Abstract: Each one of a plurality of parallel horizontal beams has longitudinally spaced vertical slots extending transversely through it, with the lower wall of each slot formed by the top of the free end portion of a tongue struck out of the beam. Resting on these tongues are parallel cross bars that extend through the slots and have notches extending downwardly from the tops of the bars receiving the portions of the beams above the slots, with those portions substantially engaging the bottoms of the notches. The height of the tongues is great enough to permit the bars during assembly to be inserted in the slots while the tongues are bent laterally out of the beams, whereupon the bars can be raised in the slots and the tongues bent back beneath the bars to support them.