| 摘要 |
A polygon in a tabular portion includes first and second axes of symmetry substantially orthogonal to each other. A top surface of the tabular portion has a swelling portion (13) and an outline of the swelling portion is line-symmetric with respect to the first and second axes of symmetry. In a lateral half portion of the outline of the swelling portion, a short-side side crest portion (21A) that is convex toward the polygonal outline in the direction of the first axis of symmetry, concave corner-side trough portions (22A, 22B) connected to each end of the short-side side crest portion (21A) and long-side side crest portions (23A, 23B) that are connected to each corner-side trough portion (22A, 22B) and convex toward the polygonal outline are formed. On one of a top surface and an undersurface of the swelling portion, a pair of lateral ribs (31A, 31 B) or a pair of longitudinal ribs (32A, 32B) that spreads toward the polygonal outline and at least transversely crosses the swelling portion is formed. A structure that mechanically and effectively improves strength of a resin-made substantially polygonal (four or more corners) tabular portion is thus provided. |
