As hacks go, this is an inexpensive solution to a common problem. Billy bookcase shelves have a reputation for sagging. It’s easy to strengthen the shelves with this hack. How much more weight can the shelves bear? I don’t know, but mine were holding full and even double-shelved text books with no problems.
Hacksaw or metal cutting blade of some sort
File to smooth the cut ends
Aluminium “U-channel” strips from the local home improvement store (One 8-foot strip can stiffen three 30-inch shelves.)
Billy bookcase or other shelves you want to reinforce.
Take a shelf with you and buy a channel that snugly slips over the shelf edge.
My Billy shelves used 3/4 inch channel, but test before you buy.
Cut the channel into pieces about 6 inches shorter than your shelf.
Use the file to smooth the rough edges on the cut ends.
Slide the channel over the back edge of the shelf, center it, and place the shelf on the pegs.