Strength of a shelf
In woodworking, we build different kinds of shelves, for different purposes. Take a very simple one as described here. The strength of this shelf depends on the 2 things. Firstly, the base – how much load it can bear till it breaks. Usually, we use 18 mm or more thick plywood or solid wood. Therefore, chances are less that it will break.
If we still need more strength, see below picture. You have to cut 2 slots in both support pieces. Then fit hard wood or aluminium connecting these slots. If there is risk of making the support blocks weaker when you cut the slot, you can make slot as small as possible and then fit aluminium of steel rods.
More stronger
If you still think that the shelf needs more strength, look at the below diagrams.
Here in the back of the support blocks, we have cut slot in each. Why? It can be used to fit the mounting block. The more accurate the mounting block size, the more the strength will be. 2-3 mounting screws will be ok for a 2′ long shelf. Below are 2 different mounting blocks. Second one will give more strength for the base as well.
To get more strength, use anchor screws in the wall.