The device is of the type in which each transverse rib (14) is supported by a pair of lateral uprights (10) and comprises, at each end, a leg (16) which can slide telescopically with respect to the upright (10), and in which each upright (10) has raising means associated thereto for placing the leg (16) in either a lowered or a raised position, which correspond respectively to the unloaded and fully loaded state of the body. The raising means essentially comprise a toggle formed by a manually operated lever (26), which is pivoted at one end to the upright (10) and the other end (30a) of which acts as a handle, and by a connecting rod (28) which is articulated at one end to an intermediate point on the lever (26) and at the other end to the leg (16) of the rib (14). The arrangement is such that, when the lever (26) is lowered against the upright (10) with the handle (30a) down, the leg (16) is lowered, and when the lever (26) is raised against the upright (10) with the handle (30a) high, the leg (16) is raised. Means are provided for locking the lever (26) in both positions.