A shelf is connected to risers that are screwthreaded together to achieve a clamping effect between the shelves and the risers. Each shelf may comprise a plastic shelf portion and at least one reinforcement portion secured to the plastic shelf portion to reinforce the plastic shelf portion along its length. In one embodiment a plurality of metal bars are used as the reinforcement portion. The reinforcement portions may be inserted into channels formed in the plastic shelve portions and may be attached to the shelve portions using a separate attachment mechanism.