Is a shelf with a shelf joint as strong as an ordinary shelf?
Yes, a shelf that has been extended, or shortened, with a shelf joint is as strong as an ordinary shelf. Please note that execution is key if you want a reliable shelf. Always start with a shelf that is somewhat longer than the length of the finished product.
In this example we will shorten the length of a shelf.
Cut the shelf to the required length.
Mount the shelf joint.
Make sure that the edges of the shelves are aligned next to each other.
Avoid this at all cost. If you want to extend a shelf use two shelves that together has a length that is greater then the finished product.