A structural element includes an internal rib and mounting structure having end ribs joined by rails to a plurality of intermediate ribs. A cylindrical skin positioned about the cylindrical rib and mounting structure allows the structure to slide in and out of the skin. A cylindrical male end joint can be a fastening to the end rib of the mounting structure to retain the structure within the skin. Likewise, a female end joint is secured on the other end rib of the structure to complete the structural element. The end joints are sealed against the skin to prevent leakage. The rails can be provided as rail sections joining adjacent ribs.