Many of the metal shelving systems you can purchase have a very thin piece of particle board, also known as pressboard, for the shelf itself.  As you start to put weight on these shelves they end up sagging, especially if you have the shelf setup in a hot/humid garage, shed, barn, etc.  This bracket helps to reduce/remove/prevent the sag using standard 2"X3" lumber found at the local box store for most Western areas of the world.

To install this bracket/brace you need the following per shelf:

• 6 X shelf brackets
• 3 X 2" X 3" (cut to depth of your shelf) with 40 degree bevel on the top of both ends
• 6 X 3" wood screws

The braces have a 40 degree bevel on the top to match the metal shelf runner, after you cut the 2X3 to length you need to bevel the top of each board to match it so the wood will slide inside the printed shelf brace.  Space out the supports equally across the metal shelf runner and using the printed brace as a template mark where you need to drill holes, alternatively you could use self-tapping screws but you will need to be careful how far you drill these in as they will strip the wood easily.  After you have the holes drilled and the wood cut you simply put the 3" long screw from the outside and fasten it all together.

With these braces I have successfully put the full weight on the each shelf (500lbs for one and 400lbs for another) and the thin wood shelf no longer bowed.