outer arched support faces 12 of the substructure part 2 and which can abut against the same help prevent sidewise tilting of the apparatus substructure part 2 has a rectangular shape and stands on four legs or supports 16 see fig4 and 5 of antiskid material arranged beneath the corners four depressions 17 are located at the upper side of the substructure part 2 precisely above the legs 16 and receive the legs 16 of the additional stacked substructure pat 2 the substructure part 2 always remains in a horizontal position the top side of the substructure part has upwardly arched or curved support or contact faces 12 in the end area between these an additional continuous upper support face 10 arched towards the top in an arc shaped manner and located at a higher level is arranged along the longitudinal axis this support face has a smaller arc radius than the two outer support faces 12 the two end sides 19 serve as a side contact for the footboard 1 placed thereon the substructure part 2 has a rectangular or quadratic aperture at the bottom side for fitting additional substructure parts 2 into each other when stacking the base area of the substructure part has two outwardly arranged horizontal support faces 18 in between which it extends to be arched upwards the geometry of the base area corresponds to the geometry of the support face arranged on top of it an upper support face which has already been described is additionally arranged on top of this the footboard 1 has additional recesses 13 at the bottom at its four sides open towards the ground downwardly oriented.