短语词组短语main steel girder 钢主梁main box girder 箱形主梁main hull girder 主船体桁main longitudinal girder 主纵梁single -main-girder 单主梁box -main-girder 箱形主梁main girder webs 主梁腹板fetching material main girder 取料主梁main girder web 主梁腹板 返回 main girder