Knockdown steel shelving is disclosed wherein each corner of a shelf is provided with a slidably mounted hook member that extends into the cut-out corner of the shelf. The hook members are guided within the box flange for retraction to clear an upright support member for level adjustment and for extension to a position for engagement within an opening in an upright support member and then tightened to a retracted position by a cam whereby the shelf corner and support are brought into rigid, weight-supporting relationship. The cam operates within a slot in the hook member and can have one of several locked positions including a position wherein the cam is rotated so that the lobe retracts the hook member into a locked position with a bottom edge of the hook member against the bottom wall of the box flange or with a top edge of the hook member against the top wall of the box flange. In either position, the cam may lock at a side-deadcenter position in relation to the other parts. The hook attaching means and cam are essentially concealed, easily installed and adapted for use with various designs of shelves and corner supports. Other embodiments are disclosed.