-shaped plates. A first (second) channel-shaped plate includes an elongated first (second) main plate, a first (second) upper flange, and a first (second) lower flange. The first channel-shaped plate includes a first extension at an end thereof, the first extension extending from the first lower flange and a lower part of the first main plate, and extending in a longitudinal direction of the channel-shaped plate. The second channel-shaped plate includes a second extension same as the first extension. The ends of the first and the second channel-shaped plates are assembled and joined such that the first extension is located outside the second channel-shaped plate and the second extension is located outside the first channel-shaped plate. A reinforcement plate is joined to both of the first main plate and the second main plate.