Abstract: A reinforcement device comprises a rigid post with hook members at spaced positions therein for selectively mounting the rigid post alongside mounting posts within a conventional trailer. The hook members permit the reinforcement device to be selectively removed so that the rigid posts can be used when loose cargo, which imposes lateral forces on the sidewalls, is carried to resist bending of the sidewalls while permitting the maximum interior dimension of the trailer to be used when carrying cargo within containers by simply removing the rigid posts.